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00oD AGARD REPORT No.7 13N

The Fatigue in Aircraft CorrosionTesting (FACT) Programme

DTIC_____ _____ ____ELECTEL J~R~UTON TATEnmEN X MAY 15.1989

Approved &ez public teleasggDiuio Vtnnizted

D)WVRUTION *M0 AVAILASILIION SACK coVEs

8.95 1~5 090

AG(ARD)R-713

NORTH ATLANTIC TREATY ORGiANIZATION

ADVISORY GROUP FOR,\AE-ROSPACE RE:SEA RCH AND) DEVEIAWME NT

(ORGANISATION DU TRAITE DE LAT%'LANTIQUE NORD)

AGARD Report No.713

THlE FATIGUE IN AIRCRAF[ CORROSION TESTING (FACT) PROGRAMME

by

R.J.H.Wanhill J.J. De LucciaNational Aerospace Laboratory NLR Naval Air Development Center

Emmeloord Warminster. PaThe Netherlands adUSA

MiTRussoNaval Air Development Center--

Warminster. Pa -

USA 4

This Programme was sponsored by the Structures and Materials Panel of AGARD,

THE MISSION OF AGARD

According to its Charter, thc mission of AGARD is to bring together the leading personalities of the NATO nations inthe fields of science and technology relating to acrospace flor the following purposes:

- Recommending effective ways for the member nations to use their research and development capabilities for thecommon benefit of the NATO community:

- Providing scientific and technical advice and assistance to the Military Committee in the field of acrospace researchand development (with particular regard to its military application);

- Continuously stimulating advances in the aerospace sciences relevant to strengthening the common defence posiurc:

- Improving the co-operation among member nations in aerospace research and development:

- Exchange of scientific and technical information:

- Providing assistance to member nations for the purpose of iet sing their scientific and technical potential:

- Rendering scientific and technical assistance, as requested. to other NATO bodies and to member nations inconnection with resear -h and development problems in the aerospace field.

The highest authority within AGARD is the National Delegates Board consisting of officially appointed seniorrepresentatives from each mem~er nation. The mission of AGARD is carried out through the Panels which are composed ofexperts appointed by the Nationvl Delegates, the Consultant and Exchange Programme and the Aerospace ApplicationsStudies Programme. The results of AGARD work are reported to the member nations and the NATO Authorities throughthe AGARD series of publication, or whicit this is one.

Participation in AGARD activities is by invitation only and is normally limited to citizens of the NATO nations.

The content of this publication has been reproduceddirectly from material supplied by AGARD or the authors.

Published February 1989

Copyright C AGARD 1989All Rights Reserved

ISBN 92-835-0495-X

Printed by Specialised Prnting Serices Limited40 Chigwell Lane, Loughton, Essex 1GiO 31Z

ti

PREFACE

In accordance with the mission of AGARD the Structures and Materials Panel (SMP) has always kept an open eye furthe possibility of sponsoring collaborative programmes of research. AGARD is unique in its ability to realise the cooperationof laboratories in up to sixteen nations. In this way AGARD distinguishes itself from other international scientific andtechnical organisations.

In the I1970s the SMP decided to embark on collaborative research activities in the area of fatigue. One of the firstactivities was the Corrosion Fatigue Cooperative Testing Programme (CFCTP). the precursor ito the Fatigue in AircraftCorrosion Testing (FACT) programme. Both programmes arc described in this report.

Failure by fatigue and degradation by corrosion continue to be major considerations in aircraft design. Environmentaleffects influence both initiation and propagation of fa tigue cracks, and dynamic loading may cause more rapid deteriorationofc-crrosion prc tcction sy' :.cm& Thcrcfotc tlw ...,.4..L c6u.. f4dynamic ioadtng and environmental attack. i.e. corrostonfatigue. requires special attention.

Many corrosion fatigue tests have been done on aluminium alloys. However, few included critical structural details likejoints, under realistic cyclic load histories and in service-like environments. Even fewer used practical corrosion protectionsystems. These aspects are specifically addressed by the CFCTP and FACT progammes. The results provide a significantcontribution to the understanding of aircraft corrosion fatigue and should encourage further investigation in this difficultand challenging area of aerospace technology.

H.P. VAN LEEUWEN

f Chairman, Subcommittee onFatigue in AircraftCorrosion Testing (FACT)

Conform~ment a la mission dc IAGARD, Ie Panel des Structures et Matcriaux ISMP) a toujours veillti ass possibilitesdle parrainage dle programmes collaboratifs dle recherche.

La capacit6 d'AGARD de coordonner des ptogrammes dc cooperation entre laboratoires dans les seize pass membresde l'OTAN est unique. Ainsi, AGARD ye distingue de bous 1es autres organismes scientifiques et techniques internationaux.

Au cours des anindes 1970, Ic Panel SMP a pris la ddeision d'entreprendre des activitos de recherche collaborative dansIc domaine dle Ia fatigue. L'une des premi~res initiatives dansce Sens a &t6 Ie Programme Collaboratif dl'ssais dc Fatiguesous Corrosion (CFCTP), pr&urseur du Programme d'essais des interactions fasigueicorrosion des matertaux eonstitutifsdes avions (FACT). Ce rapport donne Ia description des deus programmes.

La rupture de fatigue et Ia d6gradation sous corrosion sont toujours des questions daewalite dans Ia conception desa~ronefs. Les conditions d'ambiance influent Sur Ie d6but et Ia propagation de la fissure. et ['imposition des charges

dynamiques; peut condluire A Ia deterioration aeceldr6e des syst~mes de protection contre Ia corrosion. 11 s'ensuit que ['actionconjointe dle charges dynamnique et de conditions d'ambiance aggr6ssives. cest A dtire Ia fatigue SOLIS corrosion. demande uneattention particuli~re.

De nombreux essais de fatigue SOuIS corrosion ont dt6 effectuds Sur des alliages d'aluminium. mais tr~s peur Sur les6d6ments de structure critiques tels que 1es assemblages dlans des conditions qui simulent 1es conditions rdelles de service, enappliquant des s6qucnces de charges rdclles. L'emplol dle syst~mes pratiques de protection contre Ia corrosion s'av~e mn~meplus rare. Ce sont prtcisement ces aspects qui sont examin6s par les programmes CFCTP et FACT. Les rdsultats obtenusrepr~sentent une contribution importante A ['effort consacrd AI 'analyse de Ia corrosion SOuIS fatigue des mattiriauxaerospatiaux. et ils devraient conduie des travaux dle recherche plus approfondis dans cc domaine difficile et exigeant de Iatechnologie adrospatiale.

i

iii

STRUCTURES AND MATERIALS PANEL

Chairman: Professor Paolo Santini Deputy Chairman: Prof. Dr-lug.Hl.FiorschingDipartimento Aerospaziale lDirektor der DFVLR InstitutUniversita degli Studi di Roma fur Aeroclastik'La Sapienza" Bunsenstrasse. 1t)

Via Eudossiana. 16 D)-340)) 6ttingcn. (icrmanv0185 Roma, Italy

Sub-Committee oilFATIGUE IN AIRCRAFr CORROSION TESTING

Chairman: Dr-h. H.P. van LeeuwenHead of Structures & Materials LaboratoryNational Aerospace Lahoratory-NI.RP.OBox 1538300t Al) Emmelourd. Netherlands

SNIP Members

Wg Cdr D.M.F.Brigh, - UK Mir R.DJ.Maowell - UKDr 1IM.Burte - US Mr W.Ci.Molyneux - UKDr-lug. H.G.J.Carvalhinhos - Po Mr R.Schmidt - USProfessor A.IDeruyttere - Be Cot Dr S.Signoretti - ItProfessor M.Doruk - Tu Dr W.Wallace - CaMr D.AFanner - UK Mr P.R.Wedden - UKProfessor W.G.lleath - UJK Mr H.Zoeher - GeMrJ11..Le - UK

PANEL EXECUTIVE

Mr MC.McConnell

AGARD-OTAN From USA and Canada7, rue Ancelle AGARD-NATO92200 Neuilly sur Seine Attn: SMP ExecutiveFrance APO New York 119777

Tel:(I)4738 5791& 5792 Telex: 611)176F

CONTENTS

PREFACF Page

by H.P. van Leeuwen iii

STRUCTURES AND MATERIALS PANEL iv

ACRONYMS AND TRADE NAMES ix

PART I: PROGRAM-r OBJECTIVES AND DEFINITION

1. INTRODUCTION I

2. OVERVIEW OF THE CORE PROGRAMME (CFCTP) I

3. OVERVIEW OF THE SUPPLEMENTAL PROGRAMME (FACT)

4. REFERENCES 3

PART II: REASSESSMENT OF THE CFCTP CORE PROGRAMME 7

1. INTRODUCTION 7

2. DESCRIPTION OF THE CFCTP CORE PROGRAMME2.1 Material and Specimen Configuration 7

2.2 Protection System a,] Specimen Assembly 7

2.3 Mechnial Testing Conditions (Static Prestressing and Fatigue) 72.4 Environmental Conditions (Pre-exposure, Fatigue and Corrosion Fatigue) 82.5 Summary of the Test Procedure 82.6 Summary of the Statistical Methods for Data Analysis 8

3. RESULTS 93.1 Presentation of Fatigue Life Data 9

3.2 Statistical Analysis of Fatigue Life Data 9

3.2.1 Checking for normality 9

3.2.2 Checking for homogeneity of variances q3.2.3 Main statistical analysis: analysis of variance q3.2.4 "Fine tuning" with the least significant difference test o

3.2.5 "Fine tuning" with Duncan's new multiple range test I,

3.2.6 Checking for adequate sample size and differences in data scatter I,3.3 Presentation of Primary Fatigue Origin Data I1

3.3.1 Classification of all primary fatigue origins 11

3.3.2 Statistical analysis of primary fatigue origin data 12

3.4 Correlation of Fatigue Lives and Primary Origins of Fatigue

4. DISCUSSION 134.1 Primary Purpose of the CFCTP Core Programme

4.2 Environmental Effects 134.2.1 Dependence of environmental effects on stress level 14

4.2.2 Environmental effects and fatigue life data scatter 14

4.3 Primary Fatigue Origin Locations 14

5. CONCLUSIONS 1.

6. ACKNOWLEDGEMENTS 14

7. REFERENCES 15

PART III: THE FACT SUPPLEMENTAL PROGRAMME 37

1. INTRODUCTION 371.1 Overview 371.2 Recommended Specimen Configuration 37

1.3 Flight qlmil1ation Fatigue Testing 371.3.1 Short description of the manoeuvre spectrum FALSTAFF 371.3.2 Short description of the gust spectrum MINITWIST 37

1.4 Establishment of Fatigue Stress Levels for the 1j Dogbone Specimen i8

1.5 References 38

2. THE VOUGHT CONTRIBUTION TO THE FACT PROGRAMME 44

2.1 Introduction 44

2.2 The Test Programme 442.2.1 Material, specimen configuration and protection system 44

2.2.2 Mechanical testing conditions 44

2.2.3 Environmental conditions 44

2.3 Results 45

2.3.1 Fatigue life and tatigue crack initiation and propagation life data 452.3.2 Fatigue crack growth rate data 45

2.4 Discussion 45

2.5 Conclusions 45

2.6 Rec rmmendat ions for Further I net ig..tion 46

2.7 Referencen 46

3. THE SAAB CONTRIBtTItoN TiO HF AtL I i'iRG..!E 533.1 Introduction 533.2 The Test Progr.amme 53

3.2.1 Materials, specimen configurations and protection systems 53

3.2.2 Mechanical testing conditions (static prestressing and fatigue) 533.2.3 Environmental conditions (pre-exposure, fatigue and corrosion fatigue) 53

3.3 Results 54

3.3.1 Statistical methods for analysing the data 54

3.3.2 Fatigue life data 54

3.3.3 Primary fatigue origin data 54

3.4 Discussion 54

3.5 Conclusions 553.6 References 55

4. THE NADC CONTRIBUTION TO THE FACT PROGRAMME 674.1 introduction 674.2 The Test Programme 67

4.2.1 Protection systems and specimen assembly 674.2.2 Mechanical testing conditions (static prestressing and fatigue) 674.2.3 Environmental conditions (pre-exposure, fatigue and corrosion fatigue) 67

4.3 ResuiLa 67

4.3.1 Statistical methods for analysing the data 674.3.2 Fatigue life data 68

4.3.3 Primary fatigue origin data 684.4 Discussion 684.5 Conclusions 484.6 References 69

5. 3HE AFWAL CONTRIBUTION TO THE FACT PROGRAMME 775.1 lntroduction 775.2 The Test Programme 77

5.2.1 Fastener systems 775.2.2 Mechanical testing conditions (static prestressing and fatigue) 77

5.2.3 Environmei.tal condiLions (pre-exposure. fatigue and corrosion fatigue) 775.3 Results 78

5.3.1 Statistical methods for analysing the data 785.3.2 Fatigue life data 78

5.3.3 Primary fatigue origin data 7P5.4 Discussion 785.5 Conclusions 795.6 References 7q

6. THE NDRE CONIRIBUTION TO TIHE FACT PROGRAIMME 8616.1 Introduction 86

6.2 The Test Programme 866.2.1 Material properties 866.2.2 Mechanical testing conditions (static prestressing and fatigue) 8€6.2.3 Environmental conditions (pre-anposure. fatigue and corrosion fatigue) 86

6.3 ilsults 876.3.1 Statistical methods for analysing the data 87

6.3.2 Fatigue life data 876.3.3 Primary fatigue origin data 87

6.4 Discussion 88b.5 Conclusions6.6 References 88

7. THE NLR AND LRTH CONTRIBUTION TO THE FACT PROGRAMME 977.1 Introductio 977.2 The Test Programme q7

7.2.1 Materials and specimen configuration 977.2.2 Protection systems and specimen assembly 977.2.3 Mechanical testing conditions (static prestressing and tatigue) 977.2.4 Environmental conditions (pre-exposure, fatigue aid corr.sion iatigue) 98

7.3 Results 987.3.1 Statistical methods for analysing the data 98

7.3.2 Constant amplitude fatigue life data 987.3.3 Gust spectrum (MINITWISr) fatigue life data Q07.3.4 Manoeuvre spectrum (FALSTAFF) fatigue life data qQi.3.5 Primary fatigue origin data I0

7.3.6 Fatigue lives and primary fatigue origins 1007.4 Discussion 101

7.4.1 Fatigue lives at higher stress levels lol7.4.2 Fatigue lives at lower stre-s Ivels li07.4.3 Primary fatigue origins 101

7.5 Conclusions 101

7.6 References 102

8. THE IABC CONTRIBUTION TO THE FACT PROGRAMME 124

8.1 Introduction 124

8.2 The Test Programme 124

8.2.1 M.L..ials and properties 124

8.2.2 Protection systems and specimen assembly 124

8.2.3 Mechanical testing conditions (static prestressing and fatigue) 124

8.2.4 Environmental conditions (pre-exposure, fatigue and corrosion fatigue) 125

8.3 Results 125




8.4 Discussion 126

8.5 Conclusions 126

8.6 References 127

9. THE RAE CONTRIBUTION TO THE FACT PROGRAMME 138


9.2 The Test Programmes 138

9.2.1 Materials and properties 138

9.2.2 Specimen configurations 138

9.2.3 i dogbone protection systems and specimen assembly 138

9.2.4 Mechanical testing conditions (static prestressing and fatigue) 139

9.2.5 Environmental conditiins (pre-exposure, fatigue and corrosion fatigue) 13


9.3 Results of the Fatigue Strength Test Programme 140

9.3.1 Constant amplitude fatigue tests 140

9.3.2 Manoeuvre spectrum (FALSTAFF) fatigue tests 140

9.4 Results of the Fatigue Crack Growth Resistance Test Programme 141

9.4.1 Constant amplitude fatigue crack growth tests 141

9.4.2 Manoeuvre spectrum (FALSTAFF) fatigue crack growth tests 141

9.5 Results of the Programme on the Effect of Chromate in Primers 141



9.6 Discussion 142

9.6.1 Fatigue in air/fatigue in salt spray 142

9.6.2 Comparisons of materials 142

9.6.3 Effect of chromate in primers 1'2

9.7 Conclusio:s 143

9.7.1 Conclusions for the fatigue strength programme 143

9.7.2 Conclusions for the fatigue crack growth reaistance programme 143

9.7.3 Conclusions for the programme on the effect of chromate in primers 143

9.7.4 Additional conclusions 143

9.8 References 144

10. IHE NRC CONTRIBUTION TO THE FACT PROGRAMME 162


10.2 The Retrogression and Reageing (RRA) Process 162

10.3 The Test Programme !A2

10.3.1 Materials, heat treatments and specimen configurations 162

10.3.2 Fatigue testing conditions 143

10.4 Re"Ilt, 163

10.4.1 Effects of RRA on microstructure, mechanical properties and

electrical conductivity 163

10.4.2 Stress corrosion crack growth rates 164

10.4.3 Fatigue and corrosion fatigue crack growth rates for

centre cracked tension (CCT) specimens 164

10.4.4 Fatigue and corrosion fatigue crack growth rates for

compact tension (CT) specimens 164

10.5 Discussion and Conclusions 164

10.6 References 164

PART IV: EVALUATION OF THU CFCTP AND FACT PROGRAMMES 175

I. INTRODUCTION 175

2. THE CFCTP CORE PROGRAMME 175

3. THE FACT SUPPI.EMENTAL PROGRAMME 175

3.1 Inter-ParticIpant Comparisons of Materials, Protection Systems and Fasteners 176

3.2 Inter-Participant Comparisons of Environmental Fatigue Effects 176

4. CONCLUSIONS AND RECOMMENDATIONS 177

5. REFFRFNCFS 177

APPENDIX i: LOAD TRANSFER AND SECONDARY BEHDIING IN SHE 1h DOGBONE SPECIMEN 187

1. INTRODUCTION 187

2. OVERVIEW OF THE LOAD TRANSFER AND SECONDARY BENDING RATIO PO3P RXME 187

3. RESULTS 187

4. ESTIMATION OF LOAD TRANSFER AND SECONDARY RENDING RATIO IN THE CFCTP AND FACT SPECIMENS 127

5. REFERENCES 188

APPENDIX II: STATISTICAL METHODS 113

1. INTRODUCTION I I

2. CHECKING FOR NORMALITY 19)

2.1 Graphical Procedure for Test of Normality I)

2.2 X2 Test for Goodness of Fit I4

3. TESTING FOR HOMOGENEITY OF VARIANCES 7q

4. ANALYSIS OF VARIANCE I 9$

5. "FINE TUNING" WITH THE LEAST SIGNIFICANT DIFFER;ZIE TEST I !$

6. "FINE TUNING" WITH DUNCAN'S NEW MCLTIPLE RANGE TEST P"

7. LIPSON AND SHETH METHOD FOR ADEQUATE SAMPLE SIZE I"

8. X2 TEST OF INDEPENDENCE. YATES' CORRECTED X2 TEST AND FISHER'S EXACT TEST i-7

8.1 X2 Test of Independence 17

8.1.1 Analysis of primary fatigue origin data 1-;

8.2 Yates' Corrected S2

Test 197

8.2.1 Correlation of fatigue lives and primary fatigue origins 197

8.3 Fisher's Exact Test 19.R

9. REFERENCES I'm

'iii

ACRONYMS AND ERASE NAMES

AEWAI. Air Fore right Aoo''littt !-at It d-oa t-

ACARII Advo r . ; -o Ao act. En or.. r E d t-e 'tt

ALCOA tl~pal AImnu ttt'ittnAt1,d ite 'thr-aer tlners iin -atug cit attiluinr , to.'uulo C: .aI 'T

V4L,;U'ARD twater dtni to cig -trt'sii1'l preu v p

AkALI, Aratoa Re iut' ctA F rot lue icint-eAD!M \me r ical S-tit e tofr Iru ti.,g and X, -t er ithIC CT tmre Crac ked Fenlsion Itypt' :I tat i gueuspec ei:i

-cr~-at for -r 'iont rt. u't

FCTF torrocitt Fat igue ''oporat itt en,1ti IuPit grcrllrle1) FV1, lie D- t Fr,-thii g,0 - n,t Crr-thlns t a~t~lt 16r til uI t 'tt r

Oilt trace oftocti --lat do.' Ktpott

EFiti F tite Ela 'it tht tiinpiter pri-gri rjr. fo r' Ta ,In! r lSACTFA t ii nc Ati - rat t i orrt's i on rou ittE

FAL'iIAFF F I gt,-I A iroI t iad i t.-F[nda Io Fr a tigi .tr- -r1-

FFA.gtektt tikt F' ru ' '-ta I totl t.Ugottt' to h, , F tuugworE

Hi _g ai L ri t it-inro 'i ytorn

1 ,I .1 - at' I-a toneta ti (Lppe r diatttL AtFItrn, t ,a tn, rt t-,o A ltt !trca~' i it i n

E'i In , Ititilil, ,tta'ti'nn''L, timer -. tttitg

Litg onar - iS 1:nI, ,l,a I i t-fl:'c it te

It' Fit to T

>ItIt.r. Spec itIcai- -n Airpan tI~~-ll'riot'Itc-

lee ! i la I , Of'I 'l ' I to l" i '

I- :t- 'id 'o t 'Lit i: .Ir ittr -, fi I tttn't It , ,it

Eta- -S 'lil t, ' tC ;t-t i A n' uIt t;11 'lp

N '1i ';:d cjg 'Ji t tS' Nt ,u te roau Ii t t ta r-utI

NA t e ont Io r oIt a F i o en

N tait -oor ti-unuiti rutw

R , t i - a tr uo , tcr a b ll

"F - , tI Air Fo C1 K,ret oro ti:ti atn ec 1' 1ii ta n re IL I l I I' tt i i;i-

-<ISR -ud Benrd ing EonC''StA't[A ''r' mt to~p tlpodee nt ut-.tri t to '<'ntitS 1 R tructu~rat lttgoitiv Fat u d Fott' en, rtl at't it':CErbo It 'ttterterece it aircraft- taccttot ''semNIP F St rucotu re anti Mat tir" itann t ft ACAEI

SPATE t treuu, PatterncAnatoeio by Inraucrenont of tto aluist

STI tSpua tibratI P itbfttion of the Aneohcannoetc to looti, cWlt -- -hoT~Etlerer Of Re tereie for nting up an AAi) ,t io tin,

ittEItdin in it of the t. Ieo- ugh ti So oil tpOO antI Itoel f cit hop l

FARE i

i'RiGiA yh iii'hi ThsI ANTIDE INli li

1.INT RiODUCTION

Aircraft structures arc Su scept iblr toi corrosion and fat igoo. Corrosion can ocr uinder bothss'-ndil and during msin.Tints the co,,joint action of cro onand cyclii loading, I. r-sfat igoue, is poss ible. Corrosion begins when the applited protection sytems become d~ecizded acdd'.lei

D)egradation occurs swing to esposure, e.g. to iltrasinint light and oz-00, and nouistore-idtond mac hicg finhibitors from primers and sealants. Damage mav bn incidental, for esample an a i:'csnqoencn o impact hoI ,'rnigs 1 obj ects or may occor as crackling doe to snrvice loads or hecase the sndrrising netil ha, crajel.

Lcrrosion and it igon daago tond tot concentratc at 'joints , which in consnt i-s:al al1siiiiisn u . -

strsctsrns possess most or all at rheo illowing detrimental fea tures:

*t St ress colic etra t I, cs and t aoingso rtacdLecnnt ac ts thlat ocrachk and waor away t le p rost e05s:em

* res toes for moiitsre entrapment

* possible go mvanic coopirs when Steel or titaniun tasterors ire osed

* tatigon critical locations. e.g. fastener boles and their nicinitins.

t It past as-ir it. it coirrosi on ft tigne tests b ase been conds's C d wi thI a isnminis m al 1 i-%s . Ntr I wasetlis sis base isdtit cted rivs ironnenlta t IIIect S to) he o inf Icoinit. Hiunsi r , fswilvt a ,I ioisdnd t he tes t ing cc crit ,icoI s t rotsralI dent Iiis, . Ich as ,inats ssudcr rc- lt icct11,ri,ind ins, inlIat ed -er - esi rolnetcs . hce err base ons idtered t he e itsc trise- ct oar ions co Itstcs

prn- toot is -sotems. Ciisseq-jenrlInv. a da ta base I r asness is th Ie in tIles - , .: trs hI. e 1 ;fof a i rsa ft St rsotstres has not been acq i rnd .

I reco'gnitio I oi, this stlt ,of at lairis it was decfided at the, sth Mleet ing "I iiie AGAis tir-tsre ,s dlaterlai1S Easel In Ap ril 19" 7 t, iormt a Ssb-C.-mnittee and tppcint Es rpean and North Anetoar -rirlt tr

fr i'pr t ceprgramme ono orc o aiiec es.c ateriols I atio~ Iiesrt t , S,"stre.At that t ine the obiost ises Ii the yerannrme were Icrinsiated aS Ii low-s

*',aseeis '' tI'e '-Ieo Ics., ati-isi-r pon sonIc lsw

rset tie ots lt Igtoe toil oi--r-isIs rat igos

*t linlattI i. ! tie denrlimes 'It new p,.tleo i-o pr''dsts. yr--c -,4 at ~hiqs*binging I gethie ree 7chr on t. ies .I She Atlanis is a, -trsos testing effort that would

ori aeros- yat srocttoal materiais,

* iihl log part ic i pat intl lahoratiries to ad t, thleir tI( igie test i-g capabilit ies I hr singa,-iti lied issosphe ri --. r r.,si-i enn iri,-,net .

he ciperat is'. p-ipninne was issnied r, lhe otIroeId out is In .L41Stages . The first 5fewas to De a cornor-igr-smmof r isod - i I iis tnvt Ist:g t e-tibi ' It whether yartlicipant, could 'brim onidenoe in 'r ttlte'tat Igor telt Ing capab ii t Ies At sis sanme t ime ibhis -re ptro gra mme was des igned to he sltfti entt 1I

str"IighI'.rw11Ird t ,essitrige part IcLIpIation part iconlar1i:,is tho-se witi rovlIa t ise Iv lit tlIe esxpe r isocotIoro ii tat Igsne ten[tog.

trig-tiali s here were eight part ici pits to the err proigramme., which wan completed is 1081 andpub, Ishii as an AiADl report. ri-Ierelce ,II , 'iwener , n eo that t Ine two rmir part icipants hiave circled

cot -ire pro'grammne test Inig. the results, toigether with "fI in, tonhing'' of ithe stat ist foal method, 'm--! z,,aIna lyse the , ire p itg rmie daIta , warrant a rc oisosment , thle core pr'grame. Thits mass sosiet ispresented in Part i1 of this repoet.

The -mcd stage' of t he coipe rat ine prograe, was to cons ist o f sopp lementalI t es ti1ng directedi to tilerequir ements of indinviduai participants bitt still1 with much connotiality. This ne."ni stage also ino~ilerdten p articipants. foure of whim had sot t akes, part Iii the core prngr none, and was completed isi 198r). tecesus. in she Porn if citreibt li oteprt Icipants. a-c presenee" in Parr !it ,1 ci repiert

A sotmary enalnatlion of the entire programme is gieen in Part IV. In this part sthe coordinators baseedeanesred to establish cotri trends I ron tie resulIts is order tto place them in a broader contest.

Recommen da.tions Pe r furth er Insesrigat ion are made also.

OVERV;PI OFli THlE CIRE Pllth,ltA)*th iCCP)

The coe programe of round-robin testing was entitled the Corrosion Fatigue icoperaive TestingProgr amme, hereinafter referred to as the CPCTP. An overview of the l:PPTP is glves i tabte I . The CFll'

specified identical nonuiltioss for the Iillowing parameterst

m2

* material and heat treatment: 7075-T76 aluminium alloy sheet

* specimen configuration: 1i doghone joint

* protection system: chromate conversio., primer and topcoat

mechanical testing: static prestressing and fatigue (constant amplitude only)

* environments: pre-exposure, fatigue and corrosion fatigue.

To achieve these identical conditions it was necessary to obtain a batch of 7075-T76 aluminium alicofrom one heat, to manufacture all prior-to-assembly specimen parts at one location, to apply the protecionsystem asd assemble the specimens at one location, and to prepare a technical manual for mechanical -ndenvironmental testing.

The technical manual was published in reference (1). An impression of its scope and the kind of detailnecessary to try and ensure identical testing conditions is provided by the summary in table 2. Most ot thechapter headings are self-ovldent, but the -14 box requires some explanation. This is an envlroonmentalchamber for statically loading the specimens at low temperature in order to crack the yrotection system(paint) near the fasteners.

As indicated in the introduction, the main purpose of the CFCTP core programme was t, establishwhether participants could obtain confidence in one anothers fatigue testing capabilities, with the adleddimension of a controlled atmosphetic corrosion environment. That is to say, results from all participntswere to be analysed to determine whether one or more laboratories had obtained data signiicantl> dilcerontfrom those of the remaining laboratories.

3. OVERVIEW OF THE SUPPLESMENTAL PROGRAMME (FACT)

The supplemental programme was entitled Fatigue in Aircraft Corrosion Testing (FACT). This progra-oewas included so that individual participants could investigate corrosion fatigue problems ot partic ulrrelevance to their own interests and vet within a broader context. To achieve this it was emphasized tinttesting should be done with as much commonality as possible. in particular, it was recommended that

* the sane specimen configuration (i dogbone joint) be used as for the CFCTP core programme

* mechanical testing conditions be identical

a environmental conditions (pro-exposure, fatigue and corrosioo fatigue) be identical to those Irthe CFCTP

* efforts be made to obtain materials of mutual interest from one heat.

Concerniog the first three points the technical manual required for the CFCTP also included supplemntaltesting guidelines for specimen manufacture, application of protection systems. specimen assembly,pre-exposure, and fatigue and corrosion fatigue under flight simulation loading, see table 2.

An overview of 'he FACT programme is given in table 3. There were ten participants. Four had not takenpart in the CFCTP -o: programme, namely

(I) SAAB-SCANIA Aerospace Division, Linkping, Sweden.

(2) Delft University of Technology LRTH, Delft, The Netherlands.

(3) Industri ,alagen-Retriebsgesellschaft IABG, Ottobrunn, Germany.

(4) National Research Council NRC, Ottawa, Canada.

Table 3 shows similarities and comonalitles in the individual programmes. Most participants tested1i dogbone specimens under nominally identical mechanical and environmental conditions. The fatigueloadings were constant amplitude, as in the CFCTP, the manoeuvre spectrum FALSTAFF (references 2-4) and thegust spectrum MINITWIST (referenc, 5). The environmental conditions generally included two or mere of thosein the CFCTP. Notable exceptions were in the SAAB and NRC programmes.

The main interest of several participants was to compare - in their individual programmes - theenvironmental fatigue properties of a number of aluminium .liays in various tempers. However, owing to thecalibratory function of the CFCTP and the participants' active cooperation ir obtaining the manysimilarities and commonli

tils within the FACT programme, It was possible to make inter-participant

comparisons of materials, protection systems and fasteners as well. Furthermore, the total testing effortprovided many data for comparing environmental fatigue effects under constant amplitude and FALSTAFFloading, the latter being a realistic cyclic load history for tactical aircraft.

In retrospect we consider the objectives of the CFCTP and FACT programmes to have been achieved,though much remains to be done to increase the understanding of aircraft corrosion fatigue and theeffectiveness of protection systems. We hope this report will encourage further investigation in thisdifficult and challenging area of aerospace technology.

4. REFERENCES

1. R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordlnated corrosion fatigue cooperative testing

prograne", AGARD report No. 695, February 1982.

2. G.M. van Dijk and J.B. de Jonge, "Introduction to a fighter aircraft loading standard for fatigue

evaluation "FALSTAFF" ", Paper 3.hi. Problems with Fatigue in Aircraft, Proceedings of the Eighth ICAFSymposium, compiled and edited by J. granger and F. Berger, Swiss Federal Aircraft Establishment (F+W)1975: Ernen, Switzerland.

3. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation". Combined Report of the

F+W, LBF, NLR and IABG, March 1976.

4. J.B. de Jonge, "Additional information about FALSTAFF". NLR Technical Report TR 79056 U, June 1979.

5. H. Lowak, J.B. de Jonge, J. Franz and D. Schitz, "MINITWIST. A shortened version of TWIST", CombinedReport of the LBF and NhR, NLR Miscellaneous Publication MP 79018 U, May 1979.

TABI.E 1: OVERVIEW OF THE CFCTP CORE PROGRAMME

SCFGTP COR RG- M

MATERIAL * 3.2 m thick 7075-T76 aluoinium alloy shut

PRESS FIT H. Lok FASTENERS

SPECIMEN * P

PROTECTION SYSTEM * Chromate conversion + inhibited epoxy poly;amide primer(except fastener holes) + aliphatic polyurethate topcoat

PROT.CTION SYSTEM * Two stress cycles at low temperature (20S ± iO K) to crack primer and

DAMAGE paint around the fastener heads

FATIGUE LOADING * Constant amplitude, Smin/Smas - (.I

FATIGUE ENVIRONMENTS * Laboratory air; 5 % aqueous NaC salt spray with pi 4 at 295 V

STATIC PRE-EXPOSURE * 72 hours in 5 % aqucoss NaCi 50 at 31.5

NUMBER OF SPECIMENS ISCHEDULES CC.

210 MPa S - 14'4 MPa FREQUENCY

Fatigue in air 4 4

Pre-exposure + 4 4 2 lizTEST PROGRAMME fatigue in air

Fatigue in salt 4 4

p ray

Prc-exposure + 4 45fatigue in saltspray

STATISTICAL ANALYSIS * Fatigue lives and primasy fatigue origins

(1 Nasal Pir Oe'alpment Centre NADC, Waroinster. Pentsylvania USA.(2) University of Saskatchewan, Saskatoon, Canada.(3) Vought Corporation, Dallas, Texas, USA.(4) Air Force Wright Aeronautical Laboratories AFVAL. Dayton, Ohio.

USA.PARTICIPANTS (5) National Aerospace Laboratory NI'R, Emmeloord, Tie N1ltltds

(6) Deutsche For.hungs - und Versuchisansal r fu Ir -sod r R'mfahrt

DFVOR. Coogoe. Germany.Cl Norwegian Defence Research EstablisIment NDRE, elhr, Nrwav(8) Royal Aircraft Establishment RAP, Farnbooh Unitd Kingdom(9) University of Toronto SIFFRL, Toronto, canada,

(10) University of Pisa, Pisa, Italy

5

TABLE 2: SiMMARY OF THE TECHNICAL MANUAL FOR THE CFCTP CORE PROGRAMME AND ALSO SUPPLEMENTAL TESTING

(THE FACT PROGRAMME)

TABLEOF CONTENTS

1. INTRODUCTION 7 PRE-EXPOSURE CHAMBERS AND PROCEDURE

7.1 Core Programme Chamber

2. PROGRAMME OVERVIEW 7.2 Core Programme Pre-exposure Procedure

2.1 Core Frogramme 7.3 Supplemental Testing Programme Chamber

2.1 1 Core programme phases 7.4 Supplemental Testing Programme

2,1.2 Test schedules Pre-exposure Procedure

2.2 Supplemental Testing

2.2.1 Mechanical test conditions B. CORROSION FATIGUE SALT SPRAY CABINET

2.2.2 Pilot tests 8.1 Schematic of Salt Spray Equipment for

2.3 Milestones the GFCTP8.2 Salt Spray Cabinet with Internal

3. SPECIMEN Reservoir

3.1 Configuration 8.3 Attachment of Specimens + Grips

3.2 fastener Holes Assembly

3.3 Fasteners 8.4 Clamping Head Extension for Salt Spray

3.4 Corrosion Protection and Assembly Fatigue Testing

3.4.1 Core programme specimens 8.5 Bellows

3.4.2 Supplemental testing specimens 8.6 Sealing of the Salt Spray Cabinet

3.5 Sealing of Edges and Hi-Lok Collars 8.6.1 Access door / front frame

8.6.2 Cabinet / bellows

4. CLAMPING-iN 8.7 Sealing of Clamping Heads to Bellows

4.1 Grips

4.2 Bushings in Specimen Clamping Holes 9. SPECIMEN STORAGE AND CLEANING

4.3 Clamping-in Procedure 9.1 Storage

4.4 Alignment Guidelines for 9.2 Cleaning

Electrohydraulic Machines

10. TEST PROCEDURE

5. ADDITIONAL GUIDELINES FOR 10.1 Establishment of Stress Levels

ELECTROHYDRAULIC MACHINES 10.2 Summary of Test Procedure

5.1 Performance 10.3 Data Recording5.2 Static Calibration

5.3 Dynamic Calibration Ii. REPORTING

5.4 Hydraulic Shut-off Effects 11.1 Progress Reports

5.5 Electromagnetic Interference Effects 11.2 Final Reports

11.3 Specialists' Meeting

6. COLD BOX 12. APPENDIX: FLIGHT SIMULATION TESTING

6.1 Schematic of NLR Cold Box 12.1 The Manoeuvre Spectrum FALSTAFF

6.2 Configuration for the CFCTP 12.2 The Gust Spectra TWIST and MINITWIST

Specimen 12.3 Verification of Flight Simulation

6.3 Calibration of Cold Boxes Loading

F61 1

PI~

* s i

-~ ii

RIP

W0 git

7

PART 11

REASSESSMENT OF THE CFCTP CORE PROGRAMME

1. INTRODUCTION

The CFCTP core programme consisted of round-robin testing whose primary purpose was to establishwhether participants could obtain confidence in one another's fatigue testing capabilities with the addeddimension of a controlled atmospheric corrosion environment. The programme was designed to be sufficientlystraightforward to encourage participation, particularly by those with relatively little experience ofcorrosion fatigue testing.

Originally there were eight participants to the CFCTP. The results were published in an AGARD reportin 1982 (reference 1). Since then two more participants completed the core programme. The results havebeen included in a reassessment of the CFCTP. This reassessment involves "fine tuning" of the statisticalmethods originally used to analysed the CFCTP data and is presented here.

2. DESCRIPTION OF THE CFCTP CORE PROGRAMME

An overview of the CFCTP core programme is given in table 1. The CFCTP specified identical conditionsfor the following parameters: material and beat treatment, specimen configuration, protection system,mechanical testing and environmental conditions. These parameters are discussed in more detail in sections2.1 - 2.4. Summaries of the test procedure and statistical methods for analysing the results are given insections 2.5 and 2.6.

2.1 Material and Specimen Configuration

The material was 3.2 mm thick 7075-T76 bare aluminium alloy sheet from one heat and supplied by ALCOAespecially for the CFCTP. The engineering properties were specified as follows:

0.2 Z YIELD STRESS UTS ELONGATION CONDUCTIVITY

479 MPa (max) 550 MPa (max) 11.1 5 38 I.A.C.S.455 MPa (min) 541 MPa (min)

Figure 1 shows the specimen configuration. This was recommended for the FACT supplemental programmealso. The specimen is a lI dogbone mechanically fastened by cadmium plated steel Hi-Loks. It was designedto simulate the load transfer and secondary bending characteristics of runouts of stiffeners attached tothe outer skin of an airframe structure. The design goals were a load transfer of 40 % and a secondarybending ratio of 0.5 (reference 2). These characteristics have been checked and the actual values aregenerally lower, see Appendix I.

All prior-to-assembly specimen blanks for the CFCTP were manufactured in one batch by the U.S. AirForce Wright Aeronautical Laboratories AFWAL. The fastener holes of all specimens for the first eightparticipants listed in table I were drilled to press fit dimensions in one batch at the U.S. Naval AirDevelopment Centre NADC. However, to enable the remaining two participants to complete the core programmeit was necessary to disassemble some Interference fit supplemental programme specimens, redrill to pressfit dimensions and reassemble. It turned out that this procedure significantly influenced the fatigueresults, as will be discussed in section 3.2.

2.2 Protection System and Specimen Assembly

Application of the CFCTP protection system and specimen assembly were done by the NADC as follows:

" chromate conversion coating on all surfaces

" inhibited epoxy polyamide primer on all surfaces except fastener holes

" assembly of fatigue specimen -1 and half plate -2 with Hi-Lok fasteners and collars, see figure I

" inhibited epoxy polyamide primer on fastener head and collar areas

" aliphatic polyurethane topcoat on all exterior surfaces.

The specimens were then wrapped individually and shipped in batches to the participants.

2.3 Mechanical Testing Conditions (Static Prestressing and Fatigue)

All stresses were defined in terms of the total cross-section of the fatigue specimen -1 at thelocation of the centreline between the Hi-Lok fasteners, i.e. the fastener holes were included in thecross-sectional area.

Before environmental exposure and fatigue testing the CFCTP specimens were prestressed in cold boxesat 209 t 10 K by applying two quasi-static load cycles up to a maximum stress of 215 MPa. The purpose was

to crack the primer and paint realistically in the fastener head areas.

Fatigue testing was done using constant amplitude sinusoidal loading with a stress ratio R - Sm in/Smaxot 0.1. It was decided to test at two stress levels giving nominal fatigue lives of 20,000 and100,000 cycles for uncorroded specimens fatigued in laboratory air. From pilot tests (reference 1) thefollowing fatigue stress levels were established for the CFCTP:

NOMINAL UNCORRODED FATIGUE LIFE Smax Smin

20,000 cycles 210 MPa 21 MPa100,000 cycles 144 MPa 14.4 MPa

2.4 Environmental Conditions (Pre-exposure, Fatigue and Corrosion Fatigue)

There were four testing schedules for the CFCTP, see also table I:

* fatigue in air, cycle frequency 2 Hz

* pre-exposure + fatigue in air at 2 Hz

* fatigue in salt spray, cycle frequency 0.5 Hz

* pre-exposure + fatigue in salt spray at 0.5 Hz.

Specimens to be pre-exposed and/or fatigued in salt spray were sealed at faying surface side edges andHi-Lok collars in order to prevent corrosion except in the fastener head areas. Pre-exposure was for 72hours in 5 Z aqueous NaC1 salt solution to which a predetermined amount of 502 gas was added by reacting

Ha2503 pellets with 82504. This reaction was accomplished in vented test tubes suspended above the saltsolution, which was maintained at a temperature of 315 ± 2 K.

For fatigue testing all specimens were electrically insulated from the loading grips and bolts bypolymeric liners and bushings. The environments were laboratory air and 5 X aqueous NaCI salt sprayacidified with H 2SO4 to pH 4, both at a nominal temperature of 295 K. The salt spray tests were done inspecially constructed cabinets, fully described in reference (1).

2.5 Summary of the Test Procedure

A schematic summary of the CFCTP test procedure is show in figure 2. This gives some idea of thecomplexity of even a fairly straightforward programme, which is why a technical manual was prepared asalready mentioned in Part I of this report. The technical manual is published in reference (1).

One part of the test procedure was modified after analysis of results from the original eightparticipants. The technical manual had specified fatigue testing as soon as possible after pre-exposureand cleaning, with desiccator storage only if delay were unavoidable. However, this was later amended torequire desiccator storage for at least one week after cleaning, in order to ensure the specimens werecompletely dry before fatigue testing.

2.6 Summary of the Statistical Methods for Data Analysis

A detailed description of the statistical methods used to analyse the CFCTP fatigue life and primaryfatigue origin data is given in Appendix Ii. Statistical analysis was done with the primary purpose ofchecking whether participants could have confidence in one another's fatigue testing capabilities, i.e.the results were analysed primarily to determine whether one or more laboratories had obtained datasignificantly different from those of the remaining laboratories. The statistical analysis also had

several secondary purposes, namely to determine

" whether pre-exposure was significant for subsequent fatigue life in air or salt spray

* whether the effect of fatigue in salt spray, with or without pre-exposure, was significant comparedto fatigue in air with or without pre-exposure

* whether there were significant differences between Laboratories in the relative effects of pre-exposure and/or fatigue in salt spray (this is part of the primary purpose)

* whether the sample size (4 specimens per test condition per participant) was sufficient and whetherthere were noticeable differences in data scatter between laboratories and fatigue testingschedules

* whether there were relationships between the locations of primary fatigue origins, fatigue stresslevels, environmental conditions and fatigue lives.

A survey of the statistical methods and procedure is given in figure 3. The fatigue life data were firstchecked for normality and homogeneity of variances (approximate compliance with these conditions issufficient) as a prerequisite to further treatment. The main statistical analysis was multiple factoranalysis of variance. This was followed by "fine tuning" using the least significant difference test orDuncan's new multiple range test (references 3, 4). To avoid possible misuse the least significantdifference test was applied only when analysis of variance indicated significant effects. In addition,scatter in the data was used to check for adequate sample size (four specimens per test condition perparticipant) according to a method described in reference (5).

9

The primary fatigue origin data were analysed using the )e test of independence, Yates' corrected l2

test or Fisher's exact test, whichever was appropriate. For these tests it is sufficient to assume onlythat the data constitute a random sample (reference 6). These tests were also used (as appropriate) tocheck whether there were significant correlations between fatigue lives and primary origins for each testcondition.

3. RESULTS

The CFCTP core programme fatigue life and primary fatigue origin results are compiled in table 2,which also indicates the originally interference fit specimens that were disassembled, redrilled to pressfit dimensions and reassembled. The primary fatigue origin data are those obtained by one of the programmecoordinators (R.J.H.W.), who also supplied the remarks concerning fatigue fracture surfaces of pre-exposedspecimens tested in air.

The fatigue life results are presented and statistically analysed in sections 3.1 and 3.? respect-ively. This is followed by presentation and statistical analysis of the primary fatigue origin data insection 3,3. Correlations between fatigue lives and primary fatigue origins are discussed in section 3.4.

3.1 Presentation of Fatigue Life Data

The fatigue life data are presented in figures 4 and 5 in terms of log mean life and data range foreach fatigue testing schedule and for each participant. There is a clear separation of the data withrespect to stress level, as expected. Figure 5 shows a general tendency for shorter lives owing to fatiguein salt spray with or without pre-exposure, though individual trends vary.

3.2 Statistical Analysis of Fatigue Life Data

3.2.1 Checking for normality

In checking for normality the CFCTP core programme data were considered to belong to eight differentpopulations corresponding to each of the four fatigue testing schedules in combination with each of thetwo stress levels. The X

2 test for goodness of fit (references 6, 7 and see Appendix II) showed that data

for six of the populations were log-normally distributed and data for the other two (fatigue in air withSmax - 210 MPa and pre-exposure + fatigue in air with Smax - 144 MPa) were approximately log-normal. A

subjective impression of these results is provided by the logarithmic normal probability plots in figure6. Because of the log-normal distributions all further statistical treatment of the data used thelogarithms of the fatigue lives.

3.3.2 Checking for homogeneity of variances

As shown in figure 3, the Box test (reference 8) was used to check for interlaboratory differences invariances. To do this the data from different laboratories were considered to come from differentpopulations. This resulted in eighty populations corresponding to data from each of the ten participantsfor each of the four fatigue testing schedules in combination with each of the two stress levels. The Boxtest results are summarised in table 3. Thee were two very slight violations and one moderate violationof the criterion for homogeneity of variances.

The Bartlett test (reference 8) was used to check for differences in variances between fatigue testconditions. To do this the data for each fatigue testing schedule and stress level were treated as comingfrom the same population, i.e. no distinction was made between the data from different laboratories. Inview of the Box test results this assumption is not strictly correct. However, it is considered justified.The Barlett test results are summarised in table 4. There were three moderate violations of the criterionfor homogeneity of variances.

As mentioned in section 2.6, approximate compliance with the requirement of homogeneity of variancesis sufficient for further statistical analysis. In the present wcrk the main techniquo of statisticalanalysis was multiple factor analysis of variance. This is a very robust, i.e. "forgiving", technique.Thus the results summarised in tables 3 and 4 were considered sufficient for continuing the statisticaltreatment of the fatigue life data.

3.2.3 Main statistical analysis: analysis of variance

Multiple factor (three-way) analysis of variance was used to compare the CFCTP core programme fatiguelife data in terms of the experimental variables of stress level, fatigue testing schedule (environmentaleffects) and laboratory. The results are shown in table 5.

According to the analysis the main variables of stress level, fatigue testing schedule and performinglaboratory all had significant effects on the fatigue lives of the specimens. The significant effectattributable to stress level and fatigue testing schedule were anticipated from the way the CFCTP coreprogramme was planned. However, determination of whether there were significant effects attributable todifferences between laboratories was the primary purpose of the core programme.

A significant effect was also indicated for the interaction between stress level and fatigue testingschedule. This means that the stress level and fatigue testing schedule significantly affected the fatiguelives.

3.2.4 "Fine tuning" with the least significant difference test

As shown in figure 3, significant effects indicated by analysis of variance were investigated in moredetail ("fine tuning") using the least significant difference test. However, this was not necessary forthe effect of stress level: since there were only two stress levels it is obvious that the significant

10

difference is between them. Thus the significant effects investigated were

" environment

* laboratory

* stress: environment.

In the first instance the fatigue life data from all ten participants were analysed. This showed thatdata from two participants (SIFFRL and the University of Pisa) were significantly different from the rest.Because SIFFRL and the University of Pisa were the only participants to have tested specimens that hadbeen disassembled, redrilled to press fit dimensions and reassembled (see table 2) it was decided toconduct an additional analysis omitting the data for these specimens. The results are given in table 6.Note that omission of data for reassembled specimens resulted in unequal sample sizes, so that a modifiedversion of the least significant difference test had to be used. This modified version of the test is alsodiscussed in Appendix II.

Table 6 shows the following:

(1) The effects of different fatigue testing schedules (environmental effects) were significant andconsistent at both stress levels. The effect of pre-exposure was similar to that of changing thefatigue environment from air to salt spray.

(2) The SIFFRL and University of Piea data were significantly different from the other participants'data.

(3) A significant interleboratory difference was also found between the 4FWAL dar -d ttose f.- the

University of Saskatchewan, Vought, DFVLR and NDRE.

3.2.5 "Fine tuning" with Duncan's new multiple range test

As shown in figure 3, Duncan's new multiple range test was used to investigate in more detail theexperimental variables (in the present case their interactions) that were not found to be significant byanalysis of variance. These interactions were

* stress: laboratory

* environment: laboratory

" stress: environment: laboratory.

As before, it was found that the SIFF and University of Pisa data were significantly different from therest. Thus the fatigue life data were analysed both with and without data for specimens that had beendisassembled, redrilled to press fit dimensions and reassembled (see table 2). Table 7 lists significantdifferences indicated by Duncan's test, which in the case of omitting data for reassembled specimens wasmodified because of unequal sample sizes. This modified version of the test is also discussed inAppendix 11.

Table 7 shows the following:

(1) A clear indication that the SIFFRL and University of Pisa data were significantly different fromthe other participants' data.

Excluding the SIFFRL and University of Pisa data,

(2) At S - 144 MPa there were significant differences between the AFWAL total log mean fatiguelife mfd those for the DFVLR and MDRE.

(3) There were some significant differences in log mean fatigue lives for the fatigue testingschedules of pre-exposure + fatigue in air and fatigue in salt spray. In more detail thesesignificant differences were found only for Smax - 210 MPa.

3.2.6 Checking for adequate sample size and differences in data scatter

Scatter in the CFCTP core programme fatigue life data was used to check for adequacy of sample size(four specimens per test condition per participant) . The method used is due to Lipson and Sheth (refer-ence 5) and involves selecting an acceptable error level, usually 5 Z or 10 Z, and finding the requiredsample size for a particular confidence level. The sample size check has two purposes, namely

* to find the combination of error and confidence levels for which the actual sample size wassufficient

* to give an indication of differences in data scatter between laboratories and fatigue testconditions.

The actual sample size was sufficient for the combination of 10 % error and 90 Z confidence levels exceptfor one case: pre-exposure + fatigue in air at Smax - 144 MPs by the University of Pis. There was thus a

generally low scatter in the data and high reproducibility of the specimens and testing conditions foreach participant.

To indicate differences in data scatter the required sample sizes were determined for the combinationof 5 Z error and 90 Z confidence levels and are shown in table S. The shaded regions denote exceedance of

the actual sample size, and since a larger required sample size reflects greater scatter the resultsindicate

(1) More persistent scatter for the RAE data.

(2) The amount of scatter tended to increase with complexity of testing. This is particularlynoticeable for pre-exposure + fatigue in salt spray.

(3) For pre-exposure + fatigue in air there was much more scatter at the higher maximum stress levelof 210 MPa.

3.3 Presentation of Primary Fatigue Origin Data

As mentioned at the beginning of section 3, the primary fatigue origin data are compiled in table 2.In the last column of this table there are remarks concerning specimens pre-exposed and fatigued in air.Some of these specimens had corroded fracture surfaces near and at the primary cacugue origins. TiLLindicated that an aqueous solution was present inside the specimens during fatigue testing, even though adetailed cleaning and drying procedure was specified to follow pre-exposure (reference 1).

Table 9 classifies the fatigue life and primary fatigue o:igin data for all specimens pre-exposed andfatigued in air. For both stress levels the log mean fatigue lives of corroded specimens were significant-ly shorter than those for uncorroded specimens. This was confirmed by statistical analysis that omittedthe data for interference fit specimens disassembled, redrilled to press fit dimensions and reassembled.The statistical techniques used were a variance-ratio test to check for homogeneity of variances and thet-statistic evaluation to compare two means. These tests are described in references (9, 10).

It is concluded that an aggressive aqueous solution was present inside the specimens with corrodedfr" u~:r . MsL probably this was acidified aqueous NaCI remaining from pre-exposure. Informationon time delays between cleaning and drying pre-exposed specimens and fatigue testing in air was suppliedby the participants. There was no strong correlation between time delays and subsequent fatigue lives andcorroded fracture surfaces. However, from the NLR and AFWAL information it appeared that storing thespecimens for several days in desiccators resulted in relatively long fatigue lives and uncorrodedfracture surfaces, see table 2. This was considered sufficient ground for amending the cleaning procedureto require desiccator storage for at least one week, as mentioned in section 2.5 and specified in detailin reference (1). This amendment was made only after CFCTP core programme data had been received from theoriginal eight participants. Of the remaining two, SIFFRL included desiccator storage but the Universityof Pisa fatigue tested the specimens immediately after cleaning. Table 2 shows that corroded fracturesurfaces were not found for the SIFFRL specimens but were present in four of the University of Pisaspecimens. This is additional evidence that desiccator storage was effective in drying the specimenscompletely.

Despite the significant effect of insufficient drying on the fatigue lives of specimens pre-exposedand fatigued in air, table 9 shows there was no essential difference In the locations of primary fatigueorigins in specimens with corroded and uncorroded fracture surfaces. Thus it was felt that all the fatigueorigin data in table 2 could be classified together.

3.3.1 Classification of all primary fatigue origins

The primary fatigue origin data are classified in table 10. The table has four sub-divisions, whichwill be discussed consecutively:

(1) Listing the total numbers of each type of primary fatigue origin shows

- most failures began in the bores (E/Q) or at the bore/faying surface corners (F/R) of fastenerholes: there was no evident preference with respect to outer (EF) or inner (Q,R) sides of theholes

- failures at faying surfaces (C/S) occurred mainly to the outside of fastener holes (G) probablybecause the proximity of free edges facilitated relative displacements between the fatiguespecimen -I and half plate -2 (see figure 1), thereby promoting fretting fatigue initiation

- very few failures initiated in the countersink areas: most were at the surface edges to theoutsides of fastener holes (B).

(2) Listing the primary fatigue origins for specimens tested by each participant reveals some inter-laboratory differences. Possibly the most significant difference is that specimens tested bySIFFRL and the University of Pisa had more bore/faying surface corner (FIR) primary origins thanspecimens from other participants.

(3) The third part of table 10 gives a complete breakdown of the locations of primary fatigue originswith respect to stress level and fatigue testing schedule.

(4) The last part of table 10 adds up the total numbers of primary fatigue origins per stress leveland fatigue environment.

The data distribution in parts (3) and (4) of table 10 reveals a predominant effect of stress levelon the locations of primary fatigue eIignp Thve stress level has heen treated as the primary variable inpreparing figure 7, which supplements table 10. The table and figure show that

-- m - | -- "\\

12

1 stress level had a major effect:

- for Sma x - 210 MPa the primary fatigue origins were mainly in the bores of fastener holes

- for S... - 144 MPa the primary fatigue origins were mainly at the bore/faying surface corners and

the faying surfaces

e the effect of fatigue environment was significant: changing from fatigue in air to fatigue in saltspray promoted initiation in the bores or at the bore/faying surface corners of fastener holes andreduced the number of failures initiating at the faying surfaces

* pre-exposure resulted in several effects:

- relatively more primary fatigue origins in the bores of fastener holes

- a few primary fatigue origins at the surface edges of countersinks

- slightly fewer primary fatigue origins at the bore/faying surface corners of fastener holes

- reduction of the number of failures initlating at the faying surfaces.

The effects of pre-exposure and/or fatigue in salt spray may be summarised as especially promoting failureinitiation in the bores of fastener holes.

3.3.2 Statistical analysis of primary fatigue origin data

The X2 test of independence and the Yates' corrected X2

test were used to determine whether there wasa significant association between the locations of primary fatigue origins and the experimental variablesof stress level and fatigue testing schedule (environmental effects). The SIFFRL and University of Pisadata were omitted because they were considered non-representative. The results are summarised in table 11.This confirms the impression gained from table 10 and figure 7 that both stress level and fatigue testingschedule had significant effects on the primary fatigue origin locations.

Note that for Sma x - 210 MPa there is no significant association between environmental effects andprimary fatigue origin locations. This is because the higher stress level and changing from fatigue in airto pre-exposure and/or fatigue in salt spray had similar effects on the primary fatigue origin locations,i.e. promotion of failure initiation in the bores of fastener holes.

3.4 Correlation of Fatigue Lives and Primary Origins of Fatigue

Owing to the results of the statistical analysis of fatigue lives, section 3.2, it was decided toomit the SIFFRL and University of Pisa data from the correlation of fatlue lives nd primary origins offatigue.

Correlations of the fatigue lives and primary fatigue origins for the original eight participants inthe CFCTP core programme are given in table 12 and figures 8 and 9. Note that the two failures at thesurface edges of countersinks (B) for pre-exposurn + fatigue in air at Smax = 210 MPa have bee omitted

from figure 8 since there were no similar failures for other fatigue testing schedules at the same stresslevel. The correlations indicate the following:

(1) From figure 8 it is seen that there are no generally consistent relations between primary fatigueorigin locations and the fatigue lives for each test condition. However,

- for fatigue in air and pre-exposure + fatigue in air at SMax = 210 Ma the initiation of

failures in the bores and at the bore/faying surface corners of fastener holes tended to resultin shorter lives than failure initiation at other locations.

- for fatigue in salt spray and pre-exposure + fatigue in salt spray at S a i 4 MPa theinitiation of failures at the bore/faying surface corners of fastener holes rmed to result inshorter lives than failure initiation at other locations.

(2) From figure 9 it is seen that for S = 144 MPa the effect of pre-exposoro and/or fatigue insalt spray in reducing fatigue life was more pronounced for specimens in which failure initiatedat the bore/faying surface corners of fastener holes as compared to other locations.

Yates' corrected X2

test (reference 11) and Fisher's exact test (reference 12) were used to determinewhether there were statistically significant associations between the locations of primary fatigue originsand the fatigue lives for each test condition, i.e. each combination of stress level and fatigue test incs..eduie. The results are summarised in table 13. A significant association between primary fatigue originlocations and fatigue lives was found only for fatigue in salt spray at S - 144 MPa. This agrees withone of the trends noted from figure 8. It is concluded that the other three trends, namely an associationbetween primary fatigue origin locations and fatigue lives for fatigue in air and pre-exposure + fatiguein air at Smax - 210 MPa and pre-exposure + fatigue in salt spray at Sma x - 144 MPa, are not sufficiently

well-founded.

13

4. DISCUSSION

4.1 Primary Purpose of the CFCTP Core Programme

As mentioned at the beginning of this Part of the report, the primary purpose of the CFCTP coreprogramme was to establish whether participants could obtain confidence in one another's fatigue testingcapabilities with the added dimension of a controlled atmospheric corrosion 0nvironment.

Statistical analysis showed that the SIFFRL and University of Pisa fatigue life results weresignificantly different from those of the original eight participants. A partial explanation is available.To supply the University of Pina with CFCTP-type specimens it was necessary to disassemble interferencefit specimens, redrill to press fit dimensions and reassemble. This procedure apparently causedsignificant reductions in the fatigue lives, especially at the lower stress level of S = 144 MPa. Thesefatigue life reductions may well be related to an increased tendency for failure to initiate at

bore/faying surface corners of fastener holes in the University of Pisa specimens, see table 10.

In the case of the SIFFRL specimens, only six had been disassembled, redrilled and reassembled. Therest should have been nominally identical to the first batch of spcvimens delivered t' - .-riginal eightparticipants, but it appears they were not. It is worth noting that the SIFFRL fatigue life data weresignificantly different from those of the original eight participants mainly on the basis ofstraightforward fatigue testing in air, see figure 4 and tables 2 and 7. This means that the source of thedifference is unlikely to have been different environmental conditions (pre-exposure and/or fatigue insalt spray).

Excluding the SIFFRL and University of Pisa fatigue life data does not remove all the significantdifferences found by statistical analysis. However, the remaining significant differences were few andnot consistently found:

(1) The least significant difference rest indicated a significant interlaboratory difference betweenthe AFWAL data and those for the University of Saskatchewan, Vought, DFVLR and NDRE, table b.

(2) Duncan's new multiple range test indicated a significant interlaboratory difference between theAFWAL data with Sma x - 144 MPa and those for the DFVLR and NDRE, table 7.

(3) in more detail, Duncan's test (table 7) indicated significant differences for

- pre-exposure + fatigue in air at Smax = 210 MPa between Vought and the NADC and DFVLR; and

between the NLR and the NADC, University of Saskatchewan, DFVLR. NDRE and RAE

- fatigue in salt spray at S - 210 MPa between Vought and the University of Saskatchewan,DFVLR and RAE. man

An important factor in the significant differences found for pre-exposure + fatigue In air at Sma x -

210 MPa was insufficient drying of some specimens after pre-exposure, resulting in shorter fatigue livesand corroded fracture surfaces. The relevant data have been re-analysed by separating out the specimenswith corroded fracture surfaces. The results are given in table 14. All of the previously indicatedsignificant differences have been eliminated.

In view of there being only a very few unexplained significant differences found by statisticalanalysis and the generally low data scatter (see section 3.2.6) it is concluded that

* with the exception of the unamended cleaning and drying procedure after pre-exposure, the firstbatch of CFCTP core programme specimens and the mechanical and environmental testing conditionswere highly reproducible

" the original eight participants in the CFCTP cure programme can have confidence in each other'sresults.

In other words, with the exception of the two later participants, qIFFRL and the University of Pisa, theprimary purpose of the CFCTP core programme has been achieved.

It is most unfortunate that the SIFFRL and University of Pisa results were significantly different

from the rest. However, on the positive side these later results emphasize how important and necessary itwas to do the FCTP core programme, to pcovide a detailed technical manual for mechanical and environ-mental testing, and to supply the original eight participants with specimens from one batch.

4.2 Environmental Effects

Statistical analysis of the CFCTP core programme fatigue life data showed that the effects ofdifferent fatigue testing schedules (environmental effects) were significant and consistent at both stresslevels. Both pre-exposure and fatigue in salt spray significantly reduced the fatigue lives, especially incombination. An overall impression of these results is provided by figure I0, which also separates out thedata for specimens found to have uncorroded fracture surfaces after pre-exposure + fatigue in air. Figure10 shows two additional trends:

(1) Environmental effects were relatively greater for the higher Smax of 210 MPa: many environmental

fatigue data in the literature show that the reverse trend would be expected.

(2) The statistical rsult that the effect of pre-exposure was similar to that of changing thefatigue environment from air to salt spray (see table 6) is a consequence of including data forspecimens that had corroded fracture surfaces after pre-exposure + fatigue in air.

14

4.2.1 Dependence of environmental effects on stress level

In sections 3.3.1 and 3.3.2 it was shown that stress level was a major variable controlling thelocations of primary fatigue origins, see figure 7 and tables 10 and I1. F-c S'a - 210 MPa most failures

began in the bores of fastener holes. On the other hand, for S - L44 Pa most failures began atbore/faying surface corners and the faying surfaces. max

Pre-exposure and/or fatigue in salt spray especially promoted failure initiation in the bores offastener holes. It is most likely that environmental effects will be greater when they promote character-istic failure modes. This explains why the observed environmental effects were relatively greater for5uax - 210 MPa.

There is an important conclnsion to be drawn from this explanation of why the environmental effectswere relatively greater for a higher stress level, in contrast to many other data in the literature.Correct assessment of environmental effects requires the specimens to be realistic. The CFCTP coreprogramme specimens were designed to closely simulate a fatigue critical structural joint and theirbehaviour is more likely to be representative than that of simple coupons, which constitute the majorityof specimens used in environmental fatigue testing.

4.2.2 Environmental effects and fatigue life data scatter

As table 8 shows, there was a general trend for fatigue life data scatter to increase with complexityof testing, i.e. when the environmental variables of pre-exposure and fatigue in salt spray were includedin the testing schedules.

For pre-exposure + fatigue in air there was much more scatter in the fatigue life data at the higherSma x of 210 MPa. This is an unusual result, since scatter usually decreases with increasing stress level.

The explanation lies in the variable effect of insufficient drying of some specimens after pre-exposure.Insufficient drying caused significantly reduced fatigue lives and corroded fracture surfaces, and therewere many more such specimens fatigue tested at Sma x - 210 MPa. see table 9.

4.3 Primary Fatigue Origin Locations

As discussed previously, stress levels and fatigue testing schedules (environmental effects) hadsignificant effects on the locations of primary fatigue origins in the CFCTP core probramme specimens.This is shown in figure 7 and tables 10 and II. Also, there were some indications that for a given fatiguetesting schedule the initiation of failures in the bores and at the bore/faying surface corners otfastener holes resulted in shorter fatigue lives than failure initiation at other locations, see figure 8and table 13.

It is evident that examination with respect to primary fatigue origins and fracture surfaces wasessential for understanding the fatigue behaviour of the CFCTP core programme specimens. In fact, suchexamination should always be done when investigating the fatigue behaviour of realistic specimens.

5. CONCLUSIONS

The CFCTP core progrimme of round-robin testit: '1as domonstrated that

(i) The original eight participants may be confident in one another's environmental fatigue testingcapabilities.

(2) With the exception of the unamended cleaning and drying procedure after pre-exposure, the firstbatch of CFCTP core programme specimens and the mechanical and environmental testing conditionswere highly reproducible. (The amended cleaning and drying procedure is reproducible and shouldbe adopted in further tests).

(3) Environmental effects on fatigue lives were significant and consistent.

(4) Realist-t s;-'lmens are ner-snury for correct assessment of environmental effects.

(5) Examination with respect to fatigue origins and fracture surfaces is essential.

Finally we conclude that, for at least the original eight participants, supplemental testing pro-grammes directed to the requirements of individual participants may be carried out with confidence thatthe results from different labors ores can be compared.

6. ACKNOWLEDGiMENTS

We thank ALCOA for supplying the aluminium alloy sheet and the VOI-SHAN corporation for supplying theHi-Lok fasteners used in the CFCTP core programme. We also thank all the CFCTP participants for theirefforts.

7. REFERENCES

1. R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testing pro-grame", AGARD Report No. 695, February 1982.

2. D. Schtz and J.J. Gerharz, "Schwingfestigkeit von Ffigungen met Sonderbefestigungselementen",Fraunhofer-Institut fUr Betriebsfestigkeit Technische Mitreilungen TM 69/73, 1973.

3. R.G.D. Steel and J.H. Torrie, Principles and Procedures of Statistics, McGraw-Hill Book Company,Inc., pp. 106 - 109 (1960): New York.

4. J.C.R.Li, Statistical Inference I, Edwards Brothers, Inc., pp. 265 - 273 (1964): Ann Arbor, Michigan.

5. C. Lipson and N.J. Sheth, Statistical Design and Analysis of Engineering Experiments, McGraw-HillBook Company, Inc., pp. 2n) - 270 (1973): New York.

6. G.M. Smith, A Simplified Guide to Statistics, Twelfth Printing, Rinehart and Company, Inc.,pp. 86 - 96 (1958): New York.

7. E. Kreyszig, Advanced Engineering Mathematics, Third Edition, John Wiley and Sons, Inc., pp. 779 - 781

(1972): New York.

8. J.C.R.Li, Statistical Inference I, Edwards Brothers, Inc., pp. 552 - 556 (1964): Ann Arbor, Michigan.

9. J.C.R.Li, Statistical Inference I, Edwards Brothers, Inc., pp. 122 - 124 (1964): Ann Arbor. Michigan.

I0. J.C.R.Li, Statistical Inference I, Edwards Brothers, Inc., pp. 142 - 144 (1964): Ann Arbor, Michigan.

II. W.J. Dixon and F.J. Massey, Jr., Introduction to Statistical Analysis, Fourth Edition, McGraw-HillBook Company, Inc., p. 278 (1983): New York.

12. R. Langley, Practical Statistics, Dover Publications, Inc., pp. 292 - 313 (1971): New York.

16

TABLE 1: OVERVIEW OF THE CECTP CORE PROGRAMME

SCFCTr CORE PROGRAMME

MATERIAL * 3.2 mm thick 7075-T76 aluminium alloy sheet

PRESS FIT Hi Lk FASTENERSSPECIMEN

31 300

PROTECTION SYSTEM * Chromate conversion + inhibited epoxy polyamide primer(except fastener holes) + aliphatic polyurethane topcoat

PROTECTION SYSTEM . Two stress cycles at low temperature (209 t I0 K) to crack pric.T. ,t,DAMAGE paint around the fastener heads

FATIGUE LOADING . Constant amplitude, Sm /S x .I1

FATIGUE ENVIRONMENTS * Laboratory air; 5 % aqueoas NaCl salt spray with pH e. i

STATIC PRE-EXPOSURE * 72 hours in 5 % aqueous Naf l + So, at 315 K

NUMBER OF SPECIMENSSCHEDULES -CYCLE____

S -210 MPa S 1"- 1 MPa FREQUENCY

Fatigue in air 4 4

Pre-exposre + 4 1, 2 HzTEST PROGRAMME fatigue in air

Fatigue in s,!t 4 4spray

Pre-exposure + 4 4 0.5 Safatigue in saltspray

STATISTICAL ANALYSIS * Fatigue lives and pri,.- ' f'- ... origins

(1) Naval Air Development Centre NADC, Warminszer, Pennsvlvania iSA(2) University of Saskatchewan, Saskatoon. Canada(3) Vought Corporation, Dallas, Texas, USA,(4) Air Force Wright Aeronautical Laboratories AFVAL, Dayton, Ohio,

USA.PARTICIPANTS (5) National Aerospace Laboratory NLR, Enaneloord. The Netherlands

(6) Deutsche Forschungs- und Versuchsanstalt fu Luft und RaulfahrtDF'VLR. Cologne, Germany.

(7) Nirwegian Defence Research Establishment NDREF. Keller. Norwav(8) Royal Aircraft Establishment RAE, Farnborough, United Kingdom.(9) University of Toronto SIFFRL, Toronto, Canada.

(I0) University of Pisa, Pisa. Italy.

TABLE 2: FATEGUE LIFE AND) PRIMARY ORIGIN DATA FOR THE CFCTP CORE PROURADIME

FATIGUEI LI FE T FAILLURE C1YCI.ES AND L0, MEAN' CYCLES) 'UJCATI0NS F PRIAR )RICINS 38: .5 FOTA'

faiVeI pTepSA -- "A ' I 95;T I A IRC- g'.r g htlA I) T .

11,0S '3,0 Q 8,015 n

23 F E 1 , 0 F 1, 1

S, 31. E O

1,I5 3',81 F .1

I MI

1 i7 l, 11 iI

1~~~~~~Mi a* f A) I. 1* F I

31 1623 p 3)IOI nI I I, R:. .

FATGU SPECIMENI I

c 00 - g

FATIGE ORIGINSN

18w

TABLE 3: SUMMARY OF BOX TEST RESULTS (95 % CONFIDENCE)

FATIGUE TESTING SCHEDULES F HOMOGENEITY OF VARIANCES REMARKSFAIGETETNGSCEULSmax F

° -F 1.880)

(MPa) (F <

F0 . 0 5 ;9 7 3 9

210 1.925 no very slight violationfatigue in air 144 1.984 no very slight violation

210 1.249 yespre-exposure + fatigue air 144 3.795 no moderate violation

fatigue in salt spray 210 0.736 yes

144 0.816 yes

pre-exposure + fatigue in salt spray 210 0.655 yes

144 0.836 yes

TABLE 4: SUMMARY OF BARTLETT TEST RESULTS (95 Z CONFIDENCE)

COMPARISONS OF DATA FROM DIFFERENT S HOMOGENEITY OF VARIANCESmao ° REMARKS

FATIGUE TESTING SCHEDULES (MP) o (X1 <x6.5l - 3.841)

fatigue in air/ 210 5.151 no moderate violationpre-exposure + fatigue in air 144 0.366 yes

fatigue in air/ 210 -0.021 yesfatigue in salt spray 144 2.459 yes

fatigue in air/pre-exposure + 210 0.066 yesfatigue in salt spray 144 2.189 yes

pre-exposure + fatigue in air/ 210 5.403 no moderate violationfatigue in salt spray 144 0.921 yes

pre-exposure + fatigue in air/ 210 6.495 no moderate violationpre-exposure + fatigue in salt spray 144 0.757 yes

fatigue in salt spray/pre-exposure + 210 0.040 yesfatigue in salt spray 144 0.012 yes

TABLE 5: SUMMARY OF ANALYSIS OF VARIANCE RESULTS (95 % CONFIDENCE)

SOURCE OF VARIATION F DISTRIBUTION VALUE F SIGNIFICANT EFFECTS OF EXPERIMENTALo VARIABLES (F > F DISTRIBUTION VALUE)

* MAIN EFFECTS

stress 3.89 1572.877 yesenvironment 2.65 45.055 yeslaboratory 1.93 5.763 yes

* 2-WAY INTERACTIONS

- stress:environment 2.65 2.662 yesstress:laboratory 1.93 1.637 no

- environment:laboratory 1.54 1.397 no

* 3-WAY INTERACTIONS

stress:environment:laboratory 1.54 1.478 no

19

I M7

so 0w HI

1. - - h

MW 77

CO -4-P

-- -.

20

I IC

- C 0CC0CCLit

aCC 00 0 C 0

OC lo i.,0' C.U ~S N

21

TABLE 8: REQUIRED SAMPLE SIZES FOR 5 % ERROR AND 90 X CONFIDENCE LEVELS

S -210 MPa S -144 MPamax max

PARTICIPANTSn

X

NADC 3 6 3 6 4 4 3 4

UNIVERSITY OF SASKATCHEWAN 5 4 3 4 7 2 4 6

VOUGHT 4 5 2 5 2 2 6 4

AFWAL 3 3 , 4 4 2 5 3

NLR 3 3 4 5 3 4 4 6

DFVLR 3 5 3 5 3 4 5 5

NDRE 3 10 3 3 3 2 4 7

RAE 3 7 5 5 3 6 5 5

SIFFRL 8 3 4 3 4 3 3 7

UNIVERSITY OF PISA 3 8 4 5 4 12 3 2

22

0 a.

0

o 0

XC N - 0 -

o 0 0 0EX XC. NO

0 4CCC N C. XC.C.CCC.C.C.XC.X C. CC XC. 0 N 0

0 - V

2~

~ ~~CC

r.a a 0 CC

Pa

40000 44000CC.4000 .C.CC40C'C000~C.XC~-0a OCC.0.CXCCCOX, ~.40 CC 4~: :PC

C.CCCO 0 440 CC V

OCCC. 0 C. OCCO 0 C. 0

~ 2 ~C2~ 2 20 C. 4=0 C. -

C. C.,0 C. C.PCo 2Pa 0 CC. = 0

0 20 ____________ _______________ 0

0 0 V* 0

- C.~ N~CC.

N C.XC.C.X C. C. C. C. XC. C. CC. 0~ 2

~

CCC S S a 0 0 C.

00000CX040000000.C00C.C4 0C0CC.000C~CC.. ..C0

OXC.C. 0 0

C.' Ca

C. 0 C.CC.CO a 0~C.CC.. 0 C. OC.CC.C V C.

~C2~ 2 200 C. COO C. C.

402 C. <00 C. C.~0O C. C. 00 C. C.

23

TABLE 10: CLASSIFICATION OF CFCTP CORE PROGRAMME PRIMARY FATIGUE ORIGINS

BORE OF BORE/FAYING FAYING SURFACE EDGE COUNTERSINK/S TOTAL NUMBERS OF FASTENER HOLE SURFACE CORNER SURFACE OF COUTERIN COUNTERSINK BORE TRANSITION

EACH TYPE OF E/Q F/R G/S B/N C/O D/PPRIMARY ORIGIN

68/59 58/60 48/20 9/2 3/0 2/1

PARTICIPANTS E/Q F/R 4,S B/. (/O D/P

NADC 14 9 7 1 1 0

SASKATCHEWAN 14 12 9 0 0 0

VOUGHT 16 10 5 1 0 0

AFWAL 11 10 10 0 2 1S NUMBERS OF

PRIMARY ORIGINS NLR 15 8 7 3 0 0PER PARTICIPANT F 1

DFVLR 15 12 6 0 0 0

NDRE 15 11 4 3 0 0

RAE 8 8 11 3 0 1

SIFFRL 12 16 6 0 0 0

PISA 7 22 3 0 0 1

Sea.- 210 MPa Sma x - 144 MPaFATIGUE TESTING xoa

SCHEDULES -E/Q F/R G/S B/N E/Q F/R G/S B/N C/O D/P

• NUBERS OF - - - - - - -

PRIMARY ORIGINS fatigue in air 23 14 1 0 0 15 25 0 0 0PER STRESS LEVELAND FATIGUE pre-exposure + fatigue 23 14 2 2 4 13 16 5 1 1TESTING SCHEDULE in air

fatigue in salt spray 28 16 0 0 7 19 10 0 1 1

pre-exposure + fatigue 33 9 0 0 9 18 7 4 1 1in salt spray

all schedules 107 53 0 2 20 65 58 9 i 3 3

FATIGUE ENVIRONMENT Smax 210 MPa Sa x - 144 PaE/Q F/R G/S B/N E/Q F/R G/S B/N C/O D/P

PRIMARY ORIGINS fatigue in air 6 28 10 2 4 28 41 5 1 1PER STRESS LEVEL (with and without 4

AND FATIGUE pre-exposure)ENVIRONMENT

fatigue in salt spray 61 25 0 O 16 37 17 4 2 2(with and withoutpre -exposure)

24

TABLE I1: SUMMARY OF xo TEST OF INDEPENDENCE AND YATES' CORRECTED xo TEST FOR THE CFCTP CORE PROGRAMMEPRIMARY FATIGUE ORIGINS (95 X CONFIDENCE) OMITTING THE SIFFRL AND UNIVERSITY OF PISA DATA

SOURCE OF SIGNIFICANT ASSOCIATION BETWEEN STRESSASSOCIATION FATIGUE TESTING SCHEDULES aa.05:(r.l)(-i) Xc OR X LEVEL AND PRIMARY FATIGUE ORIGINS(x OR 0a > xO.o0 ,1r-1)(c -?)

fatigue i air 'O.05;2 - 5.99 X- 28.40 yes

STRESS pree..psute + fatigue in air x.05;2 - 5.00 02 - iONS yes

LEVEL fatigue in salt spray 06.052 - 9 " 6 17.48 ys

pre-epourea fatigue in sat spray x05;1 - 384 - 10.27 yes

SOURCE OF (MP.) 0 SIGNIFICANT ASSOCIATION BETWEEN

ASSOCIATION sax 0

>.r-i)( )PIRYFAIGUO

IFAT(I. 0' - 1 Ni IS en 1)

EVIRONMENT

0.05:3(FATIGUE 20X.5

3°78 .8n

TSTING 144 '0.0;6 - 12.51 28.56 yes

SCHEDULE) i4O0;

TABLE 12: CFCTP CORE PROGRAMME PRIMARY FATIGUE ORIGINS CORRELATED WITH FATIGUE LIVES, OMITTING THE SIFFRLAND UNIVERSITY OF PISA DATA

ONUMBERS OF PRIMARY FATIGUE ORIGINS AND LOG MEAN FATIGUE LIFE (CYCLES)Sman LOCATIONS OF

(NPa) PRIMARY ORIGINS pre-eupoure + fatigus pre-expsurer Fatiguefatigue In air in air fatigue in sult spray in sarl spray

E/Q 20 : 22.606 20 : 12.219 22 :10,583 2 : 0,i33

210 F/R 9 : 21 207 9 10.914 13 1(3 42 5 : U .9?G/S 7:2,699 2: 30,648 0 - 0: -B/N 0 : - 2 : 22.089 0 O : -

E/Q 0 : - 4 : 92.770 6 : 124.192 8 : 76.019F/R 1 162.489 N : 10.472 13 : 5.701 13 ,SS.OO

144 C/S 23 : 12N,.83 12 1 19,99J 10 Iih 6O. S : 80.906B/N 0 : - S : 117.704 0 : - 4 : 84.035C/S 0 : - L : 122.00 1 : 122.092 1 : 73.40D/P 0 1 . 788028 o i : 73,840

TABLE 13: SUMARY OF 12 TESTS FOR COMPARISON BETWEEN CFCTP CORE PROGRAMME PRIMARY FATIGUE ORIGINS ANDFATIGUE LIVES (95 % CONFIDENCE) OMITTING THE SIFFRL AND UNIVERSITY OF PISA DATA

FATIGUE TESTING SCHEDULES TYPE SIGNIFICANT ASSOCIATION BETWEEN PRIMARYFMP)a FATI 011 RIGINS . .FATIGE LIVES

fatigue in air 210 X nu144 Fisher's exact test n

710 Onopre-exposure + fatigue in air 144 2

fanig in salt spray 210 Yc144 Fisher's exact test yes

210 Fisher's exact testpre-expsuree fatigue in alt spray 144 no

Xcrmmm mm l u IRI I l lI

33 3333.0-S 3,4 = 33

~ - 33 A443333 3333 33

33 -13-

00 3,000 33H 1.3 3-~~0S~ .333 33 33 330'

33 0 - 1...3 33 433C~. 33 .0

33 03 3,33~ 330~ 03- 03 '0-~ 3, -~

33 3-33333- .040' 43,i0* 43,

H 33 003.SI3~ 3,0.

- 0 3, 0'

.4 3-33330~. H3;

33034 33 0000000033 3333 3,

.4

33 H r~3- 3,333,033 3,3, .03333.3, 13 13H ~ 0 3-3-3~3.-~~3-0- 33*

H 0H 0 3333 33003, 3, .1333 33 13. 3,3,3,3,*0~ 33

.4 - 133

33 3333 33 13 3,330.0.33H 0 03,3,0 00 3-

33 - 1.1 33333,33 33..1 33 - 3,0003,00o - 33 330033 or, 3.1 .433 33 33 . - . . - - 33z~330 33 33 r-03-0 333333 333,33 -- 333313.33333333(3

33 33HOH

330. 333,N-r~330 333,330 4433 33 ..-- --330 33 4330r~ 033 3303, 33

0HiI - - 0

33 0 ~ HH033~0~

- 33~.. 43 0 03,.---- .. N. N.~333313 0330 13 0 0 33>33333333333,33 13

1) 0

33330

33

33H 3333 03, 33 33131

133 33.4 0 333333

3333333333033 3,~. .40033033 333333333333

4 0

33 1.1 ~33.4330333,.4

0 0

26

110. 800200 50

04. B .2 .2

I)~~~ FATGUISECME

30.020

NHOLE Ui;A SENOTI I

2) HALFPLATE 3) SPACER

185.65. 0

SECTION c-c AL IESOSI

(1) HOLE DIAMETERS 6.306 ±0.044 no~FOR CORE PROGRAMME SPECIMENS (SLIGHT PRESS FIT).HOLE 0IAMETERS .248 t±0.0127 -n (INTERFERENCE FIT) OR 6.306 ±0.044rn- (SLIGHT PRESS Fl:A FOR SUPPLEMENTAL TESTING SPECIMENS.

(2) ALL HOLES MARKEDOTHUS " A "ARE 18' 0

'8

m. IAMETER.0

13) SPECIAL TOLERANCE INDICATIONS ARE~ 25D = 25±005

25 .2 =2500.1

25.6 26 f 012

Fig. I The CFCFP core programme and recommended FACT supplemental programme specimens

ALL SPECIMENS

PRESTRESS IN COLD0 BOX

DESICCATOR STORAGE

DESIGUCTOSTORAGE

16g 2 PCMN SEe~ai 16mr ofPECIMENtstprSeur

CCPF ATIGUEDUAT A

E ELIVES RIAYFTGEIINS

2TEST OF INDEPENDENCE

YTES' CORRE CTED <2

TESTOR

FISHER'S EXACT TEST

______________0 G SNS ANE STRESS LEVELS

TEST FOR NORMALITY O R G NS AND ENVIRONMENTS

* ARITHMETIC AND LOGARITHMIC 0 FATIGUE LIES AND ORIGINSNORMAL FROBABILITY PLOTS L PER TST CONSITSN

II-\ TEST FOR GOODNESS OP FIT

FORRMMAIN

STATSIA ITEST FOR HOMOGENEITY OF VARIANCES

ANALYSIS 0 DOG TEST TO CHECK FOR

INTERLABORATORY DIFFERENCES

PER FATIGUE TEST CONDITION

* BARTLETT TEST TO CHECK FOR

DIFFERENCES BETWEEN

FATIGUE TEST CONDITIONS

0 MAI EFFCTS* SAMFLE SIZE AND OATH SCATTER- stress

- environentRB

- laboratory

W 2 . WAY INTERACTIONS

MAI -Stress environmenRt

STATITICAL - tre~ss laboratoryANHLYSIS - environmen~tt: laboratory

*0 S-WAY INTERACTIONS

* - sirenS enviroment: laboratory

FINEFIAN TUNINGS'N

LE AST SIGNIFICANT DIFREC TES DOCANS NEW MULTIPLE RHNDET ET

Fig. 3 Survsey of statistical rmethods Dor ssrelyming the CFCTP fatigue Sife anda primary fatigue originr data

~[1 f 0 4~i~*1I411.' I I a W141A I

a -I-mi I~KI I - XIiI .4I' zaI ~ DL~I

I I- + 44 I 4o

4-, ~

I

4

4444w 4 p_~~ -4 4 4 -~ -~-~ ~. a c 4 4 4 4'- -4 a 4 4 -2 0 2 4 0 2 4 4 4 2 4 2 4 4 0

FT 1

0 ~ ii-~ 4

4 4

+It4

4 24 4

2 0 4' 4. 0 4 ± 4 4 4 .4 4' 2 4 4 4 2 4 2 4 4 a

TEST SCHEDULES NADC SASKATCHEWAN

fatigue i alt spry...-

preeaxa~re +a-j

=21OS_, S, -4Ma 211SM l S -14 MPa

fatigue ri satry NE 7 7/STEST SCHEDULES ]L VDUETVFEA

r

a- fa,, :,.,-

pr- exposure [144 MPs - 20

faiu nsalt pa

TEST SCHEDULES NL ISA

I Sax21 NP- IS~s14 Ma m -210 Pa

Spre- explsalre +Ifatigue in air

{ fatpe inp salt spray

aluein salt spray

TESTSCHEULES NORE RAE

Faige in aFi c "

ti ueis salt spraySfatigue in salt spry

pre-exposrn ________________

CYCLESCHTOUFAILURE CYCLES AILI~~ ~~~~~~~~~~~~ ' g. N C-CT 'or prgers ftigeie tapretcynt04totgace

- -~ -,i

I-~- vF7 3

jI 1~~

I ~I~lr ~ H~IL ~ N -

LJo 0

3 -

w.~ 4 0

0<

~0

ltF- A.r - I - C-,

o --- - C0 CC

~ iiL77i7II~iI.I~ I

~ C-4r~~~ *- .-. C-C C

COC 0OCOC a 0 0C~.COflflC1CC- CC C-C

Co

32

a EFFECT OF STRESS LEVEL nSmx 210 MPa f-l Sa 144 MPa

100

60 0

2

FATIGE 1008 % PIMAR

%PRIMARYFATIGUEORIGINSNG

80 08

20r i

b EFFECT OF FATIGUE ENVIRONMENT cj- EFFECT OF FRE E XPOSURE

flFATIGUE IN AIF nFATIGUE IN SALT SPRIAY flFATICUE fl FE EXPFOSURIE 'FATIGUE

100 1 0 -

p0 fatigue o 210NIP.

60I 60

20 20

%PRIMARY %PRIMARY

FATIGUE 0~------ FATIGUE 0 nORIGINS 100M~ ORIGINS 10Zrn.144MNIP.

0 60

40 40-f~20- 20

Fig. 7 Effects of stress level, fatigue, Clvero sent and Fre-exposure on locations of CFCTP core programmeprim~ary fa tigue o rigina

33

- 0 8

- S

CC, - I 8~ :

- S5-

z

Ir 00

n0

00. 0

S

(0 CCC C-C

Co

0 0

0 0

0

_______ ~ I21

34

E E

J-, Ll, -L UJO d

35

TEST SCHEDULES ALLTEN CFCTP PARTICIPANTS

SSrmx =210MP [Sa=14P

P. niri. Z D

a.u. ... m i

fatigueinsoftSpray _8_ UNCORRODEO UNCORRODEDfutip SP ~tSIR ECIMENS SPECIMENS

pro exposure + fatigue instsprw

1000 10,000 100.000CYCLES TO FAILURE

Fig. 10 Sumaary of the CFCTP core programee fatigue life data per testing schedule

36

37

PAR: Ill

THE FACT SUPPLEMENTAL PROGRAMME

1. INTRODUCTION

1.1 Overview

The FACT supplemental programme of fatigue testing followed on from the CFCTP core programme. TheFACT programme was included so that individual participants could investigate corrosion fatigue problemsof particular relevance to their own interests and yet within a broader context. To achieve this theindividual programmes were set up with a high degree of commonality. This is shown in the overview intable 1.1. Most participants tested l dogbone specimens (the same type of specimen used in the CFCTP)under nominally identical mechanical and environmental conditions. Concerning these aspects the technicalmanual required for the CFCTP (reference 1) also included supplemental testing guidelines for specimenmanufacture, application of protection systems, specimen assembly, pre-exposure, and fatigue and corrosionfatigue under flight simulation loading (FALSTAFF and MINITWIST, references 2 - 5).

The individual contributions to the FACT programme will be presented in this part of the report inthe same order as in table 1.1. These contributions also include summaries of the statistical methods usedto analyse the results. A detailed description of these statistical methods is given in Appendix II.

It can be seen from table 1.1 that the main interest of several participants was to compare theenvironmental fatigue properties of a number of aluminium alloys in various tempers. However, owing to thecalibratory function of the CFCTP and the participants' active cooperation in obtaining the manysimilarities and commonalities within the FACT programme, it has also been possible to make inter-participant comparisons of materials and the effects of different protection systems and fasteners. Theseinter-participant comparisons are the subject of Part IV of this report.

1.2 Recommended Specimen Configuration

The recommended specimen configuration for the FACT programme was the 1i dogbone used in the CFCTPcore programme. The specimen configuration is illustrated in figure 1.1. As mentioned in Part II of thisreport, the specimen was designed to simulate the load transfer and secondary bending characteristics ofrunouts of stiffeners attached to the outer skin of an airframe structure. The design goals were a loadtransfer of 40 % and a secondary bending ratio of 0.5 (reference 6). These characteristics have beenchecked and the actual values are generally lower, see Appendix I.

1.3 Flight Simulation Fatigue Testing

1.3.1 Short description of the manoeuvre spectrum FALSTAFF

The manoeuvre spectrum FALSTAFF (Fighter Aircraft Loading STAndard For Fatigue evaluation) wasdeveloped by several European laboratories (references 2 - 4). The spectrum is illustrated in figure 1.2.It is divided into 32 load levels. The load sequence consists of blocks of 200 different flightsclassified into three groups of mission types:

* flights with repetitive patterns of severe manoeuvring

* flights with severe manoeuvring

* flights with mainly moderate manoeuvring.

The sequence of flights and flight loads is random. Owing to the spectrum characteristics there are manyflights containing high loads, although level 32 is reached only twice, in flights 32 and 173. Anillustration of the flight-by-flight loading pattern is given in figure 1.3.

1.3.2 Short description of the gust spectrum MINITWIST

The gust spectrum MINITWIST (reference 5) is a shortened version of TWIST (Transport hing STandard)that was developed by two European laboratories (reference 7). The spectrum is approximated for testingpurposes by the stepped function shown in figure 1.4. Stresses are expressed non-dimensionally by dividirgthem by the stress pertaining to undisturbed cruising flight (Sf ). There are ten gust load levels and oneground load level.

MINITWIST consists of blocks of 4000 different flights. There are ten flight types, ranging fromstorm (A) to calm (J) conditions. Basic properties of the load sequence are:

* the flights and loads for each flight are applied in a random sequence except that clustering ofsevere flights has been avoided

* the loads within each flight are applied as a random sequence ot half-cycles in such a way that apositive half-cycle is followed by a negative half-cycle of arbitrary magnitude

* load sequences have been generated individually for each flight. This means that flights of thesame type generally have different load sequences.

The severest flights are 1656 (type A), 2856 (type B) and 501, 2936 and 3841 (type C). An illustration ofthe flight-by-flight loading pattern is given in figure 1.5.

W

38

1.4 Establishment of Fatigue Stress Levels for the 11 Doghone Specimen

Characteristic fstigue stress levels for the FACT programme were establishes on the basis of nominaltarget fatigue lives and pilot tests. This is illustrated in figure 1.6. The established fatigue stresslevels were as follows (reference 8):

TYPE OF FATIGUE LOADING NOMINAL UNCORRODED FATIGUE LIFE CHARACTERISTIC STRESS LEVEL

constant amplitude, Rl - 0.1 100,000 cycles S m - 144 Ml's

FALSTAFF 4,000 flights S.. - 289 Ml's

10,000 flights 5 a 238 Ml's

MINITVJIST 10,000 flights 5m 101 Iii'

40,000 flights 5,f 89 Ml's

There are two important points to note:

* all stresses are defined in terms of the total cross-section of the fatigue specimen -I at thelocation of the centreline between the fasteners, i.e. the fastener holes are included in thecross-sectional area

* the pilot tests used representative specimens but did not take place under exactly the sameconditions as definitive tests. This is because the definitive tests included prestressing thespecimens at low temperature to crack the paint (if possible) around the fastener holes.

1.5 References

1, R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testingprogramme", AGARD Report No. 695, February 1982.

2. G.M. van Dijk and J.B. de Jonge, "Introduction to a fighter aircraft loading standard for fatigueevaluation "FALSTAFF"". Paper 3.61, Problems with Fatigue in Aircraft, Proceedings of the Eighth ICAFSymposium, compiled and edited by J. Branger and P. Berger, Swiss Federal Aircraft Establishment(F + W) 1975: Emmen, Switzerland.

3. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation", Combined Report of theF + W, LBF, NLR and IABG, March 1976.

4. 1.8. de Songs, "Additional information about FALSTAFF", NLR Technical Report TR 79056 U, June 1979.

5. H. Lowak, 5.8. de Songs, J. Franz and D. Schatz, 'MINITIIIST. A shortened version of TWIST", Combinedreport of the L8P and NLR, NLR Miscellaneous Publication MP' 79018 U, May 1979.

6. D. Schatz and 5.5. Gerharz, "Schwingfestigkeit von Flgungen met Sonderhefestigungselemenren"Fraunhofer-institut far Betriebsfestigkeit Technische Mittellungen TM 69/73, 1973.

7. 5.8. de Jonge, D. Schr~tz, N. Lowak and J. Sciive, "A standardised load sequence for flight simula-tion tests on transport aircraft wing structures", Combined report of the LEP and NLR, MLR TechnicalReport TR 73029 U, March 1973.

8. R.S.N. Wanhill, "Establishment of CFCTP stress levels for NLR core and supplemental testingprogramme", NLR Memorandum SM-80-034, March 1980.

39

* 0

4 3

0 -3 __ __ 0a

440

44 0 0 0 0

* 0 0 6 0o

40

H// Lo~ B_ [I~~2 [f17I110.300.0

iC HNIS SIDIE ON LI

C

30.0 13500

1) FA TIGIE SPECIMEN

30012.

'A

2)L DIA. SEE NOTE IA

2HAIPPLATE 3) SPACER

0 6~.30601

106L6 300

SECTION c-c ~~M~b~ Nn

(1) HOLEODIAMETERS 6306 t*04n .(SLIGHT PRESS FIT) OR 6.248±0.0127 no(INTERFERENCE FIT).

(2) ALL HOLES MARKED THUS' A" ARE lI1010 on-DIAMETER.

13) SPECIAL TOLERANCE INDICATIONS ARE: 25-0 = 250t0.0525.0 = 25t 0.125.6 = 25 t 0.2

Fig. 1.1 The recoumended FACT su~pplemental programse specimen

41

24

FALSTAFFLOAD LEVEL

16

........ ...1 II

LOA LEE

CUUATV FRQELIGHTLCKO 20FLGT

FALST3 A rtoFALFF lih 8

42

2.0-

RELATIVESTRESS

AMPLITUDE

I60_ t_50 ±130 11 15 tO 995 t84 ±0285 __053 O375

GS IT a I I v i 11 Wm

0 GROOINO-AR-GROUND TRANSITIONdS

-0.5 GROUND LOAD LEVEL

1 10 102 i33

10i

CUMULATIVE FREQUENCY PER BLOCK OF 4000 FLIGHTS

Fig. 1.4 The gust spectrum MINITWIST

2.0 ----- ------------

FLIGHT 7 FLIGHT 8

1.0

RELATIVESTRESS

AMPLITUDE

0 00GROUNO -TO -AIR -TRANSITION

-1. Pat o f _________

Fig. 1.5 Part of MINITWIST

43

30C 1 T'T '

CONSTANT AMPLITUDE, R = 0.1250- 7075-T76 ALUMINIUM WITH CPCTP

CORE PROGRAMME PROTECTION SYSTEM

200- LABORATORY AIR, 295 K

150 - REDU I RED S055 144AMP.

MAXIMUMI

STRESS 15maon (Mpg)

100-

TARGET LIFE

5j ,, , ,,

1000 10,000 100,000 1,000,000CYCLES TO FAILURE

325

FALSTAFF

300 * 7075--T6 ALUMINIUM WITH NP-SREQUIRED S-=289 MPa RTCINSSE

LABORATORY AIR, 295 K

275 I 5HMAXIMUMST RIESS

5man IMPs)

250 -IREQUIRED

0r,,5= 230 MPa

225 - ITARGET LIVES I

100 1000 10.000 100.000FLGT OFAILURE

110

I PROTECTION SYSTEM

I LABORATORY AIR, 295 KMEAN STRESS EQRESI8ma I CYCLE FREQUENCY 15 H,

IN FLIGHT - -1 -S - - -5mf IMPs)

701

1000 10o0w 100,000 1.000,000FLIGHTS TO FAILURE

Fig. 1.6 Establishment of FACT supplemental programme fatigue stress levels for the Ij doghone specimen

from pilot tests (a)

44

2. THE COUHf CONTRiBUTION TO THE FAT PROGRAMME

K.E. Duval and A.E. Hohman, LTV Aerospace and Defence Company. Vought Missiles and Advanced Prgram;es

Division, Dallas., Texas, USA.

2.i Introduction

Experience has shown that metal fatigue is a major cause of structural tailures in aircraft. Also ithas been known for a long time that fatigue properties of structures can be greotly influenced by tl.nature of the environment in which they are operating. In the past these effects were accuonted for byapplying safety factors to designs based on data developed in non-aggressive environments. However,increasing emphasis on lower weight, higher performance and longer lasting aircraft makes it imperativethat future designs take more accurate account of the effects of adverse operatii.g environments.

Because of the relatively short time available during the design and development of an aircraft, longterm corrosion esposure to simulate the uervil, ecuironment is not practical. Accelerated testingprocedures must be employed to provide the necessary data in a timely manner. To obtain corrosion fatiguedata severe entronnenr ,,-- flo. LLal ,amrsion to salt fog hive been used. However, even with theseuevere environments considerable testing time is required because of the slow rates of cycling that vustbe used.

An even greater acceleration of corrosion effects is desirable. One possible wa to achieve this isto raise the temperature of the environment so that reaction rates increase. At present there is littleinformation On temperature effects in corrosion fatigue. The primary objective of the VOUGHT costributionto PACT w-s to obtain a limited amuunt of elevated temperature crrcsion fatigue data as part ,f acontinuing investigation ci corrosion fatigue iesting methods.r2.2 The Test Programme

An overview of the test programme is given in table 2.i. This proermme was preceded bv pilot teststo checi marker load characteristics predicted using the irEtif.: computer program, which is a crock -rawthanalysis program developed at Rockwell International. Purtier ivfrinoi about the prediction ,I markerloud characterisrics and EFFGRO is given in references (I, 2.

2.2.1 Material, specimen configuration and protection system

The material was 6.35 mm thick 7075-T1651 aluminium allot plite. Avcroge encineering properties wereas follows:

0.2 N YIELD STRESS U Is ELON.ATDT 2iNI'l ICITY

4H4 MPa 536 MPa 1, 7 37.47 IC.A.

The specimens were of the single dogbone configuration shown in figure 2',. Te dogbnes were notchedb central holes with a K value - 2.7. Note that mill finish was retained on all surfaces, i.e. includingthe central holes. The protection system was a standard CS. Navy paint schema:

* chromate conversion coating type 2 class 2 (MIL-C-5541)

* inhibited epoxy polyamido primer (Mil-P-23377)Mi.-C-81l773C

* aliphatic polyurethane topcoat. 207-9-404

This system was applied to all surfaces except the central holes.

2.2.2 Mechanical testing conditions

All stresses were defined in terms of leads on the central cross-section of the specimen andincluding the central hole in the cross-sectional area. The characteristic fatigue stress leves (S )for the"test programme hav e b een given already in table 2.1. These levels were based on the results ofxpilot tests and the CFCTP core programme.

The fatigue load history is illustrated in figure 2.2. It consisted of block, of 200 damage cycles

(H = 0.1) and 100 marker cycles (R - 0.5) with a constant S In order to avoid crack growth retardation.

The intention of this load history was to provide clearly visible marker bands which, however, should havea minimal contribution to overall crack growth. All tests were carried aut with a cycle frequency of0.5 He.

2.2.3 Environmental conditions

The fatigue tests were done in laboratory air at a nominal temperature of 297 K and in 5 7 aqueousNaCl salt spray acidified with H2SO 4 to pH 4. The tests in salt spray were done at several temperatures in

the range 297 - 339 K, see table 2.1. The salt spray cabinet met the requirements in reference (3) butwith the addition of a hot air inlet and baffles for mixing hot air with salt spray to produce elevatedtemperature fog. The environmental temperature was monitored by a thermistor temperature control probelocated 25 mm from the specimen test section. The specimen temperature was monitored by a thermocouple,and it was found that the temperature could be maintained to within ± 0.5 K.

45

During testing at elevated temperatures the air pressure in the salt spray cabinet was increased to

try to compensate for evaporatiot,. However. it was found that the solution collection rates specified in

ASTM standard B 117 - 73 (Stacdard Method of Salt Spray (Fog) Testing) could not be maintained at

temperatures above about 316 K. Furthermoro the salt fog became only a slight mist at 325 K and was not

observed at 339 K.

2.3 Results

2.3.1 Fatigue life and fatigue crack initiation and propagation life data

Fatigue life and fatigue crack initiation and propagation life data are compiled in table 2.2. Thefatigue crack initiation and propagation life data were obtained by correlating marker load bands on the

fracture surfaces with numbers of cycles and tracing the markers back to crack dimensions less than

0.3 m.

The data for fatigue in salt spray at different temperatures are presented in figure 2.3. These datawere analysed statistically accoding to the procedure shown in figure 2.4. Owing to the limited number ofdata and unequal sample sizes it had to be assumed that they at least approximated to rndom samples fromlog-normaily distributed populations with equal variance. Unequal sample sizes also meant that a modifiedversion of Duncan's new multiple range test had to be used for "fine tuning" the analysis of variance

results. More details of the statistical methods are given in Appendix 11.

Results of the statistical analysis are summarised in tables 2.3 and 2.4. According to the analysisthe temperature of the salt spray environment had no significant effect on the fatigue life and fatiguecrack initiation and propagation lives.

2.3.2 Fatigue crack growth rate data

The fatigue crock growth rate data are shown in figure 2.5. Most of the data fall into two broad

bands which indicate that the aggressiveness of salt spray first increased and then decreased withincreasing temperature, eventually becoming no more aggressive than room temperature air.

The data for tests in salt spray at 316 K appear to represent a transition in the aggressiveness of

salt spray. At high crack growth rates the environmental contribution to crack growth decreases withrespect to mechanically-induced crack growth. However, this effect had a negligible effect on crackpropagation life and total life.

2.4 Discussion

Owing to insufficient data for some test conditions and to data scatter the statistical analysis did

not indicate a signif,:ant effect of salt spray temperature on the fatigue life and fatigue crackinitiation and propagation ives. However, the data in figures 2.3 and 2.5 show the following trends:

(1) Increasing the salt spray temperature from 29? K to 316 K tended to decrease the fatigue crack

initiation and propagation lives and hence total life.

2) Further increasing the salt spray temperature from 314 K to 339 K resulted in an increase lv

fatigue crack initiation and propagation lives and a decrease in fatigue srack grooth rates ttvalues similar to those for fatigue in room temperature air.

In view of these trends and also the ciservations on collection rates and appearance of the saltspray at elevated temperatures (section 2.2.3) it seems reasonable to conclude that acceleration of saltspray corrosion fatigue testing is possible by raising the temperature of the salt spray, but only as long

as the experimental set-up permits the production of a proper salt lug.

For the test set-up in this investigation it appears that the critical temperature at which a proper

salt fog can still be maintained is 316 K. At this temperature the average fatigue life decreased by about35 % compared to the rcom temperature fatigue life, mainly because the crack initiation life decreased.This :epresents a considerable reduction in testing time which, however, must be weighed against theincreased complexity of salt spray fatigue testing at elevated 'emperatures and greater dilficulty in

obtaining reproducible test conditions.

2.5 Conclusions

Although statistical analysis did not indicate a significant effect of salt spray temper-ture on thefatigue life and fatigue crack initiation and propagation lives of -otched 7075-T5l plate specimens, the

following conclusions are drawn:

(i Increasing the salt spray temperature from 297 K to 31: K tended to decrease th, fatgue crickinitiation and propagation lives and hence total life.

(2) Further increaslng the salt spra temperature resulted in an increose in latigue crack lniti.tion

and propagation lives and a decrease in fatigue crack growth rates becanse a proper colt togcould not he maintained above 31b K.

(3) Raising the salt spray temperature can resilt in a considerable reduction in testing time. This

must be weighed against the experimental problems of obtaining and malntiding a proper salt fog

at elevated temperatures.

-i-s

4h

2.6 Recommendations for Further Investigation

The effects of temperature on corrosion fatigue should be investigated for other materials and heat

treatment conditions and also for specimen configurations representing typical aircraft structural joints.Other fatigue load histories should he considered, especially spectrum loading representing service usage

and preferably giving marker bands that ailow tracing crack growth back to small flaw sizes.

2.7 References

1. R.E. Duval, "Effect of temperature on corrosion fatigue life of 7075-T7651 aluminilim alloy plate",LTV Aerospace and Defence Company, Vought Missiles and Advanced Programmes Division Report

3-41300/4R-115, 1984.

2. J.B. Chang, M. Sza.ossi and K.-W. Liu, "Random spectrum fatigue crack life predictions with orwithout considering load interactions", Methods and Models for Predicting Fatigue Crack Growth underRandom Loading, ASTM STP 748, edited by J.B. Chang and C.M. Hudson, American Society for Testing and

Materials, pp. 115 - 132 (1981): Philadelphia.

3. R.J.H. Wanhill and J.J. De Luccla, "An &GARD - coordinated corrosion fatigue cooperative testing

programme", AGARD Report No. 695, February 1982.

47

TABLE 2.1: OVERVIEW OF THE VOUGHT TEST PROGRASE FOR FACT

MATERIAL . 6.35 mm thick 7075-T7651 aluminium alloy plate

6.03m DIAMETER UNPROTECTED OPEN HOLE

SPECIMEN

6.35mm00mm

PROTECTION SYSTEM * Chromate conversion + inhibited epoxy polyamide primer+ aliphatic polyurethane topcoat (except central hole)

FATIGUE LOADING . Blocks of constant amplitude damage cycles (Smin/Sma - 0.1)

and marker cycles (S.in/S - 0.5); cycle frequency 0.5 Hz

FATIGUE ENVIRONMENTS 0 Laboratory air; 5 % aqueous NaCl salt spray with pH 4 atvarious temperatures

ENVIRONMENTAL S (MPa)SCHEDULES TEMPERATURE max____ma

(°K) 152 148 144

Fatigue in air 297 0 0297

TEST PROGRAMME311

Fatigue in salt 311

spray at various 316temperatures

325

319 0

STATISTICAL ANALYSIS * Fatigue lives and fatigue crack initiation and propagation lives

SPECIAL CONSIDERATIONS * Fractographic determination of fatigue crack growth data frommarker load bands

- -'4 -- '-- -

48

44 4 404 444

4 Ca, 4 aa,'- 44a,4 404 aa,0'

a, C'' 4 '4 a,a, 44 4 a,4a,

0

4 4 W a, 0 a,o o $' 4 4 0- 4 ~ 0' C a,H 0 40 0 0 a, 44 a, 4

0' 2: 0CO 0

H4 00 0

0 '~ a,

0' 4 4CN 4 4

2:0~ a, a, a, a, 4

U ~ a, aca a, -4 4 0C '40. 4 0.

~ 4H 4

C 0

4 4a. 4 0

4 0- Ca,4C a,4,U 0 4 Ca,-aa, a,~-"-- a,44

0 40"'.'. 404 040

- 4 - - 4aa,4 444 a,4~'.

.~ a, a,4aa, 4-- -4-

C ~

0

0.004 4a,44 4 4440 aCC~ ~~44

4 4~4C a,40H H a,

C a, aa,40' 4r~4 444

H

4

0 44t. C 4a,~'-. 4 444 44 - a,4.-~ a, 44~" 4 aa,4 4H 4a,4 4 a,4a, 0 4-~a, 0'

bOON 4'~ a, aaa, ~ 4'. 4'044 4 a,-- - aaa, a,

4

0 '4 2 0 0 0 0 0 0 0 0a - '4 0. 0. 0. 0. 0. 0. 0. 0.

H4 2: 2: 2: 2: 2: 2: 2: 2: 2:

H a, 4 4 a, 4 4 a, 4 4'0 4 4 4 4 4 4 ~ 4 40. ~4 4 - - - - - - - -

H I I I I I I I

N 0 0~ 0 0 0 0 0 0 0a, 0 0 0 0

4 '44 4 4 4 4 4 4 4 4 4

4 4

4 0 4 0 4

H -

- U HCU4 WHOC

0'0H~H4 40.4

H H.a0.a, HO0.a,'0 '044 '04.-.0. 0.440 0.0.40

49

FABLE 2.3; SUI'IARY OF ANALYSIS OF VARIANCE RESULTS (95 % CONFIDENCE)

SOURCE OF SIGNIFICANT EFFEIT F T1FERATVREVARIATION FATIIUE LIFE FARbIERS F DISTRIBUTION VALUE F, F - F O :ITAIB1N VF

0 MIN EFFECT TOTAL LIFE 53 2 511- . .temerature-

LIFE TO A 0 3 - CRACK 9 12 1 270

LIFE FROM A0 In, CRACK I A12 2.058 n

FABLE 2.4: SUMIARY OF RESULTS USING DUNCAN'S NEW MULTIPLE RDGE TEST (95 Z CONFIDENCE)

1- MFA FATIGUE I.IVES AND SARIFLE

SAT F AY TEMErATt'E 2 1 K 311 K 31 K 35 y 3 3, v,

TOTAL LIFE 610 3 sOL5 3 IS 2

LIFE TO A O 3 = CRACF 40 (O 2 172 2 .130 1 2

LIFE FROM A o 3 .. CRACK 4 316 2 4 233 2 213 I 1 "

T DIFFERENE AEmFEN F S FIVT21/FF, EpFTIU LIVES1.1 0 TA S11R ,AM'l TF11 11 SAL 1PR-1 LO

AT DIFFERENT TEMPE,3TC6ES FAT E S -P

31 1 R

K,, I

L'5 "I K Fo.

III Fill Q

- g -Ill c F' .,.f 1 '

291 . . .. .. "-'I ..I ____- IV F/I): to __ S __2_,* I _

Owig t eqal amle izeIhueFopi on c- nas emd u/gteu oiidvrlno Di~al$ts h aersl got n,

300.0

10.0 80.0

30.6 120.0

ALLDIENSIONS IN

(1) MILL FINISH RETAINED ON ALL SURFACES.

12) ALL HOLES MARKED THUS" bk" ARE 1S0 05 m DIAMETER*0

(3) SPECIAL TOLERANCE INDICATIONS ARE. 25.0=2U ±0.05

25.0 =20 ±0.1

Fig. 2.1 Specimen configuration for the VOUGHT contribution to the FACT programme

20 YLSONE BLOCK toC:C-

Sma5

APPLIEDSTRESS

Smin im)

(1) DAMAGE CYCLE STRESS RATIO R =S0, 5Id/S- =03

(2) MARKER CYCLE STRESS RATIO R= 5, ,Io/S,,= 5.5

Fig. 2.2 Fatigue load history for the VOUGHT contribution to the FACT Frogramme

TEST TEMPERATURES BLOCKS OF DAMAGE + MARKER CYCLES. S,,x 144 MPa

L~FTO A LIEFROM A TTLLFCRACK. CRACK

L KD

LID>

10,000 100,000CYCLES

Fig. 2.3 VOIOHE 0sit spray fatigue life and fatigue crack initiation and propagation life data for7075-T7b~ 5s1in gle d ogone specimens with unprotected open holes

52

VOUGHT SALT SPRAY FATIGUE,LIFE DATA FOR FACT

ASSUME AT LEAST APPROXIMATELOG-NORMAL DISTRIBUTIONS

ASSUME AT LEAST APPROXIMATEHOMOGENEITY OF VARIANCESf1-WAY ANALYSIS OF VARIANCE

0 MAIN EFFECT- temperature

SIGNIFICANT EFFECTS

.o

DUNCAN'S NWMLILRANGE TEST

Fig. 2.4 Survey of statistical methods for analysng theVOUGHT salt spray fatigue life and fatigue crackinitiation and propagation life data for FACT

GATEMPERATURE ENVIRONMENT "O

10-5 0 297 K AIR A0 297 K a 0 aS 311K AO

o 316K SALTSPRAY a A 0V 325 K a

0 0

O 339 K 0 0

° 40O 0

0 0

DATA ENVELOPE FOR 0TESTS IN SALT SPRAY • •

da AT 27 AND 311 K A C,(d) 0 %

Im/CYCLE) A v10-6e

V*

A *

/ / \ DATA ENVELOPE FOR /T j ESTSINSALT.PRAY~AT 325 AND 339 K

10-7110 15 20 25 30

a AK (d) (MPa 4" )

Fig. 2.5 Fatigue crack growth rate data for the VOUGHT contribution to FACT: da/dn andAK were calculated as though the tests were done with damage cycles only

53

3. THE SAAB CONTRIBUTION TO THE FACT PROGRAMME

L.E. Jarfall, SAAB-SCANIA Aerospace Division, Link6ping, Sweden

3.1 Introduction

The Aerospace Division of SAAB-SCANIA participated in the FACT supplemental programme with the

assistance of the Structures Department of the Aeronautical Research Institute of Sweden FFA

(references 1, 2). The main objectives of the SAAB contribution to FACT were to develop fatigue testing

facilities for comparison of different corrosion protection systems and to compare results with those of

the CFCTP core programme.

3.2 The Test Programme

An overview of the test programme is given in table 3.1. There were two types of specimen. The

unnotched coupon specimens were used in an introductory study of the effects on fatigue life of outdoor

pre-exposure and/or environmental fatigue in chambers specially constructed by the FFA. The li dogbone

specimens were from the same batch as the CFCTP core programme specimens and provided a basis for

comparing the effects of environmental fatigue in the FFA chambers aid the CFCTP salt spray cabinets.

3.2.1 Materials, specimen configurations and protection systems

The material for the unnotched coupon specimens was 3 mm thick clad sheet of aluminium alloy 7075-T6

from two batches (I and II). The specimen configuration is shown in figure 3.1. This has a parallel sided

gauge section 25 mm x 20 mm at the centre of the specimen. Half the specimens were left as machined. The

remainder were chromic acid anodised, without hot water sealing, according to SAAB-SCANIA specifications.

The 7075-T76 aluminium alloy 1f dogbones were from the same batch as the CFCTP core programme

specimens and with the same fastener hole size (press fit) and protection system, as discussed in detail

in reference (3) and Part II of this report.

3.2.2 Mechanical testing conditions (static prestressing and fatigue)

All stresses were defined in terms of loads on the total cross-sections of the specimens in the gauge

sections, i.e. including cladding layers (unnotched coupons) and fastener holes (li dogbones).

Before environmental exposure and fatigue testing the lI dogbone specimens were prestressed at209 ± I0 K by apply-. u io- I -les up to 215 MPa. The procedare for this is discussed in reference

(3). The purpose of this low temperature prestressing was to ensure that the paint and primer layers were

brittle and would crack around the Hi-Lok fastener holes, thereby simulating service damage that enables

corrosion and corrosion fatigue to occur.

The fatigue testing of the li dogbone specimens was done using an FFA-designed 50 kN load frame with

MTS electrohydraulic equipment and load cell. Static calibration showed the load cell error to be within

± 1% and ± 50 N. A strain gauged dummy coupon was used to check alignment. Bcnding and axial strains were

determined at a tensile load of 5 kN. The in-plane bending strain was 2.3 % of the axial strain and

therefore well within the 3 % limit specified in reference (3).

The fatigue load history was constant amplItude sinusoidal loading with a stress ratio R S. /Sma.

of 0.1. The characteristic stress levels for the test programme have been indicated already in table 3.1.The stress level for the [U doghone specimens was chosen to he the same as the lower stress level for theCFCTP core programme, i.e. S.. - 144 MPa. The tests were carried out at cycle frequencies of 1.4 Hz for

the unnotched coupons and 0.5 Hz for the lU dogbone specimens.

3.2.3 Environmental conditions (pre-exposure, fatigue and corrosion fatigue)

Unnotched coupons scheduled for static exposure before fatigue testing (batch 1) were placed on aroof in a light industry area 5 km from the centre of Stockholm for 8 months (June 1977 to January 1978).Thereafter they were wrapped and stored in a freezer until required for fatigue testing.

Half of the ij doghone specimenF were pre-exposed by the U.S. Naval Air Development Centre NADC

before shipment to SAAB-SCANIA. The pre-exposure conditions were the same as in the CFCTP core programme,i.e. sealing of faying surface side edges and Hi-Lok collars to prevent corrosion except in the fastenerhead areas, followed by immersion for 72 hours in 5 % aqueous NaCl acidified by a predetermined amount ofso2 gas and maintained at 315 ± 2 K.

Before fatigue testing all |i doghone specimens were sealed at the faying surface side edges andHi-Lok collars. The fatigue tests on unnotched coupons and li aogbone specimens were done in speciallyconstructed environmental chambers capable of being stacked to enable tests in series in the load frame.

Drawings of the environmental chambers are show in figure 3.2 and their parameters during testing are

given in table 3.2.

Environmental influences on fatigue were studied by instituting alternating "wet" and "dry" phases.

The wet phases started every 12 minutes and consisted of fatigue in humid air with condensation, andfatigue during immersion in distilled water. These phases were terminated when a dew point hygrometersensed condensation on the surfaci of the cupu. s or pecimens exposed to numid air. Thus in one case the

wet phase corresponded to a continuous increase in humidity until condensation occurred, while in the

other there was immediate and continuous wetting.

The dry phase was fatigue in low humidity air. In fact this was a drying phase, whereby it is

unlikely that a relatively complicated specimen like the lj dogbone would dry out completely after

immersion in water.

S4

The conditions for fatigue in wet and dry air were established in the following way. Both chamberswere open ended and filtered laboratory air was pumped thjrough. This air was heated in the environmentalchambers with or without water injection:

P water injection resulted in wet phase testing with humid air that caused condensation on couponsand specimens and maintained their temperatures

* straightforward heating resulted in low humidity air that provided a reference environment forunnotched coupons, see table 3.2, and also dried and maintained the temperatures of coupons andspecimens during dry phase testing.

3.3 Results

The complete set of fatigue life and primary fatigue origin data for the SAAB contribution to FACTis given in table 3.3. The way in which the test programme was set up and the results had consequences forthe statistical methods used to analyse the data. his will be disuussed in section 3.3.1.

The fatigue life results are presented and statistically analysed in section 3.3.2. This is followedby statistical analysis of the primary fatigue origin data in section 3.3.3.

3.3.1 Statistical methods for analysing the data

A survey of the statistical methods for analysing the SAAB data is given in figure 3.3. Owing to thelimited number and unequal sample sizes of the data it had to be assumed that they at least approximatedto random samples from log-normally distributed populations with equal variance. Unequal sample sizes alsomeant that modified versions of the least significant difference rest and Duncan's new multiple ran&e testhad to be used for "fine tuning" the analysis of variance results. More details of the statisticalmethods are given in Appendix II.

3.3.2 Fatigue life data

The SAAB fatigue life data are shown in figure 3.4. The If dogbone specimen data are compared withCFCTP core nrogramme data in figure 3.5. Fro t!' se flgu- the following trends are observed:

if) Unnotched coupons:

- wet' ng by re--te ' ''t5ion or alternate immersion in distilled water reduced the fatiguelives

- the fatigue lives of as machined and chromic acid anodised specimens were similar.

(2) If dogbones:

- the SAAB fatigue testing in air with repeated condensation or alternating immersion indistilled water was - -vere as the CFCTP fatigue testing in salt spray.

The two trends for unnotched coupons were confirmed by two-way analysis of variance (table 3.4) and"fine tuning" using the least significant difference test (table 3.5) and Duncan's new multiple range test(table 3.6).

The SAAB If dogbone data were analysed using one-way analysis of variance (table 3.4) and Duncan'snew multiple range test (table 3.7). The analysis showed that there were io significant differences infatigue lives, i.e. the four fatigue testing schedules were equivalent in severity. These data were alsocompared with CFCTP data using one-way analysis of variance (table 3.4) and the least significantdifference test (table 3.8). This statistical comparison confirmed the forementioned trend, namely thesurprising result that fatigue testing in air with repeated condensation or alternate immersion Indistilled water was as severe as fatigue testing continuously in salt spray.

3.3.3 Primary fatigue origin data

The primary fatigue origin data for the SAAB If dogbone tests were analysed using Fisher's exacttest, table 3.9. Changing the environment (fatigue testing schedule) had no significant effects o- thelocations of primary fatigue origins.

3.4 Discussion

The results for both the unnotched coupons and If dogbone specimens showed that repeated condensationor alternate immersion in distilled water reduced the fatigue lives. On the other hand, pre-exposureeither outdoors for 8 months (unnotched coupons) or for 72 hours in acidified aqueous NaCi (if dogbones)had no significant effect.

As mentioned in section 3.3.2, comparison of SAAB and CFCTP If dogbone fatigue life data gave thesurprising result that fatigue testing in air with repeated condensation or alternate immersion indistilled water was as severe as fatigue testing continuously in salt spray. A contributing factor is thelikelihood that the "dry" phases during the SAAB tests may not have been sufficient to dry out thespecimens, especially for fatigue in air with alternating immersion.

55

Albeit, this result is still remarkable: in fact It is positive. The SAAB fatigue testing schedulesare relevant to the flight-by-flight transpiration of aircraft structures, whereby alternate wetting bycondensation and drying take place (reference 4). The humid air with repeated condensation is difficult tocontrol in a laboratory test. Salt spray testing is also complicated and rather unpleasant to use in theproximity of expensive laboratory equipment. However, the repeated immersion test is easy to control. Thesimilar effect on fatigue lives of all three environments means that the repeated immersion test is anattractive and convenient alternative for the more complicated testing procedures.

3.5 Conclusions

(I) Repeated condensation or alternate immersion in distilled water reduced the fatigue lives ofunnotched coupons and li dogbone specimens.

(2) Pre-exposure outdoors (unnotched coupons) or in acidified salt spray (Ij dogbones) had nosignificant effect on fatigue lives.

(3) For each fatigue testing schedule (environment) the lives of unnotched coupons in the as machinedor chromic acid anodised conditions were similar.

(4) Comparison of the SAAB and CFCTP II dogbone fatigue life data showed that fatigue testing in airwith repeated condensation or alternate immersion in distilled water was as severe as fatiguetesting continuously in salt spray.

(5) Changing the environment (fatigue testing schedule) had no significant effects on the locationsof primary fatigue origins in the SAAB 1j dogbone specimens.

3.6 References

1. L.E. Jarfall, "Comparison of corrosion fatigue in gaseous and liquid environments", SAAB-SCANIAProgress Report FKHU-80.21 (April 1980) plus Enclosures A-D (March 1981). The final report(in Swedish) was Aeronautical Research Institute of Sweden Technical Note FFA TN 1982-10, 1989, and

- .. . . by u s.

2. L.E. Jarfall and A. Magnusson, "Fatigue testing of bolted joints in humid air and alternatingimmersion", Aeronautical Reseach Institute of Sweden Technical Note FFA TN 1982-34, 1982.

3. R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordlnated corrosion fatigue cooperative testingprogramme", AGARD Report No. 695, February i982.

4. W.E. Anderson, "Fatigue of aircraft structures", International Metallurgical Reviews, Vol. 17,pp. 240 - 263 (1972).

56

TABLE 3.1: OVERVIEb OF THE SAAB TEST PROGRMIlIE FOR FACT

3 -m thick 7075-T6 clad

aluminium alloy sheet

MATERIALS

ANDSPECIMENS PRESS FIT His. FASTENERS\

3.2 no thick 7075-T76 aluminium

alloy sheet

(CFCTP cote programome specimens) m.

3.2 m305

7075-T6 (batches I and II): none and chromic acid anodisingPROTECTION SYSTEMS *

7075-T76: chromate conversion + inhibited epoxy polyamide primer(except fastener holes) + aliphatic polyurethane topcoat

PROTECTION SYSTEM * 7075-T76: two stress cycles at low temperature to crack paint andDAMAGE primer around the fastener heads

FATIGUE LOADING @ Constant amplitude, Smin/Smax - 0.1

FATIGUE ENVIRONMENTS e Low humidity air; air with repeated condensation: alternating imrerio-

in distilled water

7075-T6 clad (batch I): 8 mont , outdoors oar Stockholm

STATIC PRE-T :FOSU"E 0 light industry area)

7075-T76: 72 hours in 5 % aqueous NaCl + SC2 at 315 K

UNNOTCIIED COUPONS lx DOGBONES

S 150 MPa S - 144 MPaSCHEDULES

mac

CYCLE FREQUENCY 1.4 Hz CYCLE FREQUENCY

0.5 H

7075-T6 7075-T6

(BATCH il) (BATCH I)

fatigue in low humidity air

pre-exposure + fatigue in low

humidity air

fatigue in air with repeated •TEST PROGRAMME condensation

pre-exposure + fatigue in air 0 Swith repeated condensation

fatigue with alternating 5 5immersion in distilled water

pre-exposure + fatigue withalternating immersion in

distilled water

STATISTICAL ANALYSIS a Fatigue lives and primary fatigue origins

5.,

U CC CC 4, ''

40 -z 4,N -- 'C HO -C o:~ CE 06044C Z40

44404 -O 0(0 40Cs CC 4 CC 4 4440C H~ . C -

O C CC 0 44 C C

- 44N ON 44

C~ '-46 ON -0440 06 44 0600

44' HZ 4 0. I 0

C .4CC 40C 4 4V1C

CO C ON

CC C 446

4.) C 0 440 ON 4,

-6 -z ON. . . C 44N0 -44 40 H C4400ON 444.4, C

C ~4-4 CE CEO C CC 00 I N 44H 6. 0404 .- CC C

0 C 44 CC C 0(C (N OSNO'C C 4, - HZ 4.4 CC' C ...-.4,'o ~ .. 4C (NON ON CN

H C 60,040O CC C C'- 4.) C C C C C04 H .400C 0 44 -H 0 C.-0' - 44]'.. 4 C 4

H 4,N-..-4 . C ONOO -460(0 '4 CCC 06 CEH 404444 I(Nl0

C C .400

0-C C. C C ~CC C ON (N

04 4-.

0 44

44 44

40 4-4

C 0C 0.H '4C 44

0. 44 C.440 0 44 6

C 40 44..~ 0 44

C C 644 44 0 44

4,44 44 (4, 4444 -.4 (4

444444 40 44 44 44440444.. 4. 0 4444

H 44 44 44 44 44 44C 4444-,.-' .4444-4 44 N44

C 4.4444 04444 44 4444O 44-.-.4N. ... 00. 44 --. 6.C 4466 44446 44 66

04444 444444 0 4444446.44 4. .044

C '-~6 4.C 44 44 444444 0. 44 4444

44 044-44444 6 044 4,6 ".0 6 44 .. O-'0

N 4 044-4444444 044-4(40

ON 44. 444000. 44"446.

004.4, '44044446 0 4404.44

44

4-. 0-4. . 44

4. 44 44 N 0

44 44 0--~

N - - 44 -- 44 04 4.44 44 44 * N44,444 44 44 N.40440,4400.-' 0 4. 4.44

44 4444 '440444.-.'44 .4 4 .04444

CC .4.044 44,-.04 ~4444 44 N0.~

44 .04444 4 '4444.44

H

44. CUC CC

CC

CI-. CC44.04 ~44.-.1-.

0 4) -- )0 -

. 0.4) .. ..0..' 44 00 4

fA I

. -00 000 0 0 00

-- 4 oooooo o040 00o4./l/2• 000oooooo00ooooooF _ _ _

0. Ho oo o4

0 0 2 0 -

0- 0 -0 000 000

0- 0) 4) 00000II 00IIIII

TABLE 3.-: SL'MARY OF ANALYSIS US VARIANCE RESULTS (95 CONFIDEINCE)

rrPr or SPECNF I~ R.5I'~'M r

TABLE 3.5: LEAST SIGNIFiCANT DIFFERENCE TLS RESU.lS 193 CO I')ID%( FOR IL L I C ( I O E 'UN

FATIGUE LIFE OF LNNOICKED COUPONS

T .5

- I h AAA A ro AAOAA,5 I.t .,

t .%.A I' A. ,I 'A U , I -I

Are ADAIAA~r. * . t, . AI ' A h Ul t Slt Aat:li , l T hI' -A- { ':.A ; Ii ..

re e psA i :, ta lK, M in A . h ll i . .,r p [ , . , . .... T..

60

TABLE 3.6: SUMMtARY OF DUNCAN'S NEW M1JLT[PLE RANGE TEST RESULTS (95 Z CONFLUENCE) FOR UNNOTCNZD COUPONS

CORROSION PRTCINSYTM - iC i L clithde cneltind hidcee iind - i icoiie hnetind

LOGt fAN ATIGUE LIFE 5 672 55S78 5 5)8 5 622 5 523 5 424 5 430 547 1%l S1i Sit) 5 u538 t-

SAMPLE SIZEn 4 4 5 5 4

COMPARISONS OF DTA FOR TrST POL5.Ttttt TEST PARAMETERt LO,; W6) S

nneii-nd 1, (1St 02251fatigue in, In- humidity ai/r-nuuettgr in Inn huid toy .it,

41-it,h-dotu' 2 '5) 501

fatigue inl in-tuidituI,)pui.irli, h pctdndo~t

ni 0, ..... l"",

in- ~ lu. tutidit i'niu i5uleoln itun in, disijltidunt _ _____

iti l -rnur tuiroi, iu .. ....i4u'.iiu -l ~ u

pt-epnu ,.-tluI., IS.tuaiu, oo~nout niiiuiwtp.l~ .....l 41-

plirtpinrnnianihr mci rll,nnprui-,,torninn in/t-epnttuig

t~liuni ln t'ni ' Eli Il I l- P~~~~~~~~vzt-tp nu t 1-ngi 'Erii.F 11 A .1 1-,.IA AI!

fi - j7

/ h in. 1i iS tpu ,

it-epttl l nt 2i ~ I

TABLE 3.7: SUMARY OF DUNCAN'S NEW MULTIPLE RANGE TEST RESULTS (95 % CONFIDENCE) I JA SAAB It DUABONES

FATIGGE TENTING S IEDtL at N n TIiOh :Ae-NNoIr :f ti h I it tIno . esr •aN .

A -. . T. .1.. .. A.

P7TpLE SIZE n F A

DIFFEENE

E'E NIhNIIFCK T DIFFT

F-GA -1- A (ITFG DATA ANA l EENT FATIGUE TENTIG AIIEDGL p AND-

FATIGUEIN K

A-,- TINi wth repated ondna n/f.tge -h te i0r disll 2 I I31 0 TON

/diti led ~ % % Iaer o I2

f-&- i./ - sr fI tig e ith/i eso ndistilled -1-e 2 o OI '3 n

ltigue lih aeraL-1g 10rso indild w-t/pl -e-pr I faigue "th a.1nt n 11 - 8

- I,,g -oe~ aapes thes -op---on -a also b, made, uing -h u-.dified eri-n of D.-' t-s n,, -. 1e -.ul 1- baild

TABLE 3.8: LEAST SIGNIFICANT DIFFERENCE TEST RESULTS (95 % CONFIDENCE) FOR THE EFFECT OF ENVIRONMENT ONFATIGUE LIFE OF SAAB AND CFCTP I DOGBONES

FATIGUE TESTINGr sure - INEre-posue fatigue inl ar p-ou- faiguewt atgewt

ETAv. IT TINE T.. . ... ..... IN AT........fafu nar a alTN spayA conIdensNaton Icnesto itle ae r ion

LIP M EA FATIGUE 12 5 072 IQ 81N A A NT A AA A 829 NN

I TE NIlE N AT 28 3N 31 A 3

A0051& -TN IMeIdAN - 0 05

COMPARISONS OF DATA FOI DIFFERENT FATIGUE TENTING SUHEIESr DIFFERENUE

TatiNuNTINTITIANfTTTgAANTnTai IN-hiep 'IAN Ton pTaton dTNiII 2It

fANtigAue IInApe'xT r IN fNatNINAT wiNth aNter n TTting I NANNNniTN itl e ,e!2 'I

TIN-TT --'- -IAN ...... IN IN-ON d

Ite 1.o~l d- 11, tIu narttgei irwt eetdcnesto

pNT-TxpI'NIrINTTatgleT inTair/AatTTigu it ITe rE~ NeLnInTNONdNi NNile FTe 1,

PAT ......I A f- IN NAIT illAI TTIhNrea ted -oTTo n

fa u tn :aPt ipa/r xouo •ftge i i Lh rpae odn a,, Il, ¢

DTP oNTTTTaAt rA/ I I A teTnaI , lA rsiTnN Ii.TNNINd w e d1TT l,{ATTT'NedinTsalt -1esT N A ANigFN.it nat TTT rsIn. N TNi

TTINps. ATIgeiTTsaNtNpay/a T-,o n r t r NpATe G.o Tnde Nsa ti .I NOT TITAeTINAAINAIN ArIIIIIETAn A ltNNN aN/pIO Ax~ e*ftget TI T TiEwithT~ Ie~to tTd' , ,1 ..TIrNe INoue+ aiu AlA nh salNtN sra/f iu NTJTA iN~N.th-N ale tn g iT ersIT NA in disNN TIled at er[O (l 1 N

faItie TI, aiwth.epAN TedII c n TentNNN-NIn T-Iox*,,,e• i ge r a w h ,va d rh. a l ..

AatiAAeAII IAI ithNTNepeItTdTIondIsIIoI /aII u wT alt aIt.in Te.T, ATrU diTil,, TIIIIII * INTN ,N IIi rNTI

-. 4

62

N N

- 2*0

H 'S 0 -~ I H 4o - 0H 4 H 4

N ao 0 00N N N02

S ____________ - - - - - - .2

N a(4 2 0 ONz * 4, 0 NO0 00 20O 0.0 0(~ .' N

220003 0 HO(0 000 CO N N 0 4 3.-' 0O (fl3.'0V*-~00(2 0.0 (SON

0000=

0-a (S..'V aH N H N

H N oJ Z (2N .4 a Na S a 2

N N 04 0 (2N 0 2 2 0 =- N = .0 00 N -O - (2 .100.3 - H

N -- 0 0 CO 0 0 4 0 NN 0 3. ON Z ao w a ~0o a Ha N - 00100 0 (2a a H 0(20 a aa N N .. 0402 N (~4

N NN H

'C > .0--NN - - - - 0 00

a N 0 0(2 HN < +2 0N 0. N

N 0 '- 0000 NN 0 ~000 0 N 0

a a +-0.-' -, Q

a - N 0.000N < ON 0) ON 00C a 0 .O4Na .4 -~ -~- 0.2.30

N N N 0.0. 0. 0

1. N-. 0 a ON000 .4 0.

4-la a N00.- - C.C a a 4 (4 <N

O 0.0 Ha

2.30 NC

C S C.,a '100

H NN 2 NO (2N (2 - Ci C 0a (.a~ N 204. ~-, 0-~ NH ON -4 ON (2 C .2 OH

- 0 a N N 0 4 - 0NN . ~ H ON 2 2 HONO CO 0. (4 (2 0 NZOJ 0 S C000 H COO H

a N0 N.O CC NZ H - ~ 0

N <H = <H a a N4< 0 0< C

N I 00. cc.-.2 0

.20' 40 H

O N N

.4 NO HO

H HN N0>. 00-HO HOH0(2NO NOHNH -HO (20.ZN NON00 000(2(2 54-Jo

* 0 0

-w-\

3O63

300R=80

{ALMENS'ONSIoo

(1) MILL FINISH RETAINED ON SIDE SURFACES

(2) TOLERANCE INDICATIONS NOT INCLUDED.

ig. 3.1 Unnotched specimen configuration for the SAAo programme

h4

GAUI'ES FOR AIR AND

SPCIENTEPEA UR

HUMIDAI AIRUT

FWAERTAKLFERRERW LFE

FMEASURING

CONDENSIONCHER

AIRPO ORIECNE

Fig.3.2TheFFA nvionunisichaber use intheSAABconribtionto he ACT r~gal

65

SAAB FATIGUE LIFE DATA FOR FACT1

CFCTP CORE PROGRAMMESA F F DT FFATIGUE LIFE DATA EXCLUDINGSPECIMENS REDRILLED TOPRESS FIT DIMENSIONS ANDSPECIMENS WITH CORRODED

UNNOTCHES COUPONS 12 DOGBONE SPECIMENS FRACTURE SURFACES AFTERf [PRE-EXPOSURE+ FATIGUE IN AIR

ASMEEQUIVALENCE OF7075-TSl FATIGUELSVES SMEPRQMARI FASFCFCTPBATCH I AND BATCH H

]E LOG -NORMAL DISTRIBUTIONS FOR]|EACH TYP Of COUPON AND SPECIME[ 0 ORIGINSAND ENVIRONMENTS

AN SAA MEHANICAL TESTINGi I~~HOMOGENEITY OF VARIANCES FOR AN0 SAAB MECHNCL ETNAND AT LEAST APPROXIMATEE TM HOMOGENEITY OF VARIANCES

URNOTCHES COUPONS 11 t 55GRON SPECIM EN

S-WAY ANALYSIS Of VARIANCE 1 -WAY ANALYSIS OF VARIANCE

" MAIN EFFECTS •MAIN EFFECT

- environment - environment- anodising

- enironent NO

SIGNIFICANT EFFECTS

YES

LEAST SIGNIFICANT SIFFERENCETE] UNCAN'S NEW MULTIPLE RANGE TES

Fig. 3.3 Survey of statistical methods for ,alysing the SAAB fatigue life data for FACT

66

TEST

SCHEDULES CONSTANT AMPLITUDE, R = 0.1

I'1 DOGBONES UNNOTCHED COUPONSS_= 144 MP. S = 150 MPa

0 7075-TS WITH 0 A 70"75-TS CLADU.S. NAVY 0 A 7075-T6 CLADPROTECTION SYSTEM

00 AS MACHINEDA A CHROMIC ACID ANODISEC

a Iw humidity air

fa, g,. whuy, / -

r ~ p s m+fa t ig u e in a ir wt h - .0 -

repeated condensation--,-in air with

ni~pou + I natg wihatr ,-k

00 100,000 1 000,000CYCLES TO FAILURE

Fig. 3 4 SAAB fatigue life data contribution to the FACT programs

TEST SCHEDULES CONSTANT AMPLITUDE, R =01: S = 144 MPa

fatig ue in

ir (CFCTP

) 7 >

faiiua in ai, Wath CT7>S rep .e c siSAAB) >

SAABe-0th 1 -in dist"ied =r AI)

IXIW4..atgu in air (CFCTP)

fatigue in ft spay (CFC )

dgein air th________________

r=eoe ind en (SAAB)

I in with waternth($A Anana..,n

10,000 100000 1,000,000CYCLES TO FAILURE

Fig. 3.5 Coeparison of SAAB FACT contribution and CFCTP core programme data. The CFCTP core programe dataare for all speci e except those redrilled to press fit dimensions and those with corroded

fracture surfaces after pre-exposure + fatigue in air, see table. 2 and 9 in Part It of this report

07

4. THE NADC CONTRIBUTION TO THE FACT PROGRAMME

J.J. De Luccia, Naval Air Development Centre NADC, Aeronautical Materials Laboratory, Warminster,

Pennsylvania, USA

4.1 Introduction

The NADC contribution to FACT examined the effect of fastener fit (interference versus press fit) and

the use of a flexible, elastomeric primer instead of the standard non-flexible primer selected for the

CFCTP core programme. The advantage of using flexible paint systems to improve the corrosion protection of

aircraft has ben known for many years. However, there appear to be no data comparing flexible and non-

flexible paint systems with respect to fatigue and corrosion fatigue of aircraft structural joints.


An overview of the test programme is given in table 4.1. All specimens were li docbones from the same

material as the CFCTP core programme specimens but drilled to interference fit dimensions, as discussed in

detail in reference (i). Cadmium plated steel Hi-Lok fasteners were used. The diameter of the holes for

the fasteners was 6.248 t 0.0127 mm, see figure 1.1 of the introduction to this part of the report.

4.2.1 Protection systems and specimen assembly

Half the specimens had the same U.S. Navy paint scheme as in the CFCTP core programme, see reference

(1) and Part II of this report. The remaining specimens were painted and assembled in the same way except

that a flexible, elastomeric primer "Koroflex" was used instead of the standard primer. Koroflex is a

strontium chromate inhibited polyurethane primer that retains flexibility even at low temperatures(209 K).


All stresses were defined in terms of loads on the total cross-section of the fatigue specimen

dogbone at the location of the centreline between the fasteners, i.e. the fastener holes were included irthe cross-sectional area. Before environmental exposure and fatigue testing the specimens wore Prestressed

at 209 ± 10 K by applying two load cycles up to a stress of 215 PIPa. Tho .*_-ure for this is discussedin reference (1). The purpose of this low temperature prestresing was to ensure that intact non-flexible

paint and primer layers would crack around the Hi-Lok fasteners holes, thereby simulating service damagethat enables corrosion and corrosion fatigue to occur.

The fatigue load history was constant amplitude sinusoidal loading with a stress ratio R =

Smin/Sa

of 0.1 and a maximum stress of 210 MPa. Detailed procedures for fatigue testing are given in reference(I).


Specimens scheduled for static exposure to an aggressive environment before fatigue testing were

sealed at the faying surface side edges and Hi-Lok collars to prevent corrosion except in the fastenerhead areas. The procedure for static pre-exposure is described in detail in reference (I). The specimenswere immersed for 72 hours in 5 Z aqueous NaCIl acidified by a predetermined amount of SO2 gas andmaintained at 315 t 2 K. The specimen cleaning procedure after pre-exposure followed the amendment in

section 4.4 of Part 2 of reference (1).

For fatigue testing all specimens were elecrrically insulated from the loading grips and bolts bypolymeric liners and bushings. Specimens to be fatigued in salt spray were also sealed at the faying

surface side edges and Hi-Lok collars. The fatigue environments were laboratory air and 5 T aqueous NaCl

salt spray acidified with H2so4 to pH 4, both at a nominal temperature of 295 K. The salt spray tests were

done in a specially constructed cabinet, fully described in reference (i). The cyclic loading frequencieswere as follows:

a fatigue in air, 2 Hz

* fatigue in salt spray, 0.5 Hz.

4.3 Results

The complete set of fatigue life and primary fatigue origin data for the NADC contribution to FACT isgiven in table 4.2. The way in which the test programme was set up had consequences for the statisticalmethods used to analyse the data. This will be discussed in section 4.3.1.

The fatigue life results are presented and statistically analysed in section 4.3.2. This is followed

by presentation and statistical analysis of the primary fatigue origin data in section 4.3.3.


A survey of the statistical methods for analysing the NADC data is given in figure 4.1. Owing to the

limited number of data it had to be assumed that they at least approximated to random samples from log-normally distributed populations. Also, comparison of the data with CFCTP core programme data meant that

equal variances had to be assumed and that for some "fine tuning" of analysis of variance results modifiedversions of the least significant difference test and Duncan's new multiple range test had to be used.More details of the statistical methods are given in Appendix 11.

68


The fatigue life data are shown in figure 4.2. The data indicate the following trends:

(i) For specimens with interference lit Hi-Loks the use of Koraflex instead of a standard U.S. Navyprimer appears to have been beneficial in all three environments.

(2) The use of interference fit Hi-Loks instead of press fit Hi-Loks did not improve fatigue life.(Interference fit fasteners are usually considered to have a beneficial effect on fatigue life.)

(3) The salt spray environment was particularly detrimental to fatigue life.

As will be discussed, statistical analysis confirmed trend (2) and showed trends (I) and (3) to bepartly true.

The Box test was used to check for homogeneity of variances of the NADC data, see table 4.3. Thevariances were not all equal. However, analysis of variance is a very robust statistical technique, suchthat approximate compliance with the criterion of homogeneity of variances is sufficient for continuingthe statistical treatment of the fatigue life data.

Analysis of variance was carried out separately for the complete set of NADC data and a combinationat NADC and CFCTF core programme data for specimens using the standard U.S. Navy primer. The results aresummarised in table 4.4. The main effects of environment, primer and fastener fit were found to besignificant. Since thur. were cnly two types of primer and fastener fit, it is obvious that thesignificant differences were between the standard and Koroflex primers and the press and interferencefit Hi-Loks. Thus it was not necessary to "fine tune" these results using the least significant differencetest. However, this test was used to investigate the effect of environment (fatigue testing schedule).The results are given in table 4.5. Significant differences in fatigue lives were found mainly as aconsequence of fatigue in salt spray.

According to the analysis of variance the other potential sources of variation (environment : primerand environment : fastener fit interactions) were not significant. These were further investigited usingDuncan's new multiple range test. The results are listed in tables 4.6 and 4.7, and show the following:

* for specimens with interference fit Hi-Loks the use of Koroflex instead of the standard primer wassignificantly beneficial only for pre-exposure + fatigue in salt spray

* use of interference fit Hi-Loks instead of press fit Hi-Loks was either detrimental or had nosignificant effect on fatigue life

* the salt spray environment was particularly detrimental to fatigue life for specimens using thestandard primer but not for specimens using Koroflex.


The X2

test of independence, Yates' corrected X2

test and Fisher's exact test were used to analysethe primary fatigue origin data for the NADC contribution to FACT and the relevant CFCTP core progran.mespecimens. The results are given in table 4.8. Neither environment (fatigue testing schedule), type ofprimer nor fastener fit bad significant effects on the locations of fatigue origins for the restconditions selected.

4.4 Discussion

The present test results show that use of the flexible, elastomeric primer "Koroflex" was signifi-cantly beneficial to the fatigue life of lj dogbone specimens assembled with interference fit Hi-Loks andfatigued in salt spray. Overall the use of Koroflex appears to have been beneficial as coopared to the useof a standard, non-flexible U.S. Navy primer.

Use of interference fit Hi-Loks instead of press fit Hi-Loks did not improve fatigue life. The reasonis that under load the II dogbone specimen exhibits secondary bending that increases when the clearancebetween fasteners and holes is reduced, see references (2, 3) and Appendix 1. This characteristicbehaviour tends to nullify the usually beneficial effect on fatigue life of using interference fitfasteners.

Changing from fatigue in air, with or without pre-exposure, to pre-exposure + fatigue in salt sprayresulted in significantly shorter fatigue lives for specimens using the standard primer. irrespective offastener fit. The use of Koroflex resulted in the detrimental effect of salt spro, becoming statisticallyinsignificant.

4.5 Conclusions

(1) Use of the flexible, elastomeric primer "loroflex" instead of a standard, non-flexible U.S. Navyprimer was beneficial to fatigue life, notably in a salt spray environment.

(2) Use of interference fit Hi-Loks instead of press fit Hi-Loks did not improve the fatigue life ofli dogbone specimens.

(3) Changing from fatigue in air, with or without pre-exposure, to pre-exposure + fatigue in saltspray resulted in significantly shorter fatigue lives for specimens using the standard U.S. Navyprimer. However, the use of Koroflex resulted in statistically equivalent fatigue lives.

(4) Neither environment (fatigue testing schedule), type of primer nor fastener fit had significanteffects on the locations of primary origins of fatigue.

69

4.6 References

1. R..H. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testingprogramme", AGARD Report No. 695, February 1982.

2. HRH. van der Linden, "Fatigue rated fastener systems", AGARD Report No. 721, November 1985.

3. H.H. van der Linden, L. Lazzeri and A. Lanciotti, "Fatigue rated fastener systems in Ij dogbonespecimens". NLR Technical Report TR 86082 U. August 1986.

70

TABLE 4.1: OVERVIEW OF THE NADC TEST PROGRA OIE FOR FACT

MATERIAL * 3.2 mm thick 7075-T76 alsuiniun alloy sheet (CFCTP core programmematerial)

INTERFERENCE FIT H~itok FASTENERS

SPECIMEN

3.2. 3005I m

PROTECTION SYSTEMS . Chromate conversion + inhibited epoxy polyamide primer (except fastenerholes) + aliphatic polyurethane topcoat:

Chromate conversion + Koroflex elastomeric inhibited polyurethaneprimer + aliphatic polyurethane topcoat

PROTECTION SYSTEM * Two stress cycles at low temperature to crack non-flexible paintDAMAGE and primer around the fastener heads

FATIGUE LOADING * Constant amplitude, Smin/Sma x - 0.1. 5max - 210 MPa

FATIGUE ENVIRONMENTS * Laboratory air; 5 % aqueous NaCI salt spray with pH 4

STATIC PRE-EXPOSURE * 72 hours in 5 % aqueous NaCl + SO2 at 315 K

STANDARD KOROFLEXPRIMER PRIMER

fatigue in airTEST PROGRAMME

pre-exposure + fatigue in air •

pre-exposure + fatigue in salt spray

STATISTICAL ANALYSIS F iatigue lives and primary fatigue origins

- - -'4

71

2040040 0 -1,202

+ 0

4,4 04*0<40 40404040

20- C*--*--.'4' 4*->-'4

4 20-~<'4 4.2040 20 0

H 240 40 40 - - -

0- 0-400*0- ~0..~" -''

2K ::4.3 40 40

4,-COO 0 0\ ,,

'4

2 40020-0* 02 tWO*

0<t~0'C'. 40444'0-

t2 40 400<..".0'400 4040

40 *-400*0'P* 40404040

02 0-

-C _______ __________0- -

'40 4 -40 40 0240- -40 0202 ~0- H 0< 440

40 4002 0-'-'

4002

*0

40 4

2 0

4 4

S 22

'40 -404040

4040'440 40

>40

00~

72

TABLE 4.3: BOX TEST FOR HOMOGENEITY OF VARIANCES OF NADC CONTRIBUTION TO FACT (95 Z CINFII)!NCF)

FAT IGUE 1A1% .EI DECREES OF SAMPLE VARiANcFKTEST ING SAPE SML IE sum OF SQUARES RUEDU ,s o.' .'

SEDULE NUMBEER 5 I 6 ~ -, T

ISTA D U 0084 1 o DOOR - b2 - 40 55

2KOkOFLEK 4 0 002 3 o 0001 -(2z. -S'S

k - 2 0 E6suM 0 6 22

POOLED 0 6 6 0 161 001.. - 0... -1

Eatiguei, ar DIFFEAUNCE 0 -U - 440 - - ( 204

K 2 E3026 D - 7 506 L------- 1- 6 I[ -k-I-I ,-

D- - K(21 -K- 120 F1 - I ;I' I

FOR -. 5 N &ND 1 SOD 109 DEGREES OF FREEDOM F - S 0 1, 14( ' 3 4. THE PP-TIN 'ARIAN ; AI-- N T A:

STANDARO U 0 05 3 ! ) .OROFLEX I 0 1R I I

6-0 I ____I _______

kC -2

FOOLED -. (

DIFFERENCE - ,1

1a.01u i~ (4Air

K - 2 3026 0 1421 L - D - 0 1- - 'U -k I

02

'2D - - 12'41 3F - wITI , A.DI' -, DE , (,1ES "I

I1 121.2)D,.

FOR a - 5t AND I AND 109 DECREES OF FREEOM F - 4 04 SINCE 0 I1,l < rHF Y FI

I STOSUR 4, 1I1 1 3U2 . .ROF. E. 4 007. 1 ,

k-2

50- --," D1SEEI

KOlE - (2 21 15 D0 IkSo I, !1, T.

-26D0 -21(0 I. - -4I'' -6-2-I " "

2 1,-)' i

D - (221 - K - 125 721. F - 1- 8- 1 ' iTH I A- I III , ,F LFI.

FOR _=5 tAND I AND 109 DECREES OF FREEDOM F - 1 4 SINCE l 1 3 94 THE POPT N VARIANCES F U 'AL

.I ......

-~ -. ~-- -a

S

0

___ I _____

_________ I _________________ j

PARIS .4-~ SOLYRY OF ILINCAN'S NEW mULMILF KVNGE TESI RESELLFS (95 2 ONVILNCE) IOE 1111 NADC FACL DAIS

(5iAXXDSDi R 2.2. LAD rLTR PSCiL' '0 Si)

1.

Cs

NAUC FACT DATA

CFCTP CORE PROGRAMME PRIMARY FATIGUE ORIGINS FATIGUE LIVES I CFCTP CORE PROGRAMMEPRIMARY FATIGUE ORIGIN P FATIGUE LIFE DATA EXCLUDATA EXCLUDING SPECIMENS DING SPECIMENS REDRILLEOREORILLED TO PRESS FIT 1 TO PRESS FIT DIMENSIONSDIMENSIONS AND SPECIMENS AND SPECIMENS WI (HWITH CORRODED FRACTURE CORRODED rIACTURESURFACES AFTER PRE- ASSUME AT LEAST APPRO MATE SURFACES AFTER PREEXPOSURE + FATIGUE IN AIR LOG-NORMAL DISTRIUINS EXPOSURE - FATIGUE IN AIR

TS TEST OF INDEPENDENCE, TEST FOR HOMOGENEITY ASSUME EDDIVALENCELOF CFCTFYATES' CORRECTED N TEST YATES' CORRECTED \2 TEST R OF VARIANCES AND ADC MECHANIC TE G

FISHER'S EXACT TEST AND AT LEAST APPROXIMATEHOMOGENEITY OF VARIANCES

* ORIGINS AND FASTENER FIT 6 ORIGINS AND ENVIRONMENTS S BOX TEST TO CHECK FORDIFFERENCES BETWEEN PRIMER

ORIGINS AND PRIMERS TYPESPER FATIGUE TESTCONDITIONI

2-WAY ANALYSIS OF VARIANCE

MADE FACT DATA NADC FACT DATA AND CFCTP DATA

• MAIN EFFECTS 6 2 WAY INTERiACTIONS • MAIN EFFECTS 0 2 WAY INTE RACT IONS

enionmmet environment primer - envitronment arnneat se ni tr - I

- primer fastener fit

< SIGNIFICANT EFFECTS- -

YES

ILEAST SINIFICAT DIFFERENCE TESTI NA NEWLTIPLNGEnRA

Fig. 4.1 Survey of statistical method. toe anal)sing the NADC data for FACT

KEY TO SYMBI1S

TEST SCHEDULES 0 STANDARD U.S. NAVY PRIMER 0 DKROFLEX PRIMER A CFCTP CORE PROGRAMME(INTERFERENCE FIT) (INTERFERENCE FITI (SLIGHT PRESS FIT'

CONSTANT AMPLITUDER = 0 1, S... = 210 MPa I

fpreapas . atigue In lt

pre-expore fatigue in Salt aptap

10(0S I000 100 000

CYCLES TO FAILURE

Fig. 4.2 NADC fatigue life data on triiution to the FACT programme ant 'FCTP core programme fatligue

life data. The CFCTP core programme data exlude specimens redrilled to press fit dltmnrtr nsand specimens with corroded fracture surfaces after pre-exposur. - fatigue in air

74

5. THE AFWAL CONTRIBUTION TO THE FACT PROGRAMME

N.R. Onko, Air Force Wright Aeronautical Laboratories AFWAL, Materials integrity branch, vystvis

Support Division, Dayton, Ohio, USA

5.i introduction

The AFWAL contribution to FACT concentrated on the effects of using an-" fasteaer sYstem

(SLEEVbolt) to replace the Hi-Lok system chosen for the CFCTP core programme. The SLEEVbolt fastener

system is of particular interest because it can be used to repair a structure. The system Incorporotet a

tapered pin in an internally tapered/externally straight shanked sleeve.

In addition the effects of installing press fit hi-iks with or without polysulphide sealant in the

fastener holes were investigated.


An overview of the test programme is given in table 5.1. All specimens were li doi,,tzes from the sine

batch as the CFCTP core programme specimens and with the same U.S. Navy paint scheme, as discussed indetail in reference (1) and Part II of this report. However, some of these specimens were altered byremoving the press fit Hi-Loks and either reinstalling them with sealant r replacing them b SLEEVbolts.

5.2.1 Fastener systems

The specimens were supplied to the AFWAL containing press-fit Hi-loks dry installed in chromateconversion coated fastener holes. For some of these specimens the Hi-loks were removed and either re-

installed "wet", i.e. coated with sealant, or replaced by SLEEVbolts. Figure 5.1 lllustrite. the

installation of both types of fastener system: installation details are given In refernces (1 - 4).

the sealant used in reassembling specimens with Hi-Loks was a pclysulphide with idded -hr, mates f,,rcorrsion inhibition and conforming to MIL-S-81733 B. Most of the sealant was squeezed out diriog Hi-lok

installation but some remained around the fastener head to seal off the countersink arra.

To reassemble specimens with SLEEVbolts the fastener holes were redrilled from s.0is t '.7r, mrnominal diameter. The holes were left in the as-machined condition. The SI.EEVhI't combination selected tir

installation was an aluminium coated steel bolt with an aluminium sleeve. The tasteners were pressed into

place before installation of the ili-Lok collars. Installatin resulted in a typical intenerence of0.064 mm.


All stresses were defined in terms -f loads on the total cross-sectioll of th ftigun peCt

dogbone at the location of the centreline between the fasteners, i.e. the fastevier hales were included inthe cross-sectional area. This meant that the net section stresses !r the speciens with SiEEVbolts wre

approximately 8 Z higher than those far the specimens containing Hi-loks.

Before environmental exposire and fatigue testing the specimns with HI-,,ks were prestressed ,t209 t 10 K by applying two load cycles up to either the macimum stres crurrinc In the subsequent fati-ee

test or 215 MPa, whichever was the greater. The procedure for this is discussed in reference Ii. Tiepurpose of this low temperature prestressing was to ensure that ans intact point, primer and sc-l-intlayers were brittle and would crack around the Hi-lok fastener holes, thereby s-ilating service danagethat enables corrosion and corrosion fatigue to ccur.

The specimens containing SILEEVbolts were not prestressed at lw !eperitire. This ws- ,nsideredunnecessary because the specimens had not been repainted after reissemble.

The characteristic fatigue stress levels for the test programme hive beer, indicated already it, tal-le

5.1. These stress levels were obtained from the pilot tests descriled li section I.-. ,f this part "t thereport. The fatigue load histories were constant amplitude sinisiiLal h-adine with a stress riti, RStain/S. of 0.1 and the manoeuvre spectrum FALSTAFF (references S, it. A slant des-ripti-in - thi

spectrum is given in section 1.3 ol this part of tie report.

5.2.3 Environmental conditions (pre-euposure. fatigue and corrosion latiue)

Specimens soheduled for static exposure to an aggressive envirnment iefore fatigue teting weresealed at the faing surface side edges and hi-Lok collars ti prevesit corrsii, except Ii tie fastenerhead areas. The procedure for static pre-exposure is described in detaii in reference tl). Tie specimenuwere immersed for 72 hours in 5 Z aqueous NaCl acidified by a predetermined i-rnt ot S(;, vas a-dmaintained at 315 0 2 K. The cleaning procedire after pre-exposure followed the unmended prlcedure insection 7.4 of Part I of reference (1).

Fur fatigue testing all specimens were electrically Insulated fron the loadItni grips and hilts bpolymeric liners and bushings. Specimens to be fatigued Ite salt spray were also seaied ,it tie ti,surface side edges and Hi-Lok collars. The fatigue environments were laboratory air and 5 7 aqueous (a(lsalt spray acidified with H SO

4 to pH 4, both at a nominal temperature of 195 F. lhe salt nprav tests were

done in a specially constructed cabinet, fiilly descrihed in reference (I).

lnie .oin .al Ile irequeriole, t r- -h -itibleit ion ,-f fatigue lxid histri and enri -tten t

'-;M NAI Yi bil F INK 1 MCF-A!:, h GOAPI, i,- U :I- KY -

In i, ~ .,~ in sIt up ri.I

amplItulde. . - lit Ii>He

S j Results

The complete set I in igie ife, iiid primary l-itigue crigini data te tlin-AFWhl Ceiitillitiii, L.) FAlzgiven in table 51. 1ii wa in wiih thle test ir ranewsnet up, -. i the eut a asqe cSth tatistical methods used to analyst the data. ris wil heI discus-ed I in se-Ao N. I.

The fatgu l;'I fe results are preseoted aiid stat ist ical ly analysed In sect ion 5.3.. T,is is o Ii-edby presentation and s tat ist ical onalyasi A tin primary fat igue orig in data in scnljin NJ. J.

5.J.1 I tatiutical methods t aaisieg fte data

A sinev tif thle stitiutical nethids for aitalynitig thie 5F;Al daais lvein iii t gumo N... tcgI theI iiitd number and unequal sample sizes -if thle lot igue li te datai it had to he assumd tbhat ties at lev-t

appramin.ated to random samples rarm log-nermallIv distribu ted -pplatiss wi th equalI var iance., UnelualIsmle s ies a1lue meant that modified versions of the least significant difference test ind j ,it new

n altip le rang 6e t est woul Id have to btused iir "fi nt tunling" the jaalysis oIf sarince rilsults . t e detajiisalte satistia Imetho.ds a gn gienu tc A ilinedis. I I

5.3- Fatigue lilt data

he a t igiie I Ife da ta are, shown -in ti c u re 3 . Thte se do toa inrdIca teI t ha t ureI c-: re fit i :5,salt spra resulted in srter linve, thinftiua n air. Ibis was centinmed is tw--a nalsis ,<iso.ria nce, thle results of which tee stnmarised in table 1.3. Sl~ice there were only tw. testsc dierepresen "t inIg thle effect o f one t ronmeint lit iue In air, pre-posur u at i nn I it I l a. it isobsicas that the significant difference is between then.. Thus it was no t Tac sars t, .ntecl- tint lsresult using the least signif icaiit dii terte, test.

Aecurding to fte fasaissie the ,thee py--te::! --- ,i~.' f ciriitl- ('icm-n r rs-nn-e-cnirnmnt ;lauteiier ssieia interac tisi were iot stinificant. These were fuirther nesti. iten in.r

Duitan' s mew mul1 t lyl11 rouge test . The res- -s gre iven in table N. In. l L n. ainr i .i de .dFALSAAFF load Ing and fur a given test cohidule ltneiroumeitai cord it!n i ci Ne were is sat liii"ilifferengcem in fatigue lines awing to diliitnt fastener Instillastinns.

5-.3.13 Prinars fIitigiue origin data

Yates' corrected X2 test and Fisher's enact test wsrn msed t,' analys ftirir fin ~i.,u -Iii dat..isted is t able & .2 - Owing no tile limited number of~ data it was it possitble t,- uealsstsoy eprares fr

gosh se.mbi nat ion of fat igue liad history. endriinnlent and fastener system 'intead carien -' l uunedhcombinations wer e,, aeined includiiig combiing- data for constant amplitnude and F-ALSTArF laiding. Ibis inIel to h , e justi led snce it. enuls son tIhe TLR and I.TlI cantesh~niitse to FAT)U show tin thledependence or primarN fatigue origin location em a stress level is similar nor on tantmm iid-aPATAiAFF loadieg, such that the sane trends are obtained foe a constaint anplietude i f I-.. MPi

nALSTAFF b, of 23m MPa. Tho results of the tests ore, sumlmarised in table -5. Bothuensir-ei-tmos

nest ing schedule) and fastener system bad signi ficoen eni fects oun tie I -ot inus t~ eir i on a gu~e ,ri nas fellows:

(1) Changing Itam fat igue i n aoir to pee-ysuee + fat i gue in sat spriy prom-ned f alIluec -i 1t atniiii the bores (F/0l) and ceunnrsini areas li/N, K. ) t- the ou hii oles, e-e ri . irspecimens with slight press fir ill-lbsk iiistalled dry is per Kl oyig~ne

At fa SIi NlhKbhet, prunatnd foi lure ieniation se thn n-...nn terninkiare's 10. 0i the tseeboles. T his otff et I-sospeciol no uticeable for uenesfutigued in .iii t bl

s..l"iousiun

Use of the ij dogS-ne specimen for this test pregramme is a Pains- steers ic-iL htnen slI.Ehhul t fastener- sutee. Under load ithe I deghmme specieoe esh ibits ,m-mi ,den hutn eict,! thatser eases when the c Ileoraiice be tweeni fast eners -d 1-1 es i s red- med , se , r. f c renccs , 's :JA - -:

This, nhiiracnterlisn ic hehanlIijir tends tii -nulItyI - t he sins b enet i iu Itf ect is l~t igin lie at I uing,iiterite musce fit fasteners.

Un th ohrbn, the eq11iialent fatigue l ines if peeimei- ntoin, FIl-lobs iid S; ~b -itdnas rates the iisefu Iess nf SI FF%*huLts fee repa i rfig a1 structuore .

(I Iangig fIr-i fa t igue iii air to preepoSmue + iatigie it, soL syris roInled in ii,,ntscantlys0 ,rter fatLiguIe ines, bet intalIantion (If pre- fit HIl-lot suleg inhilbitmed hislenuu!Ihimlt --ulain ml ' nd if e retice.

74

5.5 Conclusions

(1) The usefulness of SIEEVbolts for repairing aircraft structures has been demonstrated.

(2) Wet installation of press fit Hi-Loks using inhibited polysulphide sealant was not beneficial tocorrosion fatigue resistance.

(3) Changing from fatigue in air to pre-exposure + fatigue in salt spray promoted failure initiationin thn bores and countersink areas of fastener holes and reduced the number of failurescommencing at faying surfaces.

(4) Use of SLEEVbclts instead of press fit Hi-Loks promoted failure initiation In the barecountersink areas of fastener holes.

5.6 References

1. R.J.H. Wanhill and 1.1. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testingprogramme". AGARD Report No. 695, February 1982.

-. .R. Ontko, "Evaluation report; supplemental data in support of the NAT0-AGARD coordinated corrosionfatigun cooperative testing programme (CFCTP)". AFWAL Report AFWAL/MLS 82 - 48, June 1982.

3. "Hi-Lok and Hi-Lok/Hi-Tigue fasteners installation instructions", Hi-Shear Corporation, 2600 SkyparkDrive, Torrance, California, USA.

4. "SLEEVbolt interference fastening system", SLFEVbolt Systems Committee, 1700 West 132nd Street,Gardena, California, USA.

5. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation", Combined Report cf theF + W, LP, NER and lAB , March 1976,

6. 1.B. de Jonge, "Additional information about FALSTAFF". NLR Technical Report 18 79056 U, lne 1979.

7. H.H. van der Linden, "Fatigue tated fastener systems", AGARD Report No. 721, November 1985.

8. H.H. van der Linden, L. Lazoeri and A. Lanciotti, "Fatigue rated fastener systems in lj d'gbnespecimens", NLR Technical Report TR 86082 C, August 1986.

Xi

TABLE 5.1; OVFRVIE', OF TILE AFWIAL TEST PROGRA tE FOR 'ACT

MATERIAL . 3.2 mm thick 7075-T76 alsminium alloy sheet (CFCTP core programmeCa' rial)

PRESS FIT HiLkFASTENERS OR SLEEVbols

SPECIMEN*

SLEEVbolt specimens: chromate conversion + inhibited epoxy polyamideprimet (except fastener holes) + aliphatic polyurerhane tpcoet

PROTECTION SYSTEMS *Hi-Lok specimens: :hromatelconversion + inhibited epoxy polyanide

primet (except fastener holes) + aliphatic polyrethane topcoat and

with or without inhibited psiysulphide sealant in the fastener holes

, POTECTION SYSTEM . Hi-Lok specimens: two stress cycles at low temperature Zn crack

DAMAGE paint and pril-er around the fastener heads

FATIGUE LOADING * constant amplitude. Smn/Sa. - 0.1. Swax - 144 MPa; FALSTAFF,

S ma - 238 MPa

lATIGUE ENVIRONMENTS . laboratory air: 5 % aqueous NaCl salt spray with pH 4

STATIC PRS-EXPOSURE * 72 hours in 5 % aqucous NCei SO2 at 315 K

CTTTP CORE PROGRAM.MF SPTIM'NS

SCHEDULES FATIGUE LOAD HISTORY A i-Lo i' Hi- cAS REINSTALLED RPLACED B

RECEIVED WITH SEALANT SLEEkVnbIs

constant amplitude,

fatigue cycle frequency 2 Ho * STEST PROGRAMME

io atc FALSTAFF,

cycle frequency 7 Hz 0

cunstant amplitude,

pre-expusure yc I- Frequency 0.F H. * • 0+ fatigue in-sait uprav FALSTAFF,

cycle frequency 2 Hz

Previously tested in he uFCTP coare prograe

STATIST'ICAL ANALYSIS * fatiguc lives and primary fatigue origirs

-'4

141

- - r - ______ -

-2a! 0

4 0 0<2.0 ~0 000 00 0000. 0 .0

0 00

0* 00001.4 0.~0 00.4.4<04..00 0000

0 0 4 0.0,0.410 0000

o 0 0 .4000~I.4 004'. 0< 00.4 0002.,.4~ 4 00C..~&.2 0 '-'I

- + 4.4<.<fl0'..4 o..0.-. 0 0~.4.04- .4<0.4' 4.4-00 4.4~.40.-~00 H <4'0.0~O~0. .'. -, <..0.< .- ~..."O00'4< -

0 0 0!.H 0.0

.2 0 G!..~

0! 0.4000

4' 0 0'. 44

0 0 /4

4< 0

.2 0 .' 0000 0000 0000 -00- 0 0 0a! -0 H 2 0000 00C'!00J. 00000 ~44.

a! -- 0000000.04-00 0 0.4000 4'000~0<H 0 4 ~

.2 4 0.4.404.00-0<00 0 0000~ OC<.,0<N .?.40".2

4004 4 2.2 N 4/

44 <2 0

0 .4.4

0 0 0 0 1 -.- 0 0 0 .2 0 0H2 = 0 0 0 0

.2 0 .4 0 4 0 000 -4 -4 -.

0 02 - 0 - '. -

00 I I I I I

S 0 4

0 00 0 0 0 0

44 -~ .4

0 0 0 .4* 0 o /4

20 0 0 4002 4

0 a!0 H 4

H .2 4

13.4 4

4 4

42 a

H

- 4=

0 .. 0044 - 0~40. 00 4. - 00H - -0 4-4a!0 4

TABLE 5.3: SUYMARN OF ANALYSIS OF VARIANCE RE-SULTS (95 CUNFISENCE)

1 81

-NT - F IA I- T A! -'

FI F

--' I : '~ 'F'

11 q .1 -.VlAFA h

r-~

.. C o

'4 0

0, C -

F--A

'4

< C.

H C-0 0

0

/ /,

N4

INTLAION

DRILL STRAIGHT INSERT SLIGHT PRESS INSTALL COLLAR WITH COLLAR END SHEARSDIAMETER HOLE FIT Hi-Lok PIN WITH SPECIAL HATCHET WRENCH OFF AT CORRECTWITH COUNTERSINK OR WITHOUT TORQUE LEVEL

SEALANT ON PIN

INSTALLATION WITHHi-Lok COLLAR

REDHILL STRAIGHT INSERT CRITICAL PULL BOLT INTO COLLAR END SHEARS

DIAMETER HOLE CLEARANCE FIT INTERFERENCE FIT BY OFF AT CORRECTWITH COUNTERSINK SLEEVbat ASSEMBLY INSTALLING Hi.Lk COLLAR TORQUE LEVEL

(TAPERED BOLT IN WITH SPECIAL RATCHETTAPERED INSIDE WRENCH (ALTERNATIVELY

DIAMETER SLEEVE) PRESS BOLT IN AND INSTALLHi.Lok CULLAR)

Fig. 5.1 Fastener systems used -o the AFWAL contribution to FACT

F m FWAL FACT DATAO F

FA TIGUE LIVES PRIMARFATIG UE OIGNA SSUME AT LEAST APPROXIMATE Yl ATES' CORRECTED, 2TESTLOG -NORMAL DISTRIBUTIONS OR

i FISHER'S EXACT TEST

SASSUME AT LEAST APPRO0XIMATE * ORIGINS AND ENVIRONMENTS

HOMOGENEITY OF VARIANCES 0ORIGINS AND FASTENER SYSTEMSf2-WAY ANALYSIS OF VARIANCE

" MAIN EFFECTS- environment- fastener system

" 2-WAY INTERACTIONS-environment: fastener system

LEASTS.FCAN, DIFEENCE TEST [U NW LTLE OTSTIF RIEQIRED' (SEE TEXT) DUCNSNWMLTPEJN EJ

Fig. 5.2 Survey f" statisttia1 methods for analysing the AFWAL data for FACT

TEST SCHEDULES 0 SLIGHT PRESS FIT Hi-1.0 INSTALLES LIH P!ESS FIT Hi-L. k, A SLED VboktsDRY AS PER CFCTP CORE PROGRAMME RISALLED W ITH SEALANT (IN7ERFERENCE FIT)

CONSTNT APLITDE FFALSTAFF

R = .1,S.. = 14 Ms Sx =238P

___________e I

10.000 100000 1,000,000 10(00 10,000 1055 30CYCLES TO FAILURE SIMULATED FLIGHTS TO FAILURE

Fig. 5.3 AFWAL fatigue life data contribution to the FACT progranan

N6

6. THE NDRE CONfRIBUTION TO THE FACT PROGRAMIME

L. Stivold, Norwegian Defence Research Establishment NDRE, Division for Weapon and Equipment, Kjellpr,

Norway

K. Asbtll. A/S Raufoss Ammunisjonsfabrikker, Metallurgy Division, Raufoss, Norway

6.l Introduction

The NORE contribution to FACT compared the fatigue and corrosion fatigue poete f77 lmnu

Tlhee DREcotrn t oT7ad FRAC retroparedsio tbe fage)a corin. fAtigs properties of 7075 aluminiunalloy sheet in the T76 and eRA (retrogression and reage) conditions. As is well hown, the T76 conditionpronides improved resistance to stress corrosion compared to the T6 temper hut is accompanied by astrength reduction of 5 - 1D %. The RRA treatment was developed to avoid this strength loss (reference 1).

In the first instance 7075-T6 sheet was supplied to the NDRE from a common batch purchased by the NLRand also supplied to the IABG and RAE, see table 1.1 of the introduction to this part of the report. Thissheet material was subsequently converted to the T76 and RRA conditions by the A/S Raufoss Ammunisjons-

fabrikker.

Conversion to the T76 condition is achieved simply by overageing 7075-T6 for several hours (typicallyabout 10 hours) at 463 ± 3 K. The RRA treatment is a more complex two-step process. 7075-T6 is first"retrogressed" at a temperature between 473 - 533 K for a short time (I - 2 minutes at most). This Isusually accomplished in a silicone oil bath in view of the short times involved. The material is thenwater quenched and reaged at 393 K for between 16 - 48 hours.

In the present work the BRA treatment consisted of retrogression ia a salt bath at 51 K in 35 s.followed by reageing at 393 K for 24 hours.


An overview of the test programme is given in table 6.1. All specimens were ot the Ij dottaneconfiguration discussed in detail in reference (2) and recommended for the F CT programme. Cadmian toesteel Hi-Lok fasteners were used. The diameter of the holes for the fasteners was b.306 ! 0.044 mm, whioc)corresponds to a slight press fit, see figure 1.1 of the introduction to this part of the report.

After conversion of the sheet material to the T75 and SRA conditions the specimens were mamafac,ored,painced and assembled by the U.S. Naval Air Development Centre NADC. The specimens had the same U.S. Naypaint scheme as in the CFCTP core programme, see reference (2) and Part 11 of this report.

6.2.1 Material properties

Engineering property data of the 7075 sheet as supplied in the 36 temper and after conversion t, the176 and ENS conditions -o compared with data for the 7075-76 sheet used in the CFCTP core progranme as

1 folows:

MATERIALS 0.2 Z YIELD STRESS (MPa) iCTS (MPa) ELONGATION7075-T6 547 5822

7!)75-T76 (conversion) 485 582 11

7075-T6RRA (conversion) 5h2 582 16

479 (max) 550

7075-T76 (CFCTP coe ]I.G

prohrcnmF 455 (sin) 5,1 .m)


All stresses were defined in terms of loads on the total cross-section of the fatigue specimendogbone at the location of the centreline between the fasteners, i.e. the fastenc holes were included in

the cross-sectional area.

Before environmental exposure and fatigue testing all specimens were prestressed at 209 - 10 F byapplying two load cycles up to either the maximum stress occurring in the subsequent fatigue test or215 MPa, whichever was the greater. The procedure for this is discussed in reference (2). The putpose othis low temperature prestressing was to ensure that the paint and primer layers were brittle and wo.ldcrack around the Hi-Lok fastener holes, tfereby simulating service damage that enables corrosion andcorrosion fatigue to occur.

The characteristic fatigue stress levels for the test programme have been indicated already in table6.1. These stress levels were obtained from the pilot tests described in section I., o this part of thereport. Detailed procedures for fatigue testing are given in reference (2). The tatigue load historieswere constant amplitude sinusoidal loading with a stress ratio R = Sm i/Sma

x of ,1 and the manoeuvre

spectrum FALSTAFF (references 3, 4). A short description of FALSTAFF is iven in section 1.3 of this partof the report.


Specimens scheduled for static exposure to an aggressive ervirenent before fatigue testing wereseaied at the faying surface side edges and Hi-Lok collars to prevent corrosion excapt in the fastenerhead areas. The procedure for static pre-expostnre is described in deLail in reference (2). The specimenswere immersed for 72 hours in 5 5 aqueous NaCi acidified by a predetermineh amount of SO, gas and

N,

maintained at 315 ± 2 K. The specimen cleaning procedure after pre-exposure followed the amendment insection 4.4 of Part 2 of reference (2).

For fatigue testing all specimens were electrically insulated from the loading grips and bolts bypolymeric liners and bushings. Specimens to be fatigued in salt spray were also sealed at the fayingsurface side edges and Hi-Lok collars. The fatigue environments were laboratory air and 5 2 aqueous NaCIsalt spray acidified with H2So4 to pH 4, both at a nominal temperature of 295 K. The salt spray tests were

done in a specially constructed cabinet, fully described in reference (2).

The nominal cycle frequencies for each combination of fatig. load history and environment were asfollows:

NOMINAL CYCLE FREQUENCYFATIGUE LOAD HISTORY

fatigue in air fatigue in salt spray

constant amplitude, R - 0.1 2 Hz 0.5 HzFALSTAFF 15 Hz 2 Hz

6.3 Results

The complete set of fatigue life and primary fatigue origin data for the NDRE contribution to FACT isgiven in table 6.2. The way in which the test programme was set up and the results had consequences forthe statistical methods used to analyse the data. This will be discussed in section 6.3.1.

The fatigue life results are presented and statistically analysed in section 6.3.2. This is followedby presentation and statistical analysis of the primary fatigue origin data in section 6.3.3.


A survey of the statistical methods for analysing the NDRE data is given in figure 6.1. Owing to thelimited number of data it had to be assumed that they at least approximated to random samples from log-normally distributed populations. Also, unequal sample sizes for the FALSTAFF data and comparison of theconstant amplitude data with CFCTP core programme data meant that equal variances had to be assumed foranalysis of variance, and that for some "fine tuning" of analysis of variance results modified versions ofthe least significant difference test and Duncan's new multiple range test had to be used. More details ofthe statistical methods are given in Appendix II.


The fatigue life data are shown in figure 6.2. In a general way these data indicate that szress leve'(FALSTAFF), environment and material had significant effects on fatigue lives. As will be shown, this waconfirmed by sta-istical analysis.

The Box test was used to check homogeneity of variances of the NDRE constant amplitude data. Thevariances were found to be equal, see table 6.3. Analysis of variance was carried out separately for theconstant amplitude and FALSTAFF data. The results are summarised in table 6.4. The main effects of stresslevel, environment and material were found to be significant. Because there were only two stress levelsfor the FALSTAFF tests and only two test schedules representing the effect of environment (fatigue in air,pre-exposure + fatigue in salt spray) it is obvious that the significant differences were between eachstress level and each environment. For the FALSTAFF tests it is also evident that the significant effectof material is due to 7075-T6RRA specimens having longer average fatigue lives than 7075-T76 (conversion)specimens.

For the constant amplitude tests there were three material conditions. The least significantdifference test was therefore used to "fine tune" the significant material effect indicated by analysis ofvariance. The results are given in table 6.5 and show that also for constant amplitude loading the7075-T6RRA specimens had significantly longer average fatigue lives than 7075-T76 specimens.

The other potential sourc¢s of variation (2-way and

3-way interactions) were not found to be

significant by analysis of variance. These were further investigated using Duncan's new multiple rangetest. Table 6.6 lists the results, which may be described as follows:

(1) The effect of stress level (FALSTAFF) was significant for each environment and matecial.

(2) The effect of environment depended on load history, stress level and material. Changing fromfatigue In air to pre-exposure + fatigue in salt spray was especially significant in reducing thefatigue lives of 7075-T6RRA specimens tested under constant amplitude loading and FALSTAFF withS = 289 MPa.

(3) Although 7075-T6RRA specimens generally had significantly longer average fatigue lives than7075-T76 specimens, this was not true for all combinations of load history, stress level andenvironment. Changing from fatigue in air to pre-exposure + fatigue in salt spray tended toreduce the differences between materials.


Yates' corrected X2

test and Fisher's exact test were used to analyse the primary fatigue origin datalisted in table 6.2. The results are summarised in table 6.7. Only one significant effect was found,

| S

88

namely the influence of environment (fatigue testing schedule) on the locations of primary fatigue originsin specimens tested with FALSTAFF. Specifically, changing from fatigue in air to pre-exposure + fatigue insalt spray promoted failure initiation in the bores (E/Q) and countersink areas (A/M, B/N) of the fastenerholes, especially for SMax - 238 MPa.

6.4 Discussion

This test programme has shown that the retrogression and reageing (RRA) treatment for 7075 aluminiumalloy sheet has two important advantages compared to the conventional T76 overageing treatment. The staticyield and ultimate strengths of 7075-T6RRA are significantly higher, by about 15 % and 8 % respectively,and are equivalent to 7075-T6 values. This confirms the work of Cina (reference 1). Secondly, whenassembled into specimens representing realistic structural joints the fatigue and corrosion fatigueresistances of 7075-T6RRA are generally better than those of 7075-176.

6.5 Conclusions

(1) Retrogression and reageing (RRA) enabled 7075 aluminium alloy sheet to retain T6 strength levelscombined with generally better fatigue and corrosion fatigue properties than 7075-T76.

(2) The effects of stress level and environment on fatigue lives were significant. Changing fromfatigue in air to pre-exposure + fatigue in salt spray tended to reduce the differences betweon7075-T6RRA and 7075-T76.

(3) From the tests with FALSTAFF it was found that chanefng 'r-t fL;oe .. .i. L. I;-frig- in sal spcay promoted failure initiation in the bores and countersink areas of fastenerholes and reduced the number of failures commencing at faying surfaces.

6.6 References

1. B.M. Cina, "Reducing the susceptibility of alloys, particularly aluminium alloys, to stress corrosioncracking", U.S. Patent 3,856,584, December 24 (1974).

2. R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testingprogramme", AGARD Report No. 695, February 1982.

3. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation", Combined Report of the

F + W, LBF, NLR and IABG, March 1976.

4. J.B. de Jonge, "Additional information about FALSTAFF", NLR Technical Report TR 79056 U, June 1979.

89

TABLE 6.1: OVERVIEW OF THE NDRE TEST PROGRAME FOR FACT

MATERIAL . 3.2 mm thick 7075-T76 aluninium alloy sheet converted to the T76 and

RRA conditions

PRESS FIT Hi-bok FASTENERS

SPECIMEN *

3.2mm 30

PROTECTION SYSTEM . chromate conversion + inhibited epoxy polyamide primer(except fastener holes) + aliphatic polyurethane topcoat

PROTECTION SYSTEM a two stress cycles at low temperature to crackDAMAGE paint and primer around the fastener heads

FATIGUE LOADING * constant amplitude, Smin/S.ax

- 0.1; FALSTAFF

FATIGUE ENVIRONMENTS 0 laboratory air; 5 % aqueous NaC salt spray with pH 4

STATIC PRE-EXPOSURE * 72 hours in 5 % aqueous NaGI + SO2 at 315 K

FATIGUE LOAD CHARACTERISTIC MAIESAL .,i-,ONSHISTORY STRESS LEVEL 775-T 6 015 T6R A

constant amplitude S_ - 144 MPa 6 •

fatigue S - 289 MPa 0 0TEST PROGRAMME in air FALSTAFF

S - 238 MPa *constant amplitude Sma

x - 144 MPa 0

p re-exposure

+ fatigue in S - 289 MPa *alL spray FALSTAFF max

S - 238 MPa * 0

STATISTICAL ANALYSIS 0 fatigue lives and primary fatigue origins

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HOMOGENITY VARAIANCES

3-WAY ANALYSIS OF VARIANCE | 2-WAY ANALYSIS OF VARIANCE

|•MAIN EFFECTS 0 2-WAY INTERACTIONS 0 3-WAY INTERACTIONS • MAIN EFFECTS 0 2-WAY INTERACTIONS

- stress - stress: environment -Stress: environment: I- environment -environment: materialenvironment - stress: material rmaterial - materialmaterial - environment: material

• I~LEAST SIGNFICANT OFFE E T FST -71SNEW MUTIPLE RANGE TIES

Fig. 6.1 Survey of statistical methods for analysing the NDRE data for FACT

- --F 70 5-T6(C E -K E Y TO SYMBOLSTEST S.CHEDULES A 7075-TIR FROM THE

0 7075-T76 (CON'ES...0.,I-TRR CFCTP CORE PR" RAMME

COSTNT AMPITU~DE A L

!0

10,000 1l0g0 0 1,00 00 0 1000,0Ic 100CYCLES TO FAILURE SIMULATED FLIGHTS T0 FAILURE

Fig. 6.2 NORE fatigue life data cotribution to the FACT programe arI CFCTP core programme fatigue lifetdata. The CFCTP core programme data exclude specimens redrilled to press fit Iimenflons

7. THE NLR AND LRTH CONTRIBUTION 10 THE FACt PROGRAMME

R.J.H. hanhill, Tational Aerosnace Laborer y 41R, Structures and Materials Division, Emmeloord.The Netherlands

7.1 Introduction

The Structures and Materials Division of the NLR and the Department of Aerospace Engineering LRTH ofDelft University of Technology wsre joint participants in the FACT supplemental programme. Most of thework was carried out with the support of the Scientific Research Division of ihe Directorate uf Materiel,Royal Netherlands Air Force, and the .'etherlaads Agency for Aerospace Programs NIVR.

The primary objective of the NLR -' ORTS contribution to FACT was to compare the r~sistance tocorrosion iacigue of aircraft corrosoon protection systems and aluminium alloy materials in use in theNetherlands. In addition the effectiveness of AMLGUARD, a water displacing corrsan preventive compound(reference i). was examined.

To try and place the results in a broader contesL it we- arranged that two of the aluminium alloys,7075-Tb and 7475-T761 clad, came from the same batches of material tested by "she NDRE, IASC and RAE, s etable 1.1 of the introduction to this part of the report.


An overview of the test pros ammo as it i nally evolv,' is given in table 7.1. The partial filling-in

of the test matrix -os the consequence of limited number of specimens and opdating of 1-iorities withrespect to choice of fatigue testing schedules.

7.0.l Materials and specimen configuration

The alus'oium alloys used for monolithic specimens were 3.2 mm thick sheets of 2024-T3 Alclad,7077-T6 and 7475-T761 clad. The 2024-T3/aramid fibre laminates were 3.2 mm thick and buiit p fram 0.6 mr,thick 2024-T3 sheets interleaved and adhesively bonded with single aramid 143 fabric layers and theoplastically stLained 0. %. Additional details of the fabrication of these laminates, Iereioaft re rerredto as ARALL (Aramid Reinforced ALuminium Laminate) are given in reference (2).

Engi:'eering property data, based on total cros" sectional area, were as follows:

MATERIALS 0.2 % YIELD STRESS (MPa) LTS (MPa) fLCNCATIN <7

2024-73 All, 350 474 16.5

7075-T6 547 58211.2747,-T761 lad ..72 498 12.6ARALL 470 bOO 1.8

Note the low niongatio- to failure for ARALL. In a fatigue-seositive naterial this w,:ld bedisastrous, but ARALL is highly resistant to fatigue (reference 2) as will also become clear in thiscontr buton to the FACT programme.

%lI specimens were of the l dogbone configuration discussed in detail in reference (3) andrecommended for the FACT prog-amme. Cadmium pleted steel Hi-Lok fasteners were used. The diameter - theholes for the fasteners was c. 30b t 0.044 mm, which cot responds to a slight press dit, see figure I.I ofthe introduction to this part c: the report.

7.2.2 Protection systems and specimen asss bl

Corrosion protection systems were applied by Fokkr Aircraft Factories according to stoodard processsp cifications for the F-28, NF-N and F-lb aircraft. Simplified processing schedules are shown in figure7.1, which also includes Hi-Lok installation and assembly of tie specimens. These processing schedulesresulted in the Hi-Lok fastener holes belng devoid of pr rective coatings, i.e. hare alumirium alloydirectly contacted the cadmium plated fastener shanks and heads. This is a "worst ase" situation which,however, is entirely feasible.


All stresses were defined in teras ot loads on rh-s total cross-section (bt exclodiog cladding lasersif present) 0t the fatigue specimen dogbone at the location of the centreline between the fasteners, i.e.the fastener holes -,re included in the cross-sectinal area.

Before environmental exposure and fatt gle testie all specimens were prestressed at ' t 10 K bapplying two load rycles u1 to either th, maximum stress occurring in the sohsequent fatigie tcst or215 Mpa, whichever was the greater. The proce,,re for this is discussed in referen-e (3). The purpse ofthis low temperature prestressing was to ensure that the paint and primer lasers were brittle and wouldcrack around the Hi-lok fastener holes, thereby simulating service damage that enables crrosi-' ac-Icorrosion fatigue to occur.

The characteristic fatigue stress levels bar the test programme have been Indicated already in table7.1. These stress levels were obtained from the pilot tests described in sectiot. 1.4 of this part o nereport. Detailed procedures for fatigue testing ;re given in reference (3). All tests were done uslnz a900 kN load frame fitted to a WOILPERT-AMSI,ER/MT' electrohydraulic machine. The cl, ed loop system wascontrolled by an NbbR-deveoped device, MiloAS I iMagneric tape Input Digital-to-Analogue Signal).

lnformation on flight simulation load sequences was stored on magnetic taes and -ead by a KENyflif 900

re,. rder.

The fatigue load histories were constant anmclit ode sinus idol loading wit, a stress ratio R =

Smin/Smax

of 0.1, the manoeuvre spectrum FALSTAFF (references 4, 5) and the gL..t spectrua MINIIlISr

(references 6, 7). The peak loads of both spectra were untruncated. Short descriptions of these spectra

are given in section 1.3 of this part of the report.

7.2.4 Environmental condirl as (pre-exposure, fatigue and corrosion fatigue)

Specimens scheduled for static exposure to a aggressie envirotimen, before fatipt, troting - r,sealed ;t the faying surface -ide edges and tHi-Lok collars to prevent -rrosion exept posibly in thefastener head areas. Some of he specimens were then completely spray coated with A.14GJUARD 24 !,urs beforenxposure, see the test matrix in table 7.1. The procedure for static pre-exposure is described in detailin reference (3). The specimens were immersed for 72 hours in 5 7 aqueous NaCl .cidifed by a predeter-mined amount of , gas and maintained at 315 ± 2 K. The specimen cleaning procedure after prr-exposure

followed the amendment in section 4.4 of Part 2 of referenc (3).

For fatigue testing all specimens were electrically insulated from the loading grips and bolts bypoiyme:'ic iins-s and bushings. Specimens o be fatigued in salt sprar were also sealed at the fayingsurface side edges and Hi-Lok collars. The atigue environments were lruoratory air and 5 . ,.ueuus NaCisalt spray acidified with H2S04 to pH 4, hot' oL a nominal temperature o- 295 K. The salt spray tests were

done in a specially constructed cabinet, full, de-cribed in referesce (3).

The nominal cycle frequencies for each combination of fatig , load history and environment were asfollows:

FATIGUE LOAD HISTORYfatigue in air fatigue in salt spray

cstant amplitude, R =

0.[ 2 Hz 0.5 Hz

MINITWIST 15 Hz 5 HzFAL STAFF 15 Hz 2 Hz

7.3 Results

The complete set of fatigue life and primary fatigue origin data for the NiR and LRTH contributionto FACT is given in table 7.2. The way in which the test programme was set up and the results hjdconsequen.es for the statistical methods used to analyse the data. This will be di! ussed in section

7.3,1.

The fatigue life reolts are presented and statistically analysed in scotims 7.3.2 - 2.3.4. This isfollowed by presentation and statisticol analysii of the primary fatigue origin data in section 7.3.5.Correlations between fatigue lives and primary fatigue origins are discussed in section 7.3.h.


A survey of the statistical methods for analysing the XLR and RNTH data is glvei is figure 7.2. Owingto the limited number of data it had tal : assumed that they at least approximated to random u.mpl's fromlog-normally , .tributed populations. Aizo, unequal sample sizes for the MINITWIST data and comparison ofthe constant amplitude data with CFCTP core programme data meant that equal variances had to be assumedtar analysis of varlance, and that for some "fine tuning" of analysis of variance results modifiedversions of the least significant difference test and Duncan's new multiple range test had to be usedMore details of the statistical methods are given in Appendix II.

7.3.2 Constant amplitude fatigue life data

The constant amplitude fatigue life data are shown in figure 7.3. The data indicate the followin

tre-ds:

(1) 7075-T6 specimens had significantly shorter fatigue lives than other specimens.

(2) The fatigue lives of 2024-T3 specimens and 7475-T761 specimens without interfap sealant wereequivalent to those of CFCTP specimens, as indicated by the shaded bar in figure ;.3.

(3) An interfay sealant was beneficial to the atigue lives of 7475-T761 specimens.

(4) Coatilg with MLGtUARD to prevent corrosion during pre-exposure or fatigue in salt spray had

little or no beneficial effect on the fatigie lives of 2024-T3 and '075-T6 specimens.

(5) Changing the fatigue environment from air to salt spray was more detrimental to fatigue life

than pre-exposure.

As will be di cussed, statistical analysis confirmed trends (1), (3), (4) and (5) and sh-wed trend (2) to

be partly true.

Analysis of variance was carried out separately for the NLR constant amplitude data and a combinationof NLR and CFCTP core programe data. The -esults are sumarised in table 7.3. The main effects ofenvironment and material were found o be significant in both cases. The environment : material

interaction were found to be significant only when the NLR data were analysed separately.

The analysis of variance results were "fine tuned" using the least significant difference test or

Duncan's new Multiple range test, as appropriate. The results of these tests are listed in tables

7.4- 7.7, and show the following:

" 7U;5-T6 specimens had significantly shorter fatigue lives than other specimens, in agreement with

(I) above

" for fatigue in air, with or without -ze-exposure, the fatigue liven of 2024-T3 specimens and

7475-T761 specimens without interfuy sealant were equivalent to those of CFCTP specimens

* for pre-expcsure + fatigue in salt spray the fatigue lives of 2024-T3 and CFCTP specimens were

equivalent and significantly shatter than those of 7475-T7hl specimens without interfay sealant

* 7475-T7bi specimens with interfay sealant had significantly longer fatigue lives than other

specimens: this confirms the beneficial effect of sealant

" coating with AMLGLARD before pre-expusure had no significant effect on the lives of :024-T3 and

?075-Tb specimens fatigued in air and salt spray

* an indication that pre-exposure significantly affected fatigue life was found only for 7075-Tbspecimens ktable 7.5)

* changing the tatigue environment from air to salt spray significantly shortened the lives of2s24-T3, 7075-T6 and CFCTP specimens

* 7475-T76 specimens with and without interfay sealant were insensitive to pre-exposure and changing

the iatigue environment from air to salt spray.

7.3.3 Gustn spectrum (MINITWIST) fatigue life data

Tno MINIIIST fatigue life data are shown In figure 7.4. The data indicate the following:

(1) ARALL specimens were greatly superior to monolithic 2024-T3 specimens (fatigue lives more than

IOX longer at the same stress level).

(2) Stress level had a significant effect for 2024-T specimens.

i3; Coating with AMLGUAR) to prevent corrosion during pre-exposure or fatigue in salt spray had

little or no beneficial effect on the fatigue lives of 2024-T3 specimens.

F-. Fatigue in salt spray was more detrimental to fatigue life than pre-exposure.

Owing t,, the evident superiority of ARALL it was considered unnecessary to check (I) statistically.

However, ;tisticail analysis was used to check and confirm (2) - (4).

The results of two-way analysis of variance are summarised in table 7.3. The effects of stress and

environment and their interactions were found to be significant. Because there were only two stress levels

it is obvi.us that the significant difference is between them. Thus it was not necessary to "fine tune"

this result using the least significant difference test.

The stress : environment interactions were further investigated using the least significantdifference test. The results are given in table 7.8, and show that

* the effect of stress level was significant for all environments (fatigue testing schedules)

* coating with AMLGUARD before pre-exposure had no significant beneficial effeot on the lives of

2024-T3 specimens fatigued in air and salt spray

* environmental effects were more significant at the lower stress level (Smf = 89 MPa) and weremainly due to changing the fatigue environment from air to salt spray.

7.3.4 Manoeuvre spectrum (FALSTAFF) fatigue life data

The FALSTAFF fatigue life data are shown in figure 7.5. The following trends can be observed:

(1) Stress level had a significant effect.

(2) At the higher stress level (S... = 289 MPa) the 7575-T6 specimens had significantly shorter

fatigue lives than 7475-T761 specimens with interfay sealant. However, the ranges in fatiguelives of 7075-T6 specimens and 7475-T761 specimens without interfay sealant tended to overlap.

(3) At the lower stress level (Smax

- 238 MPa) the 7075-T6 specimens had significantly shorter

fatigue lives than both types of 7475-T761 specimens.

(4) An Intertay sealant -s beneficial to the fatigue lives of 7475-T7hl specimens only at the higher

stress level,

(5) Coating with AMLGUAKD to prevent corrosion during pre-exposure or fatigue in salt spray hadlittle or no beneficial effect on the fatigue lives of 7075-T6 specimens.

(6) 70 75-T6 specimens were more sensitive to envirouiental effects than 7475-T7ki specimens. At thelower strcs level (S... - 238 MPa) 7475-T761 specimens were completely insensitive to

environmental effects, as shown by the shaded bar in figure 7.5.

(7) When environmental effects were present, notably for 7075-T6 specimens, the change from fatiguein air to fatigue in salt spray was more detrimental to fatigue life than pre-exposure.

As will be discussed, statistical analysis confirmed all these trends, with minor refinements.

The Box test was used to check for homogeneity of variances of the FALSTAFF fatigue life data. Thevariances were found to be equal, see tables 7.9 and 7.10. The results of thret-way analysis of varianceare summarised in table 7.3. The effects of stress, environment .,d material and most of their inter-actions were found to be significant. Because there were only two stress levels it is obvious that thesignificant difference is between them. Thus it was not necessary to "fine tune" this result using theleast significant difference test.

The remaining analysis of variance results were "fine tuned" using the least significant differencetest or isnoan's new multiple range test, as appropriate. The results of these tests are given in tables7.1! - 7.13, and show the following:

* the effect of stress level was significant for all environments (fatigue testing s-hedules

* at the higher stress level (S... = 289 MPa) the 7075-T6 specimens had significantly shorter fatigue

lives than 7475-T7ie specimens with interfay sealant; but the fatigue lives of 7075-T specimens

and 7475-T761 specimens without interfav sealant were equivalent for two of the three fatiguetesting schedules (fatigue in air, pre-exposure + fatigue in salt spray)

* at tie lower stress level (5 23 MPia) the 7075-T6 specimens had significantly siort,r f t

lives than both types of 7475-T76l specimens, whose livs were equivalent

o an interfay sealant was significantly beneficial to the fatigue lives of 7475-T7l specimenu ol

at the higher str,63 1001

* coationg with ill.GUARD before pre-exposure had no significant effect on the lives oi 707 -Tispecimens latigued in air and salt spray

* at the higher stress level 7075-T6 specimens and 7475-Tbl specimens without interf. sealantshowed equivalent sensitivity to environmental eifects; 7475-T7i specimens with interfay sealant

were i'isensitive to chinging the fatigue testing sohedul

* at the lower stress level 7075-Ti specimeus were signilicantly sensitive to enviroomental lfiectsbut 7475-T76l specimens were completely insensitive

* significant environmental effects were due to changing the fatigue environment from air to siltspray: pre-exposure had no significant effect by itself.


The Xa test of independence and Yates' corrected n2 test were sed to analyse the primary fatigueorigin data listed in table 7.2. owing to the limited number of data it was not possible to analyseseparately for eacoi combination of types of primary fatigue origin, stress level, environment andmaterial. Instead various "lumped" combinations were examined, The results of the tests are summarised intable 7.14 and qualitatively compared in figures 7.6 - 7.8. Stress level, environment and material usuallyhad significant effects on the locations of primar> fatigue origins. it nore detoI:

(I) Under constant amplitude fatigue a change from fatigue In air to pre-exposure + fatigue in saltspray promoted failure initiation at the bore/faying surface corners (F/R) of the fastener holesand reduced the number of faying surface (;/S) failures. The effect of changing the material andprotection system was also significant: 7475-T761 specimens with or without interfas sealant hadno failure initiations at bore/faying surface corners (F/R) ad had many more faying curface(G/S) failures as compared to 2024-T3 and 7075-T6 specimens.

(2) For MINITWIST fatigue of monolithic 2024-T3 specimens the effect of a higher stress level was topromote failure initiation in the bores (E/Q) of the fastener holes and reduce the number ofbore/faying surface corner (F/R) and faying surface (G/IS) failures. However, changing the fatigueenvironment from air to salt spray did not have a significant effect on the locations of primaryfatigue origins.

(3) For FALSTAFF fatigue the effect of a higher stress level and changing the fatigue environmentfrom air to salt spray was to promote failure initiation at the bore/laying surface corners (F/Rlof the fastener holes and reduce the number of faying surface (G/S) failures. The effect ofchanging the material and protection system was also significant: 7475-T761 specimens had morelaying surface (G/S) failures than 7075-T6 specimens.

7.3.6 Fatigue lives and primary fatigue origins

Some unusual locations for primary fatigue origins were observed for 7475-T76 specimens withinterfay sealant, see table 7.2. This was especially true for constant amplitude fatigue. The specimenshad very long fatigue lives both in air and salt spray. For fatigue in air all the specimens foiled atlaying surface locations remote from the fastener holes. For fatigue in salt spray three out of fourspecimens failed near the top of the countersink area (B/N) as a consequence of paint cracking andcorrosion attack of the underlying metal during the fatigue tests.

]I0

:L, view of these results it seems reasonable to conclude that for constant amplitude loading the useof interfay sealant prevented fatigue crack initiation at the more usual locations, i.e. bore/fayingsurface corners (FR) and faying surfaces (G/S) close to the fastener holes. This resulted in prolongationof the fatigue lives until failures became possible at the other initiation sites.

It is unfortunate that under FALSTAFF loading, which is more realistic than constant amplitudeloading, similar changes in primary fatigue origin locations (presumably owing to the use of interfaysealant) did not result in significantly longer fatigue lives.

These contrasting results illustrate the complexity of environmental fatigue in aircraft structuraljoints and the necessity for realistic testing.

7.4 Discussion

As mentioned in the introduction (section 7.1) the primary oijective of the NLR and LRTH contributionto FACT was to compare the resistance to corrosion fatigue of aircraft corrosion protection systems andaluminium alloy materials in use in the Netherlands. The results of this test programme have shown thatthere were significant differences in environmental fatigue pertormance of 11 dogbone specimens made fromdifferent terials and with different protection systems. An overview of the results is given in figure7.9. This will be helpful in the following discussion.

7.4.1 Fatigue lives at higher stress levels

For MINITWIST loading the fatigue performance of ARALL (2024-T3/aramid fibre laminates) was muchsuperior to that of monolithic 2024-T3 Alelad. This is most encouraging for the use of ARAIL in advancedaircraft structures. Subsequent testing of a full-scale wing panel has confirmed this (reference 8).

For FALSTAFF loading the fatigue resistance of 7475-T761 clad specimens with interfay was superior tothat of 7075-T6 specimens and 7475-T761 clad specimens without interfay. Thus the Interfay sealant wasbeneficial to fatigue lives.

Use of the water displacing corrosion preventive compound AMLGUARD, which was applied before pre-exposure and fatigue testing, proved to be ineffective in prolonging fatigue life. However, this does notinvalidate the use of AMLGUARD or similar compounds for inhibiting corrosion.

Pre-exposure was not detrimental to fatigue life. Changing the fatigue environment from air to saltspray was detrimental for 2024-T3 Alclad, ARALL, 7075-T6 and 7475-T761 clad specimens without interfay,but not for 7475-T761 clad specimens with interfay. Thus the 7475-T761 clad specimens in combination withthe F-16 paint system and interfay sealant were more resistant to envirormental effects. This is animportant result, since it means that corrosion-related fatigue problems for F-16 aircraft based in theNetherlands should be less severe than those for the previous generation of aircraft.

7.4.2 Fatigue lives at lower stress levels

At lower stress levels the fatigue resistances of 7075-T76, 2024-T3 Alclad and 7475-T761 cladspecimens were equivalent. 7075-T6 was consistently inferior, thus differences in susceptibility tocorrosion (pre-exposure) and corrosion fatigue were not primarily responsible. Possible reasons for theinferiority of 7075-T6 are a lower resistance to fatigue crack initiation, with or without fretting, andgreater susceptibility of the relatively thick sulphuric acid anodisation layer to cracking as compared tothe chromic acid anodisation layers on other specimens (see figure 7.1). However, it has been shown thatanodisation layers are beneficial to the fatigue resistance of aircraft structural joints because theyprovide wear resistant coatings that delay the onset of fretting (reference 9).

With regard to corrosion protection systems, an interfay sealant was beneficial for 7475-T761 clad

specimens tested under constant amplitude loading, but not under FALSTAFF loading. The reason for this isun-l

-. especially because th interfay elant was beneficial at the higher FALSTAFF stress level, see

the previous section and figure 7.9. As at higher stress levels, AMLGUARD was not effective in prolongingfatigue life.

Except for 7075-T6 specimens, pre-exposure was not detrimental to fatigue life. Changing the fatigueenvironment from air to salt spray was detrimental for 7075-T76, 2024-T3 Adclad and 7075-T6 specimens, butnot for 7475-T761 clad specimens with or without interfay. This confirms that 7475-T761 clad specimens incombination with the F-l6 corrosion protection system were more resistant to environmental fatigueeffects.

7.4.3 Primary fatigue origins

An overview of the main influences on locations of primary fatigue origins in the ii dogbonespecimens is given in figure 7.10. These influences may be summarised as follows:

(I) Higher stress levels, pre-exposure and/or changing the fatigue environment from air to salt spraypromoted fatigue crack Initiation in the bores and at bore/faying surface corners of the fastenerholes. This means that the number of failures at the faying surfaces decreased.

(2) Lower stress levels and the absence of corrosion or corrosion fatigue favoured fatigue crackinitiation at the faying surfaces close to the fastener holes. This means that there were fewerfailures in the bores and at bore/fayfng surface corners of the fastener holes.

7.5 Conclusions

(1) Significant differences in environmental fatigue performance were found for Ii dogbone specimensmade from different materials and with different corrosion protection systems.

I 2

(2) Under gust spectrum (MINITWIST) loading the fatigue performance of ARALL (2024-T3/aramid Iibrelaminates) was much superior to that of monolithic 2024-T3 Alclad.

(3) Under constant amplitude and manoeuvre spectrum (FALSTAFF) loading the fatigue performance of7475-T761 clad specimens was equivalent to, or better than that of 707! -76, 2024-T3 Alclad ind7075-T6 specimens.

(4) An Interfay sealant was beneficial to the fatigue lives of 7475-T761 clad specimens tested underconstant amplitude loading at a lower stress jevel and FALSTAFF at a higher stress level, butnot for FALSTAFF at a lower stress level.

(5) The water displacing corrosion preventive compound AMLGUARD was not beneficial to fatigue lives.

(6) Environmental effects were mainly due to changing the fatigue environment from air to saltspray: pre-exposure had no significant effect except for 7075-T6 specimens fatigued In air <iderconstant amplitude loading.

(7) It may be concluded that AMLGUARD's ineffectiveness in prolonging fatigue life can he associatedwith the lack of effect of pre-exposure on fatigue lives, It should be noted that AMIGUARD wasdeveloped specifically for combatting corrosion under static conditions, which it does veryeffectively. The present results therefore show that extension of corrosion protection tofatigue conditions will probably require a dynamic inhibitor system capable of being deliveredto growing cracks.

(8) 7475-T761 clad specimens in combination with the F-l6 corrosion protection system were moreresistant to environmental fatigue effects than other combinations of materials and corrosionprotection systems.

(9) Higher stress levels, pre-exposure and/or changing the fatigue environment from air to saltspray promoted fatigue crack initiation in the bores and at bore/faying surface corners of thefastener holes and reduced the number of failures at the faying surfaces.

(IO) Lower stress levels and the absence of corrosion or corrosion fatigue favoured fatigue crackinitiation at the faying surfaces. Thus there were fewer failures in the bores and at bore/faying surface corners of the fastener holes.

7.6 References

I. G.J. Pills, "The development of AMLGUARD, a clear, water displacing, corrosion preventive compound",Naval Air Development Centre Report NADC-78220-60, May 1979.

2. L.B. Vogelesang and J.W. Gunnink, "ARALL, a material for the next generation of aircraft. A state ofthe art", Department of Aerospace Engineering. Delft University of Technology Report LR-4D, August1983.

3. R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testingprogramme", AGARD Report No. 695, February 1982.

4. "Description on a Fighter Aircraft Loading STAndard For Fatigue evaluation", Combined Report of the


5. 3.B. de Jonge, "Additional information about FALSTAFF", NLR Technical Report TR 79056 U, June 1979.

6. H. Lowak, J.B. de Jonge, J. Franz and D. Schiltz, "MINITWIST. A shortened version of TWIST", CombinedReport of the LBF and NLR, NLR Miscellaneous Publication MP 79018 U, May 1979.

7. J.B. de Jonge, D. Schitz, H. Lowak and J. Schijve, "A standardised load sequence for flightsimulation tests on transport aircraft wing structures", Combined Report of the LBF and NIR, .RTechnical Report TR 73029 U, March 1973.

8. LN. van Veggel, A.A. Jongebreur and J.W. Gunnink, "Damage tolerance aspects of an experimental ARALLF-27 lower wing skin panel", New Materials and Fatigue Resistant Aircraft Design, Proceedings of the14th Symposium of the International Committee on Aeronautical Fatigue, Engineering Materials AdvisoryServices Ltd., pp. 465 - 502 (1987): Warley, U.K.

9. R.J.H. Wanhill, "Effects of cladding and anodising on flight simulation fatigue of 2024-T3 and7475-T761 aluminium alloys", Fatigue Prevention and Design, Engineering Materials Advisory ServicesLtd., pp. 323 - 332 (1986): Warley, U.K.

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TABLE 7.2: FATIGUE LIFE AND PRIMARY FATIGUE ORIGIN DATA FOR THE NLR AND LRTH CONTRIBUTION TO FACT

FATIGUE LIFE TO FAILAR (CYCLES OR FIIGHTS. A2D O VAJ4ALUES)/LOCATTIONS OF FRIWeEy FATIGUE 8IGIN

MTEI.LS 8.0 FATIGUEGONEOSIMo FROTECIOE OAT C8 CTERISTIC A"LAJ on:' EJ- coat *

S T NI H ISTORY STRESS LEOEL Fa t .0, PoO '08p :*A I* F cost * fa tigo e l s1 t Ire x

220.285 5 101.039 N 95.517 0 08648 F 73.48 Rcosat138.733 0 217.112 0 10.000EF 7313 F 118.AA7 S

..lod. 7 -144 HP. a:4A14 8 1200,3 GE 66.795 F SA842 0.1 '' 76.274 F.R 150.745 G 61.61 F 84.745 F

175,214 14.212 107.391 70.777 86.219

0::6 6 NAI 5.1 672 5:,56 E .S 7801.

I FS - 101 OP. 7.533 Q 6.856 EE 5,656 A .66 6 F 56.030#ZhF28poecin Sm O1Ha 7.3 .6$6E 15 5,6,6 E 5 :6 56 Z 5.656 E

..... 11,721 Q 14.856 3,636 F 5.66 R 6.682 E

8.889 3.79 9.036 5.656 3.656 7.072

14,856 c 31.656 F 103,17 F.G 17.656 F 13.008 R 14 95 F

38.856 0 14.856 N 40,420 0 37.636 E 03.378 N 8.100 F.E

- 89 mp. 13,840 4 38.850 R 48.10 G G.013 E.0 11.936 F 73.7015.856 R 27 .320 G 26.956 E . .0.846 F

6,4-8 32.699 31,928 27.146 i0.366 1,971

24>IN -2O 220.000 .250,000 - 134,371 - 97510

010... 150 0 - 101 OF 2 0,000 226.02 - 108.30t1o7o syste8 .o50,000 > 250.000 174.526 128,181

38.101 3 31.142 8 42.045 R 14.008 F 02.707 4

40.10 1, 31.17' I 29.614 0 15,3 29 6I G 40"a..103774: 5 - 144 NP. 30,028 0 28,017 R 31.080 2 7,890 R 15.113 R

R - 0 1 38,165 S 15,459 R 36,724 G 12.11 F41,047 25.117 16.411 16.S31 15,76

4.372 S 3.372 8 6.92 0 2,831 8.0 2.824 F1,972 1 3.372 P 4.031 G 1.080 F 420 P

7075-T6 with NF-5 S - 289 M' 0,211 G 5.512 F 5,211 4 1.172 R 2.3'2 P

pro0e7077n sys.e. 60210 0 1.172 R 5.o1 G 2.831 N 1.1'2 83.096 1.820 5.215 1,330 2,40

F.771 4 0.572 F 8.824 S 0.759 R 0.172 F

10.372 G 10,520 G 0,631 A 4,330 F 1.011 F

IA - 018 . 0,313 S 9.831 N 8,120 I I01 F I.411 F

8,000 5 0.031 10.031 G 3,0 F 4.031 F

9,651 8,792 9.179 5,778 3,002

1q9? oI C 1 5 117 G 77,."41 0141 063 G 108.741 G 14.2 8

-liud. ,s - 144 MPa 147.22 G 021.473 G 188,24o G

P - 0.0 x 152.2 6 G 108.130 G 262.870 8.S1,49 157.780 175.241

wtouto 7.0'2 A 11.500 1 2.824 F4.024 I 10,211 1.511 F

sealant0* 0~ "0810 Pa 8631CEC 4,377 F 1,024FE.F

60,87 F 10.072 0 3.912 0.R

6.028 8,613 2,950FALSTAFF ___________

17.480 0,s 16,372 G 10,701 410,480 1 13,314 0 77,]92 C

S - 038 OF. 1,072 0 74,372 0 20,106 G

7-773, 21.230 S 15 538 S 18,4.0 47112d w7th 18,200 17,554 18.732F-16

rtcin442,110 1 . 210,220 08§

syste-const-nt 377,252 G* 202.301 8apit-., s - 774 8PA 373,014 4 . 401,376 8 §

I - 0.1 411.851 S . 136,846 G44.303 310.174

7,431 3,424 Ft1:yC' 7.372 81 10.996

S - 289 O. 12.700 4.172 R12,72 0S 12.24 G

0.747 7 .37

10.086 4 17.831 0

21,523 4 04,025 4[, - 238 OF. 12,772 G 17.55 s

21,780 G 21.23219.969 .7

FATIGUE A UEClIWENA 8N1/M" M N A2 A,

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Th.. f8l1.e -. 1 .. . N. .nr 1-0h. I,4... h.1- ntd l T b t t. -- ti .:1 't5 h th. h1f plt .77ded

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SULPHURIC ACID ANODISEO CHROMIC ACID ANDDISEO

CHRMI ACDADSE AND HOT WATER SEALED AND HDT WATER SD ALED

APPLICATION OF CHROMATE APPLICATION OF CHROMATECONTAINING CORROSION INHIBITING CONTAINING CORROSION INUHIBITINGEPOXY PHENOL IC ADHESIVE EPOXY POLYAMIDE PAINTBONDING PRIMER BRI27 PRIMER SlfE

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CFCTP CORE PROGRAMME ATGE LIVES PMARY FATIGUE ORIGINS

SPECMEN WIH CORODD xTEST OF INDEPENDENCE ORFRACURE URFAES ATERYATES* CORRECTED k2 TESTPRE XPOURE FATGUE N AR 9ORIGINS AND STRESS LEVELS

ASSUME AT LEAST APPROXIMATE OIINANEV10UET1.0NRA ITRIOIN ORiGIS AUG MATERIALS

LEAST SIOMFICRAT DIFFERENCE S TETBEAd' E OTWEERNGTE

Vvg.7.2 urve of tatitvca aetoda or aalysng Mte TRAdLT ADat forTECTIO

I119

TEST SCHEDULES CONSTANT AMPLITUDE, R 0.1: Sinus 144 MWe

penrexfatigue in air

AMIGUARD Coat KEY TO SYMBOLSpre-expasare + fatigue in air 0 1075-T76 WITH U.S. NAVY

PROTECTION SYSTEM(CFCTP CURE PROGRAMME)

fatgueinsal spay0 2024-T3 ALCLAO, WITH F-2U

A 1 015-TO WITH NF-5PROTECTION SYSTEM

pre-exposare.+ fatigue insalt pray Cl 7475-T761 CLAD. WITH F-16-a------PROTECTION SYSTEMEXCLUDING INTERFAY SEALANT

--------- 7475-T761 CLAD, WITH FPIEAMIGUARD coat P -0- ROTECTION SYSTEMpre-exposure + fatigue in salt spray> INCLUDING INTERPAY SEALANT

100010,000 tOSO t00000 10,000000CYCLES TO FAI LURE

Fig. 7.3 Comparison of NLR FACT contribution and CFCTP core programme (a) constant amplitude fatigue life

data. The CFCTP core programme data exclude specimens redriled to press fit dimensions andspecimens with corrodes fractare surfaces after pee-exposure + fatigue in sir. The shaded barshows that the data fall into three groups with significant differences In fatigue life (see tent)

TEST SCHEDULES MINITWfIST

fatigue in air A

pre-expowure + fatigue in air A-.

fatigue in salt spray

pre-erpsure , faftigue.. in1 sat pra

AMIGUARD Coat+crerparefaigue in salt spray

1000 1t,000 tO000 1,000,000SIMULATED PLIGHTS TO FAILURE

Fi.7. :L nd.LRTH. atgue li fe data for 2024-T3 Aiclad and ARALI. with an F-28 corrosion protectionsyst. ad t ed under guat spectrum loading (MINITWIST)

1201

TEST SCI4EDULE FALSTAFF

fatigue in air

pro-exposure . fatigue In air -a---

AMLGUARD coat. KEY TO SYMBOLSpmn-expsosrn . fatigue in air S axIUpI MATE RIALS AND

CORROSION281 238 PROTECTION SYSTEMS

pre-xpoure faiquA A 7075-T6 WITH NF-5pse-epunur o faigue n sal spra -0- ROTECTIONSTE

7475-T761 CLAO.WITH FA16* ROTECTION SYSTEM8 3EXCLUDING I NTERFAY

AMLGUARD coal. SEALANTpre-exposure + fatigue in salt &pro,> 7475-Ti6l CLAD, WITH F-16t

u IPROTECTION SYSTEMEs~ INCLUDINI NTERFAY

SEALANT

I -1001 10,000 100,000

SIMULATED FLIGHTfS TO FAI LURE

Fig. 7.5 NL~itfatigue l ife data foroteotingounder manoeuvre spectru.Mloading (FALSTAFF). The shaded barIndicates 7475-T76 1 lad opecimeno tes6ted with S 238 SPa

121

[]FATIGUE IN AIR WITH OR WITHOUT AMLGUARD COATING Fl- 2024-T3 ALCLAD -2 POTECTION SYSTEMADPRE-EXPOSURE ri- 7075-TO WITH NF-U PROTECTION SYSTEM

PI0REWHOSUT FAIGU ICATINRA 7475-T761 CLAD, WITH F-IS PROTECTION SYSTEM ANDI IHORWTOT MUAD OTN WI TH OR WITHOUT INTERFAY SEALANT

100-

% PRIMARY 8

FATIGUEORIGINS

60

40

20

Fig. 7.6 Effects of en vironment and material on locations of -mary fatigue origins for the NLR constantamplitud e f atigue (R 0.1, Sma = 144 U[Ps) contribotion to FACT

S., 101 ps H FATIGUE IN AIR WITH OR WITHOUT AMLSUARO COATINGAND PRE-EXPOSURES., 89MP. H FATIGUE INHSALT SPRAY WITH OR WITHOUT AML GUARD[1 S~=RPCOATING AND PE-EXPOSURE

100++

0%PRIMARY

FATIGUEO RIOGINS

40-

20

C -- l - L

Fig. 7.7 Ecfects of stress snd environment on locationa of primary origins for monolithic 2024-T3 Atcisdspecimens teated as part of the ALA and LATH USNITWIST fatigue contribution to FACT

F]FATIGUE IN AIR WITH OR WITHOUT F-j7075-TG WITH NF-5S =29M.AMLGARO COATING AND F1PROTECTION SYSTEM

S.. =238 NP. HPREE XPOSU RE .FATIG UE IN PRTCTO SYSTEM

H] SALT3MP H PA IHO WITHOUT 7475-T761 C.WITHF-16 PROTEC.

100-

%PRIMARYFATI GUEORIGINS

0

40-

20-

CJIF-- 1

Fig. 7.8 Effects of stress, environment and material on locations of primary fatigue origins for the lILAFALSTAFF fatigue contribution to FACT

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8. THE IABG CONTRIBUTION TO THE FACT PROGRAMME

W. SchUtz and W. Oberparlelter, Industrieanlagen - Betriebsgesellschaft IABG, Ottobrunn, Germany

8.1 Introduction

The IABG contribution to FACT compared the fatigue and corrosion fatigue properties of 7475-T761 clad

and 7075-T6 aluminiom alloy sheet under the realistic manoeuvre load history FALSTAFF (references 1, 2).

To try and place the results in a broader content it was arranged that the 7075-T6 material came from a

conmon batch purchased by the NLR and supplied also to the NDRE and RAE. and the 7475-T761 clad matral

was shared with the NLR, see table 1.1 of the introduction to this part o the report.

8.2 The Test Programm

An overview of the test programme is given in table 8.1. All specimens were of the li dogbone

configuration discussed in detail in reference (3) and recommended for the FACT programne. Note that some

tests on 7075-T76 specimens from the same batch as the CFCTP core programme specimens were included. In

all cases cadmium plated steel Hi-Lok fasteners were ued. The diameter of the holes lot the fasteners was

6.306 ± 0.044 mm, which corresponds to a slight press fit, see ligure 1.1 of the introduction to this port

of the report.

8.2.1 Materials and properties

The materials were 3.2 mm thick sheets of aluminium alloys 7075-T76 (CFCTP core pegramme material,,

7475-T76l clad and 7075-T6. Engineering propety data were as follows:

MATERIALS 0.2 7 Y*FEI STRESS (MPa) ITS tMPa) ELflNGATION (7)

479 (nan) 550 (max)

7075-T76 IT.455 (min) 541 min)

7475-T7bl clad 422 498 12.6

7075-Tb 541 582 i1.2

8.2.2 Protection systems and specimen assembly

The 7075-T76 specimens had the same U.S. Navy paint scheme as in the CFtTP core programme, see

reference (3) and Part 11 of this report. The 7475-T7hI and 7075-To specimens were manufactured, paintedand assembled by Messerschmitt-Blhkow-Blohm MBB in Augsburg, according to the following procedure:

" specimen parts machined, drilled and degreased

" chromate conversion coating "Alodine 1200" on all surfaces

* inhibited epoxy polyamide primer on all surfaces except fastener holes

* application of chromate-containing sealant "Celloseal" to facing surfaces and fastener holes

" Hi-lok installation and wet assembly ot fatigue specimen dogbones and half pates

* application of polyurethan topcoat.

The protection system applied by .BB was representative for the European Malti Role Combat Aircraft MR A.During wet assembly the sealant was forced through the lastener holes, but post-test examination showed

that the holes had remained coated with a layer of sealant.


All stresses were defined in terms of the total cross-section (but exclding the cladding laver:.7475-T761) of the fatigue specimen doghone at the location of the centerline between the fasteners, i.

the fastener holes were Included in the cross-sectional area.

Before environmental exposure and fatigue testing all specimens were prestessed at 20q - 10 K byapplying two load cycles up to 238 MPa. The procedure for this is discussed in reference k3). The psrpose

of this low temperature prestresslng was to ensure that the paint and primer layers were brittle and would

crack around the Hi-Lok fastener holes, thereby simulating service damage that enables c-rrosion andcorrosion fatigue to occur.

The characteristic fatigue stress levels for the test programme hav been indicated already in table8.1. These stress levels were obtained from the pilot tests described in section 1.4 of this part o1 the

report. The fatigue load history was the manoceuvre spectrum FALSIAFF (references 1, 2). A shcrt dcscrlpt-

ion of this spectrum is given in section 1.3 of this part of the report.

Detailed procedures for fatigue testing are given in reference (3). All tests were done using a 64 kN

load frame fitted to a SCHENCK electrohydraulic macnine. The closed loop system was controlled by aSCHENCK GA-16/440 digital control computer. The generated load sequence was checked for each specimen type

(7075-T76. 7475-T76t and 7075-T6) by classifying and comparing the actual and specified peak stresses for

one complete block of 200 flights. Agreement between the actual and specified peak stresses was good.

I _ l l I I IS

S.2.4 Environmental conditions (pre-exposure, fatigue and corrosin fatigue)

Specimens scheduled for static exposure to an aggressive envirnment before fatigue testing weresealed it the fayling surface side edges and Hi-Lok collars to prevent corrosion except in the lastencrhead areas. The sealant used was a silicone type and n-. Permagum as recommended in reference (3).The procedure for static pre-exposure is described in detail in reference (3). Most specimens were

immersed fur the recommended time of 72 hours in 5 % aqueous NaCl acidified by a predetermined amountof SO, gas and maintained at 315 ± 2 K. However, for comparison purposes some specimens were pre- exposedtar multiples (3X, 4K and 5X) of 72 hours. The cleaning procedure after pre exposure followed theunamended procedure in section 7.4 of Part I of reference (3).

For fatigue testing all specimens were electrically insulated from the loading grips and bolts bypolymeric liners and bushings. Specimens to be fatigued in salt spray were also sealed at the fayingsurface side .dges and Hi-Lok collars. The fatigue environments were laboratory air and 5 Z aqueous NaClsalt spray acidified with H2so4 to pH 4, both at a nominal temperature of 295 K. The salt spray tests were

done in a specially constructed cabinet, fully described in reference (3). The nominal cycle frequenciesfor fatigue testing with FALSTAFF were 15 Hz in air and 2 Hz Sn salt spray.

8.3 Results

The complete set of fatigue life and primary fatigue origin data for the lABG contribution to FACT isgiven in table 8.2. The way in which the test programme was set up and the results had consequences forthe statistical methods used to analyse the data. This will be discussed in section 8.1.1.

The fatigue life results are presented and statistically analysed in section 8.1.2. This is followedby presentation and statistical analysis of the primary fatigue origin data in section 8.3.3.


A survey of the statistical methods for analysing the IABG data is given in figure b i. Owing to thelimited number and unequal sample sizes of the fatigue life data it bad to be assumed that they at leistapproximated t random samples from log-normally distributed populations with equal variance. Unequalsample sizes a. - meant that modified versions of the least sign.ficot difference test and lioncan's newmultiple range test had to be used for "fine tuning" the analysis of variance results. More details of thestatistical methods are given in Appendix 1I.


The fatigue life data are shown in figure 8.2. In a general w, these data indicate that stress level-d environment had significant effects on fatigue lives (note that extended pre-exposures are consid redrquivalent). With regard to maerlal. the only obvious differences were at Smax -89 MPa, namely the

shorter fatigue lives of 7075-T76 specimens compared to the 7475-T7b and 7075-T6 specimens.

The results of three-way analysis of variance of the dita are summarised in table 8.3. According torte analysis the main variables of stress level, environment and material and their two-way interactionsall had significant effects on the fatigue lives of the specimens. Since there were only two stress levelsit is obeious that the significant difference is between them. Thus it was not necessary to "fine tune"this rcsult using tbo least significant difference test. Also, owing to the dominating effect of stresslevel (compare the P and F distribution values in table 8.3) it was not worthwhile using the leastsigiificant ilfferencu test to compare environments and materials withouc iitroducing stress level:o. Inther words it was better to proceed dires'ly to the twc-aay interactions, which will be discussed in thetllwiog order:

" elect of stress level per enhrnment (fatigue testing schedule) and material

* nlfect of environment at each stress level

* etest of material at each stress level

* etlect ,i environment per material

lett f ..aterial per environment.

* imprtcivce of stress was confirmed in detail by using the least significant difference tost totri. itect f stress lenw -c fatigue life per environment and material. The results are given in

htis I... I: ever. case the effect of stress level was significant, as would be expected.

:east ,:ignilicant diiference test results for the effects of environment and material at each stresslevel -e smma'lsed in tobles 8.5 and 8.:j. Significant environmental effects were fOrd for botn stresss"els, b-v apart from this thei, was no general trend. Significant differences between materials occurred,1v at tile higher stress level and were the result of shorter fatigue lives of 7175-T7h specimens

compared ti: the 7475-T7b and 7075-16 specimens.

For completeness t _. 'east significant difference test results concerning environment : materialinteractions are listed in table 8.7. There is a problem with interpreting these interactions. Nodistinction could be made between stress levels, since three-was interactions were not tound significantby analysis of variance, sce table 8.3. To include the effect of stress the data had to be analysed usingDuncan's new multiple range test. The results are given in tables 8.8 and 8.9 and compared In table 810with the least signficant difference test results for environment : material Interactions. There were

severai distrepancies, shown shaded, between the test indications. In all cases the discrepancies could "eattributed to the impe rance of taking stress level into account. In other words, only the results fromDoncan's test should be considered. These may be described as follows:

ki) igniti:ant environmental effects were fou-d for each material sit there was in overall trend.

I2) For 7475-T76I and 7075-Tib the ,igniticant environmental effects were confined to fatigue with

289 MPa.

k3) Extended pre-exposure generally had no additional effect on fatigue life. An explanation of the

one case for which extended pre-exposure was significant (7075-r6 fatigued with S.. 238 MPa)in provided by the primary fatigue origin data in tab. 8.2. Extended pre-nxposure sometimesresulted in enhanced corrosion attack at specimen corners remote from fastener holes. For 7075-T6specimens fatigued at the lower stress level this corrosion was sufficiently severe to causeearly initiation of fatigue crcking. St the higher stress level fatfge crack .nitiation wasdetermined mainly by the stress concentrating effect of the fastener holes.

(4) Significant differences between materials occurred for fatigue with S = 289 MPa and for each

environment in which all th-ee materials wer, tested. As stated previously, these differenceswere due to shorter fatigue lives of 7075-T76 specimens compared to 7475-T761 and 7075-T6spnc imens.

h.3.3 Primary fatigue origin data

The primary fatigue origin data were axalysed using the (2

test of independence and the results aresummarised in table 8.11. All three main variables of stress level, environment (fatigue testing schedule)and material had significant effects an the locations .f priorry fatigue origins, as follows:

0i) For S = 289 MPa most failures initiated in the bores (E/Q) and at the bore/faying surfaceMor(erm (fiR) of the fastener holes in the specimens. For S 238 MPa most failures initiated

at the in/S) faying surface locations. ax

(2) i ith or without pre-euposure the change from fatigue in air to fatigue in saL spray reduced the

r.umber of lay'ing surface (GS) failures. Pre-exposure and/or fatigue in -alt spray promoted[dilures at the bare/faying surface corners (F/R) of the fastener holes and also promoted

failures at specimen corners remote from the fastener holes.

i3) For IUi5-T7 specimens ti-re were relatively more faclures in the bores (E/Q) and at the bore/

taying surface corners (F/R) of the fasener holes. For i74i-T761 and 775-T6 specimens a number

ot lailures ccurred remote from tie fastener hoiss. This was not observed for the 775-T76specimens either in this investigation or in the CFC P core programme (see tble 2 in Part II ofchis report)

8.4 Piscussinn

Tfhis investigation has shown that 7475-T76l and 7075-Tb specimens assembled using the MRCA pr,:tectionsystem have equivalent fatigue and corrosion fatigue properties when tested with a realistic load history(FALSIAFF) . coe the other hand, 7075-T76 CFCTP core programme-type specimens had significantl. shorterlives at Sa x = 289 MPa but not at Sa x z 238 MPa.

lhe reason for this difference is not obvious. However, the primary fatigue origin data provide aclue. For the 7075-T76 specimens tested "t Sma = 289 MPa there were relatively more failures in the bores

and at the bore/faying surface ccrners of the fastener holes. In the CFCTP core programme it was foundthat those failure locations tended to result in shorter fatigue lives than ether 'ocitlons, see section1.4 of Part iL of this report.

The question now arises as to why there were relativel) more failures in the bores and at the herelraying surface corners of cte fastener holes in the 7075-T76 specimens tested at S = 289 MPa. it is our

opinion that the use of Ceiloseal sealant in assembling the 7475-T761 and 7075-T6 specimens was

toponsible. In other wards, Cellasea1 prevented or p-stponed fail: initiation at the characteristicshorter life locations and enabled the 7475-17I and 7075-T6 specness to reach significantly longerfatigue lives Lhan the 7u,5-in specimens.

'1.5 Conclus ions

(!) 7-75-i7tl and 1075-Tb specimens assembled using the M,,A protection s -,em had equivalent fatigue-.,d corrosion fatigue properties under FALSTAFF loading. 7U75-176 FC u core programue-typeopecimenu were significantly inferior at the higher stress level (S x = 279 MPa) bt ev'ivalentat the lower stress level 1 = 239 PiPa).

21 Set assembly with Celioseal sealant can be beneficial to fatigue ond corrosion fatigue life.

) Stress level had a predominant effect on latigue life.(4) Significant environlmental effects occurred at both stress levels, but there was no overall trend.

5) Extending t he pre-exposure period to multiples of the specified 72 hours generally had noadditional effect On fatigue life.

6) All three main vatriables )f stress level , envirmnent , and mate. i1 + protect ion systemLombinations had significant effects on the locations of primary fatigue origins. Cellosealprevented or postponed failure initiation at locations which are characteristically associatedwith shorter fatigue lives.

8.6 References

1. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation", Combined Report of theF + W, LBF, NLR and IABG, March 1976.

2. J.B. de Jonge, "Additional information about FALSTAFF", NLR Technical Report TR 79056 U June 1979.

3. R.J.H. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testingprogramme". AGAPD Report No. 695, February 1982.

12,

TABLE 8. I: OVERVIEW oi Il IABC i1FSI PROCI8A'll FO8 FACI

MATERIALS * 3.2 - thick 7075-T76 (CFCTP core programme material), 7475-T761

clad and 7075-T6 alumnium alloy sheets

PRESS FIT H, .k FASTENERS

SPECIMEN

3 2 n1 e

7075-T76 . chromate conversion + inhibited epoxy polyamide primer(except fastener holes) + aliphatic polyurethane topcoat

PROTECTION SYSTEMS

7415-T761 and: chromate conversion * inhibited epox polvaide primer7075-T6 (except fastener holes) + celloseal in iastener halls

and at faying surfaces s polvurethane ropcoat

PROTECTION SYSTEM . two stress cycles at low tempcrature to crakDAMA(;E paint and primer arourd the fastcn, hiads

FATIGUE LOADING . FALSTAFF

FATIGUE ENVIRONMENTS . labeatory air; 5 % aqu,.ous NaCI s, sprav wit, p

STATIC PRE-EXPOSURE * multiples of 72 hours in , ,uc ,Cu N I 3S .i: 31 1E

SCHEDULES CIIARACTERISTIC 7,75 T61 ,T6 T.77STRESS LEVEl.

S - 239 SPa Sfatigue in air Sa -_2_9MP.

- 238 MPa

S5 289 Mea •pre-exposure + max-fatigue in air S - 238 MPa 0

TEST PROGRAMRME __________ a .fatigue in

pre-exposure n Sma - 289 MPa • •

fatigue in______

salt spray Smax

- 238 MPa •

extended pre- S - 289 IPa * •exposure + fatigue

in salt spray S - 238 MPa *

STATISTICAL ANALYSIS 0 fatigue lives and primary fatigue origins

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I I-I I -.-

9. THE RAE CONTRIBUTION TO THE FACT PROGRAMME

R.M.J. Kemp, Royal ircraft Establishment RAE, Materials and Structures Department, Farnborough,United Kingdom

9.1 Introduction

The high strength AIZnMgCuZr alloy 7010 has been developed for aerospace structural applications withthe aim of combining high strength with resistance to corrosion and stress corrosion. The RAE contributionto the FACT programme concentrated on the fatigue and corrosion fatigue properties (fatigue strength andcrack growth resistance) of 7010 in the T7651 and T7451 tempers.

In addition, the effectiveness of chromate-containing and non-chromate-containing primers inmitigating corrosion fatigue was compared using Ij dogbone specimens of 7075-T6 aluminium alloy sheet.This material came from the same batch tested by the NDRE, NLR and IABG, see table 1.1 of the introductionto this part of the report.

9.2 The Test Programanes

An overview of the test programmes is given in table 9.1. There were three test programmes to compare

" fatigue and corrosion fatigue strengths of 7010-T7651 and 7010-T7451

* fatigue and corrosion fatigue crack growth resistances of 7010-T7651, 7010-T7451, 7475-T7351 and7050-T7451

* corrosion fatigue resistance of 7075-T6 with protection systems including chromate-containing andnon-chromate-containing primers.

9.2.1 Materials and properties

Engineering property data for all the materials were as follows:

MATERIALS 0.2 % YIELD STRESS (MPa) UTS (MPa) FLONGATION (%)

T7651 472 (L) 536 (L) 13.3 (L)488 (T) 548 (T) 11.8 (T)

25 mm thick 7010 plate413 (L) 491 (L) 14.0 (L)425 (T) 505 (T) 13.0 (T)

25 - thick 7475-T7351 plat. 451 (L) 519 (L) 11.0 (L)449 (T) 519 (T) 12.0 (T)

488 (L) 548 (L) 10.5 (L)40 mm thick 7050-T7451 plate 486 (T) 545 (T) 12.0 (T)

3.2 - thick 7075-T6 sheet 547 582 11.2

Note that the two heat treatment conditions of 7010 were from the same plate.

9.2.2 Specimen configurations

The specimen configuration for the fatigue strength test programme is shown in figure 9.1. Thisspecimen has a stress concentration factor K = 2.52. The burrs around the drilled holes were removed bygentle abrasion. Note that the specimen long taxis is normal to the plate rolling direction.

The specimen configuration for the fatigue crack growth resistance test programme is shown in figure9.2. The specimens were machined from the centre sections of the plates and with the long axis normal tothe plate rolling direction.

The specimens for comparing chromate-containing and non-chromate-containing primers were of theIi dogbone configuration discussed in detail in reference (1) and recommended for the FACT programme.Cadmium plated steel Hi-Lok fasteners were used. The diameter of the holes for the fasteners was6.248 ± 0.0127 mm, which corresponds to an interference fit, see figure 1.1 of the introduction to this

part of the report.

9.2.3 1l dogbone protection systems and specimen assembly

The 7075-T6 Ij doghone specimens were originally manufactured, painted and assembled according to thefollowing procedure:

* specimen parts machined

* chromic acid anodising and hot water sealing of all surfaces

* application of chromate-contsining or non-chromate-conrtaining epoxy primer on all surfaces

* fastener holes drilled

* application of polysulphide sealant to faying surfaces

" Hi-Lok installation and wet assembly of fatigue specimen dogbones and hali plates

" appication of acrylic topcoat.

During the test programme it became evident that the protection system with non-ch, ate-cortairingprimer was insufficiently resistant to static pro-exposure. Drastic decoheslon of the topcoat + primerindicated faulty application of the primer. Therefore some specimens were reprocessed according to RoyalAir Force (RAF) practice as follows:

* solvent or chemical stripping, without disassembly, down to the anodised layers on extnrior

surfaces

* re-application of chromatr-containing or non-chromate-containing epoxy primer on exter ir surfices

a application of polyurethane topcoat.

Finally, it should be noted that although in the original processing the polysulphide sealant wasapplied intentionally only to faying surfaces, post-test examination showed traces oi sealant in thefastener holes of both dogbones and half plates.


All stresses were defined in terms of loads on the total cross-sections of the specimens in the gaugelength. This means that the central holes and notches in the fatigue strength and crack growth resistancespecimens and the fastener holes in the dogbone specimens were included in the cross-sectional area.

Before environmental exposure and fatigue testing all li dogbone specimens were prestressed at209 ± 5 K by applying two load cycles up to 215 MPa. The procedure for this is discussed in reference I).The purpose of this low temperature prestressing was to ensure that the paint and primer layers werebrittle and would crack around the Hi-Lok fastener holes, thereby simul. ting service damage that enablescorrosion and corrosion fatigue to occur.

All fatigue Lasts were done using an INSTION 1342 eiectrohydraulic machine. The fatigue loadhistories were constant amplitude sinusoidal loading with a stress ratio R = Smn/Sa of 1.i and the

manoeuvre spectrum FALSIAFF (references 2, 3). A short description of this spectrum is given in section1.? of this part of the report.

Details of the fatigue testing conditions are as follows:

(1) Fatigue strength tests were carried out over a range of stress levels at a coninal cyclefrequency of 15 Hz for both constant amplitude and FALSTAFF loading.

(2) Fatigue crack growth resistance tests were done at similar load levels for constant amplitudeloading and at a constant gross section S of 75 MPa for FALSTAFF loading. As shown in tableman9.1, the cycle frequencies were i Hz and I Hz for constant amplitude loading and 10 Hz fyrFALSTAFF loading. Crack growth was monitored using a 2-wire pulsed direct current potential dropmethod. The current and voltagn leads were taken out of the salt spray chamber vi.: a sealedporthole as shown in figure 9.3. A microcomputer was used for data storage and analysis.

(3) The l dogbone specimen fatigue tests were done with constant amplitude loading at an S of

210 MPa and cycle frequencies of 2 Hz in air and 0.5 Hz in salt spray. These testing conditionswere based on those of the CFCTP core programme.


1 dogbone specimens scheduled for static exposure to an aggressive environment belore fatiguetesting were sealed at the faying surface side edges and Hi-ink collars to try and prevent corrosionexcept in the fastener head areas. The procedure for static pre-exposure is described in detail inreference (1). The specimens were immersed for 72 hours in 5 % aqueous NaCI acidified by a predeterminedamount of SO gas and maintained at 315 ± 2 K. The cleaning procedure after pre-uxposure followed theamendment in section 4.4 of Part 2 of refe-'nce (I).

For fatigue testing the fatigue strength specimens were sealed off from the environment at theclamping area, while the crack growth resistance and 13 dogbene specimens were electrically insulated frocthe loading grips and bolts by polymeric liners and bushings. 13 dogbone specimens to be fatigued in saltspray were also sealed at the faying surface side edges and Hi-Lok collars. The fatigue environments werelaboratory air (relative humidity - 50 %) and salt spray, both at a nominal tempcrat:re of 295 K.Depending on the test programme, the salt spray environment had different compositions and acidity. asshown in table 9.1 and listed here also:

SALT SPRAY PARAIMFTERS

FATIGUE TEST PROf;RATME

weight % NaCl pH

(1) fatigue strength 3.5 7(2) fatigue crack growth resistanc, 5 4,}(3) effect of chromate in primers 5 4

The salt spray tests were done in a specially constructed cabinet illustrated in figure 9.3. Adescription of the cabinet, except for the sealed porthole for the crack growth monitoring leads, is given

140l

in roterenc i:.

The nominal cycle reque. aeo for each combinat ion of fat igu. load history and envlronmet;t and I-each test programme are given in table 9.1 and listed -r0 also:

NOMINA . CYCLE FREQUENCY

FAI1GUf TEST PROGRAMME FATIGUE LOAD HISTORY fatigue in salt spray

fatigue In airPH 4 i pit 7

() fatigue strength constant amplitude, K = 0.1 15 lz I15 lieFALSIAFF 15 He I5 he

(2) fatigue crack growth re constant amplitude, R = 0.f IS Hz Ib Iz, I Hz: I He2 o tanc FALSTAFF 10 Hz 01 lIz

k3) effect ot chromate in primers constant arplitude, R = 0.1 2 Hz 0.5 He

9.2.b Statistical methods for analysing the data

The way in which the test programmes were set up and the results had consequences for the statisticalmethods used to analyse tie data. A survey of the statistical methods is given in figure 9.4. Only thenotched fatigue strength data for FALSTAFF loading and the 1 dogbone fatigue life and primary fat ise"'rigin data were readily amenable to statistical analysis. Nevertheless, owing to the limited number andunequal sample sizes of the fatigu strength and life data it had to be assumed that they at leastapproximated to rando samples from log-normally distributed populationa with equal variance. Unequalsample sizes also meant that modified versi ns of the least significant difference test and Duncan's newmultiple range test had to be used for "fine tuning" the arolysis of variance resuIts. More details of thestatistical methods are given in Appendix II.

9.3 Results of the Fatigue Strengta Test Programme

The complete set of fatigue life data for the RAE fatigue strength contributIon tc FACT is given intable 9.2.

9.3.1 Constant .mplitude fatigue tests

The constant amplitude data are plotted is figure 9.5 and show the followlng:

o Fatigue in air: the high cycle notched fatigue strength of 7010-T7451 was slightlIv greater tha:that of 7010-T7651. However, at higher stress levels 7OO-T7651 was super .r.

* Fat igue in nestral pH 7) salt spray: the high cycle notche fatigue strengths were reduced to asimilar levei less than half the fatioe strengths in air.

9.3.2 Manoeuvre spectrum (FLSTAFF) fatigue tests

The data for FALSTAFF loading are shown in figure 9.6. These data indicaee that stress level had asignificant effe.t or fatigue life and that 7015-T7b5l was superior to 7010-'17451[ at higher stress levels,as was the case for constant amplitude loading. However, neutral salt spray apparently !ad no significareffect on the fatigue ies. This is a remarkable result, especially for tests at lower stress levels, inview of the relatively long testing times, tar example, 50,000 FALSTAFF flights require about 80 hours oftesting at a nominal cycle frequency of 15 Hz.

The results of three-way analysis of varia e of the data are summarised in table 9.3. According tothe analysis the main variables of stress level, material and their two-way intekaccins had signilicanteffects on the fatigue lives of the specimens. Since there were only two materials it is obvious that thesignificant difference is between them. Thus it was not necessary to "fine tune" this result using theleast significant difference test.

The least sig uilicant difference test was used to "fine tune" the effect of stress level and the

stress : material interactions. The results are given in table 9.4. Changlhg the stress levelsigellcantly altered the fat'gue lives, as would be expected. At the two higher stress levels 7Sf0-T7651specimens had significantly longer fatigue lives titan 7010-T7451 specimens.

The potential sources of variation not found to be significant by analysis of variance were:

* effect of environment

* effect of environment at each stress level{ effec t of material in eaeh environment

* effect of material per stress level and environment.

Because there were only two envirosments the lack of a significant difference between them did not requirefurther analyis. The remaining potsntial sources of variation were investigated using Duncan's newmultiple range test. Th, results are listed in table 9.5 and show:

| ! _ _ _ _ _ _ _ _ _ _ _ _-_ -

141

I() At each stress level the overall fatigue lives were unaftected by changing from fatigue in air

to fatigue in salt spray. However, f. one case there was a significant difference. namely for

7010-I701S tnted with S .5 0U Mpa.

The previously mentioned result tht ;010-T7651 specimens had significantIly longer fatIgue lives

rhan 7010-T7.51 specimens or the two higher stress leveIs must be quaIifIed. Significant

differences were found only for fatigue in air at S - 3100 MPa and fatigue in salt spray at

S = 250 MPa. nax

9.4 Results of the Fatigue Crack Growth Resistance Test Programme

9.4.1 Constant amplitude iatigue crack growth tests

The coo-ant amplitude fatigue crack growth data are shown in figures 9.7 and 9.H. Figure 9.7

compares the fatigue crack growth resistances of 7010-T7k51, 7010-T7451, 7475-T7351 and 7050-T7451 plate

materials in air and acidified (p1

4) salt spray at a cycle frequency of 10 Hz. It is seen that

" in air the crack growth resistances t 70iO-T76SI, 7010-T7451 and 7475-17351 were equivalent hut7050-T7451 had significantly higher crack growth rates

" acidified salt spray resulted in increased crack growth rates for 7010-T7651. 7010-T7451 and7.75-T7351 but not for 7050-T7451. The greatest sensitivity to changing the environment was shown

by 7010-1741.

Figure 9.8 shows the effects of changing the salt spray acidity and cycle frequency or the crack

growth resistances of 7010-T7651 and 7010-T7451. For 7010-T7451 these effects were regligible, but701U-F7651 crack growth rates depended strongly oil salt spray pH and cycle frequency. These results can beexplained partly by the generally higher sensitivity of 7010-T7451 to changing from fatigue in air totatigue in salt spray. i.e. there is a strong environmental effect even at a cycle frequency of 10 Hz.However, it was unexpected that crack growth rates in neutral salt spray would be higher than in acidified

salt spray.

9.4.2 Manoeuvre spectrum (FALSTAFF) fatigue crack growth tests

The FALSTAFF fatigue crack growth data are given in figure 9.9. 7010-T7651 and 7010-T7451 were tested

in air and neutral salt spray at a nominal cycle frequency of 10 Hz. The results show

" no significant influence of the environment on crack growth

* similar crack growth rates and lives for 7010-T7651 and 7010-T745i

* the occurrence of tensile crack jumping (static crack extension during peak loads) at half cracklengths beyond about .1 mm. This corresponds to Km.. ' 22 MPa'm, which is significantly less thanthe fracture toughnesses of the two tempers (Kic in the appropriate T-L orientation was 31.9 ond

37.7 MPafm for 7010-T7651 and 7010-T7451 respectively). Similar results have been reported and

explained in reference (4).

The lack of an effect of environment on crack growth cannot he explained solely as a cycle frequency

effect, since constant amplitude tests on 70i"-T7451 at 10 Hz showed large differences in crack growthrates for fatigue in air and salt spray, see figure 9.8. Also, it is somewhat surprising that themanoeuvre spectrum crack growth rates and lives of 7010-T7651 and 7010-T7451 were similar. Generally it is

found that within a class of materials the alloys and tempers with lower yield strengths (in this case7010-17451) exhibit more crack growth retardation following peak tensile loads and hence lower overall

crack growth rates and longer lives.

9.5 Results of the Programme on the Effect of Chromate in Primers

The complete set of fatigue life and primary fatigue origin data for the RAE contribution to FACT on

the effect of chromate in primers is given in table 9.6.


The tatigue life data are plotted in figure 9.10 and indicate that environment (fatigue testing

schedule) had a significant effect on life. Pre-exposure + fatigue in salt spray was especially

detrimental to specimens with non-chromate-containing primer + acrylic topcoat. As mentioned in section9.2.3. the problem with these specimens was attributed to faulty application of the primer leading to

drastic decobesion of the topcoat + primer during pre-exposure.

lhe results of two-way analysis of variance of the data are summarised in table 9.7. According to the

analysis the main variablos of environment and protection system had significant effects on the fatigue

lives of the specimens. These effects were "fine tuned" by the least significant difference test. The

results are given in table 9.8 and show that:

(1) Fatigue lives in air and salt spray were equivalent. This unusual result agrees with the FALSTAFFtests -'n unprotected notched specimens oi 7010-T76Sl and 7010-T7451 (see section 9.3.2).

(2) Pre-exposure + fatigue in salt spray significantly reduced the fatigue lives.

(3) the combination of chromate primer + .crylic topcoat appeared to be better than the otherprotection systems. There is a complication with this result: specimens with ciromate primer +polyurethane topcoat were tested only by pre-exposure and fatigue in salt spray, so that a p'neralcomparison with specimens having other protection systems is not justified.

r

A more detailed analysis of environment : protection system interactions had to be done usingDuncan's new multiple range test. The results are summarised in table 9.9 and can be described as follows:

(4) Pre-exposure + fatigue in salt spray significantly reduced the fatigue lives of specimens withacrylic topcoats whether or not the primers contained chromates.

(5) The only significant differences in fatigue lives per testing schedule occurred for pre-enposure+ fatigue in salt spray and were due to the shorter lives of specimens with non-chromane-containing primer + acrylic topcoat.


The primary fatigue origin data were analysed using Yates' corrected X2 test. The esults are listed

in table 9.10. Only the protection system was found to have a significant effect on the locations ofprimary fatigue origins. Specimens with non-chromate-cntalnlng primers had relatively more failures inthe bores (E/Q) of the fastener holes and fewer failures at faying surface (/Is) locations as compared tospecimens with chromate-containing primers.

9.6 Discussion

As shown in table 9.1, the present contribution to FACT consisted of three test programmes. The scopeis broad and therefore the topics for discussion will be addressed separately in sections 9.h.1 - 9.f.3.These topics are:

* the effects of changing from fatigue in air to fatigue in salt spray

* oomparisons of materials with respect to fatigue and corrosion fatigue strengths, lives and crackgrowth resistances

* the effect of chromate in primers.

9.6.1 Fatigue in air/fatigue in salt spray

in general it is to be expected that changing the fatigue environment from air to salt spray or saltwater will result in lower fatigue strengths, shorter lives and higher crack growth rates, see for examplereferences (5, 6). In the present test programmes several exceptions to this general trend were found.Table 9.11 reviews the comparisons of data for fatigue in air and salt spray. The results may be describedas follows:

(1) High cycle notched fatigue strengths were significantly reduces by a salt spray environment.

k2) Fatigue lives of some specimens were unaffected by changing the environment from air to salspray. In particular, it is remarkable that under manoeuvro spectrum (FALSTAFF) loading thefatigue lives of unprotected notched specimens were unaffected tp to 6i,000 simulated flights,corresponding to about 93 hours in the salt spray environment.

(3) For most of the materials tested, including 7010-T7651 and 7010-T7451, the constant amplitudefatigue crack growth rates were significantly increased by changing the environment from air tosalt spray. But at the same nominal cycle frequency the fatigue crack growth rates of 70l0-T7651and 7010-T7451 under FALSTAFF loading were virtually the same in air and salt spray.

These results demonstrate the importance of conducting environmental fatigue tests with realisticload histories.

9.6.2 Comparisons of materials

The fatigue strength and life tests on 7lO-Tibbl and 7O-T7T51 in air and salt spray showed that athigher stress levels 7010-T7651 was generally superior and at lower stress levels the alloys wereequivalent. 7010-T7651 was also equivalent or superio to 7iuiO-T7-5l in fatigue and corrosion fatiguecrack growth resistance at a cycle frequency of I0 Hz. However, from figure 9.8 it is seen that thecorrosion fatigue crack growth resistance of 7010-T7651 was strongly affected by salt spray ph and cyclefrequency. Reducing the cycle frequency to I He caused 7010-T7651 to have higher crack growth rates than7010-T7451 over a wide range of AK.

Thus it is concluded that besides using realistic load histories isee section 9.6.1) it is importantto conduct environmental fatigue tests with realistic stress levels and cycle frequencies.

Constant amplitude fatigue and corrosion fatigue crack growth tests were carried out for 7475-T7351and 7050-T7h51 us well as 7110-T7651 and 7010-T745i an a cycle frequency of IS He. In air the crack growth

resistances of 7010-T7651, 7010-T7451 and 7475-T7351 were equivalent but 705O-T745l had significantlyhigher crack growth rates. In salt spray 7010-T7651 was superior and 7010-T7451 was the least resistant.These latter results cannot be generalised because of the previously montloned effects if salt spray pHand cycle frequency.

9.6.3 Effect of chromate in primers

The results of this test programme indicate that the absence of chromate in properly applied primerhad no significant detrimental effect on the resistance to pre-exposure and/or fatigue in salt spray oipainted 1i dogbone specimens containing interference fit Hl-Loks. However there are some caveats. Owing tothe high constant amplitude fatigue stress level (Smax - 210 Mpa) most specimens failed in the bores or at

hure/faying surface corners of the fastener holes where unprimed metal was present. Also the fatigue testsin salt spray lasted less than 14 hours.

143

It is possible that an effect of chrumate in primers will be found for corrosion fatigue conditions

under which failures initiate in areas where primer is more or less continuously present and there is

plenty of time for chromate to leach out into the corrodent. With respect to the 03 dogbone specimen the

results of the CFCTP core programme (reference 1) indicate that these conditions can be obtained by

lowering the constant amplitude fatigue stress level. Flight simulation loading should also be used, since

besides being more realistic it also gives much longer testing times, see for example table 9.11.

9.7 Conclusions

The present investigation consisted of three test programmes to compare

* fatigue and corrosion fatigue strengths of 7010-T7651 and 7010-T7451

* fatigue and corrosion fatigue crack growth reofstanes of 7010-T~h5l, 7ili-T7451, 7475-T7351 and

7050-T7451

* corrosion fatigue resistance of 7075-T6 with protection systems including chromate-containing and

non-chromate-containing primers.

Conclusions drawn from the results of each programme are given in sections 9.7.1 - 9.7.3. Some additional

and more general conclusions are given in section 9.7.4.

9.7.1 Conclusions for the fatigue strength programme

tI) In air the high cycle notched fatigue strength of 7010-T7451 was slightly greater than that of

7010-T7651.

(2) In salt spray the high cycle notched fatigue strengths of 7010-T7651 and 7010-7451 were reduced

to a similar level less than half the fatigue strengths in air.

(3) Under manoeuvre spectrum (FALSTAFF) loading in both air and salt spray the notched fatigue lives

of 7010-T7651 specimens were equivalent to or longer than those of 7010-T7451 specimens.

(4) The notched fatigue lives of 7010-T7651 and 7010-T7451 under FALSTAFF loading were unaffected by

changing the environment from air to salt =pray.

9.7.2 Conclusions for the fatigue crack growth resistance programme

(5) In air the constant amplitude fatigue crack growth resistances of 7010-T7651, 7010-T7451 and

7475-T7351 were equivalent at a cycle frequency of 10 Hz. 7050-T7451 had significantly higher

crack growth rates.

(6) Acidified salt spray increased the constant amplitude fatigue crack growth rates for 7010-T7651,

7010-T7451 and 7475-T7351, hut not for 7050-T7451, at a cycle frequency of I0 Hz. The greatest

sensitivity to environmental change was shown by 7010-T7451.

(7) Changing the salt spray acidity from pH 4 to pH 7 and the cycle frequency from 10 Hz to I Hz had

negligible effects on constant amplitude fatigue crack growth rates for 7010-T7451. However,7010-T7651 crack growth rates depended strongly on salt spray pH and cycle frequency.

(H) Under manoeuvre spectrum (FALSTAFF) loading the fatigue crack growth rates and lives of

7010-T7651 and 7010-T7451 specimens were similar. Changing the environment from air to neutralsalt spray had no significant influence.

9.7.3 Conclusions for the programme on the effect of chromate in primers

(9) The absence of chromate in properly applied primer had no significant detrimental effect on the

constant amplitude fatigue lives of painted l dogbone specimens of 7075-Tb subjected to pre-

exposure and/or fatigue in salt spray. However, owing the the high stress level the testing

times were short and most specimens failed in the bores or at bore/faying surface corners of thefastener holes where unprimed metal was present.

(LO) Fatigue lives in air and salt spray were equivalent, but pre-exposure + fatigue in salt spray

significantly reduced the fatigue lives of specimens with acrylic topcoats.

(I) Specimens with non-chromate-containing primers had relatively more failures in the bores of the

fastener holes and fewer failures at faying surface locations as compared to specimens with

chromate-containing primers.

9.7.4 Additional conclusions

(12) The fatigue strength and life tests on 7010-T7651 and 7010-T7451 in air and salt spray at a

cycle frequency of 15 Hz showed that at higher stress levels 7010-T7651 was generally superior

and at lower stress levels the alloys were equivalent. 7010-T765I was also equivalent or

superior to 7010-T7451 in fatigue and corrosion fatigue crack growth resistance at a cycle

frequency of 10 Hz. However, the results of changing cycle frequency and salt spray pH for crack

growth tests show that a conclusion as to the overall superiority of 7010-T765i cannot be made.

144

(13) Changing the fatigue environment from air to salt spray does not necessarily result in shorterfatigue lives and higher crack growth rates. Significant variables In this respect include the

type of test; fatigue load history and stress level; cycle frequency; environmental pH andmaterial response. Environmental fatigue tests should thezefore be conducted with realistic loadhistories, stress levels and cycle frequencies. (There still remains the difficult problem ofdeciding what are the most realistic environments.)

k14) Further investigation of the effect of chromates in primers should include flight simulation

fatigue tests on realistic specimens at stress levels that result in fatigue crack initiation inareas where primer is more or less continuously present. The tests should be of sufficientduration to allow time for chromate to leach out of chromate-containing primers into the

corrodent.

9.8 References

i. R.J.. Wanhill and J.J. De Luccia, "An AGARD-coordinated corrosion fatigue cooperative testing

programme", AGARD Report No. 695, February 1982.

-. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation", Combined Report of the


3. J.B. de Jonge, "Additional information about FALSTAFF", NLR Technical Report TR 79056 C, June 1979.

4. R.J.H, Wanhill, H.J. Kolkman and L. Schra, "Fatigue crack propagation and fracture in 7050 and 7091aluminium alloy forgings", AGARD Conference Proceedings No. 376, Paper 5, November 1984.

5. Collected Papers in "Corrosion fatigue of aircraft materials", AGARD Report No. 659, October 1977.

6. Collected Papers in "Corrosion fatigue", AGARD Conference Proceedings No. 316, October 1981.

145

TABLE 9. 1: OVERVIEW OF THE KAE TEST PROGRAM ES FOR FACT

SPECINM* e

703050005 ostan sploosds. S5*n/,o5*, -0 (01A 00505,,.! (os~...oo 10 05 10 .1t,1l~

5 0 0 3,ba0,t,00.3, 3 O t aqueous 0.03 s.lo sprsy silh PH 0

T1rT?- gf-, in t.

-TI- -L ........ -- -1 - 1i-

FAT U Nn .p , 50 '000 0H 1 FALTAFF

-1 - TISI,

055TETI0 8 0 5yS * Ihopic FAT.d P

I.00 Ao Hs3 T-11'.. -1 1

00035001 330.05 Ool. les. '. p, s sosoxd slob,, (5. 0050 , 5 .yu~h * o,,,

;4, Il o'. 's o 00 p, o o S 0.s 0

..... * '00 01,1

1SIT0 I E

/'T500E 2 0 '0 . 0 0 -

1p, i 0l(0, 0i

146

TABLE 9.2: FATIGUE LIFE DATA FOR THE RAE FATIGUE STRENGIH CONTRIBUTION TO FACT

MATERIAL HEAT FATIGUE CHARACTERISTIC FATIGUE LIFE TO FAILURE (CYCLES OR FLIGHTS)TREATMENT LOAD STRESS LEVELCONDITION HISTORY S .. (MPa) fatigue in air fatgue in salt spray*

250 14,035

200 44,850 24,329175 71,837150 171,266 130,782

constant 135 181,586amplitude, 125 305,809R - 0.1 120 246,393

100 > 9,141,016 1,044,501

7010-T7651 75 1,480,73850 6.346,696

300 5,311 4,077

300 5,497 4,826250 6,734 14,825

FALSTAFF 250 10,996 22,217

175 36.352 59,735175 56.047 57,598175 52,823

250 1,270

200 25,780 13,892175 7,800 25,779160 40,912 88,799

constant 150 83,220

amplitude, 140 96,849R - 0.1 125 >12,673,484 188,026

100 701,17075 1,848,773

7010-T7451 7 ,4,750 4,999,171

300 1,914 2,283300 2,334 3,764250 6,958 3,431

FALSTAFF 250 5,825 10,987175 60,727 33,713175 118,766 52,52387 94,659

Neutral salt spray solution with pH 7

j-0

- 00S Os= 0 -

C-C CC

'C S :-~- 0 -~

- CC-SOC 4 -~ -

~~C~C-C C- --,05C0S H

-~ C- 'C

0 S S

'CS C

CC 0'

48

21

149~

0 (

Ox 0 11

0 0 -. 1(04 4 -- 01 C

(4z o

to <

= 00 0

40 0 .

9 Ca

Hto 0

c\ ,

0.

0( o w.19

-25

I 54.

H4~~ 0

4))~ ~~ ~~ 044 0 - S)4

-024~ ~~~~ 4))44 0 ,4 .

0) Ca 044 a . ) ) 0 0

040 0+

240004~ 0) 0400 .:4~1 K4) SC 24 0t

H ) L

00 0C 2 0 Z

:4 ca0 0)

'5'

C 4. .4 0

4~~ 0, L

ol L 0.

tS 2

0

H o.o40C 0.40

-- <HOHA

o .- ' <-0 00 000H 0.40 00 00 0 00 <.000 0 0

00 00. 0 00 0,0-0.0 0 0

~40 0 00 40~40

- 0< 00

C <0.4 .0-0 0404 -?

40 HH HH H HH H(-.HH H H

H 00 00 00 00400 0 0

0< 0< 0, 4

4 - -

0. 0, 0

-. 40 0 0 N- 0. 0 0 0 0 4 0<

40 0 0 0 0 <00 0 0 0 4

0 H 4 4 0 0 -

40 0 0 4

- 40 -0-~ 40 40

40 -0. 40 40

- 40 0< 40 0 0 0

0 I I 0< 0

.40 0< 0 - 40 0< 4 I40 <00 - .- o

U0

H 0- 0 0 40

H 0 0 o

<.5 <.5 0 0 0

0< 40 40 U - 4040 0 0

H 0 40 - - 40 CO 4040

*0 4040 V * V V

0- -- 40 0 0 0 00 40 0 0 0 0 0 00

-~ > -~ *~ 0 ~ 0

C U ,. 4040 -40 40 -

0< U H40 40- 0 <A H- 40 4040 4040 -0 40 40 40

H HZ 0(0 40U 40 o.

0< 0 4040 0 ~ 40H 0 0

0- 40 4040 HU 040 U UU 0 0 0

0< 40 4040 40 AU 0 0 0

40 40 -H H 040 C

± ~ .0. 40 440

40H 0. 0 4 0 4

0 40 0

4040 0. OH

2 2 2 2

H 2~

0<

.t. 0<

0- I 1<

-, 0 40 H 400 H

~ <I I A40 40 040 -0

j 0 _____ 0.

40 -0~

0< 0

U 4<40 0 0H 0C .40(0

190.5

139.715

19.1

9=38.1 PARA LLEL

ROLL ING~m

(1) ALL HOLES REAMED.

(2) AL L HO LES MARKED THUS W' ARHE 12.3 DIAMETER.

(3) TOLERANCE INDICATIONS NOT INCLUDED

Fig. 9.1 Notched specimen configuration for the RAE fatigue strength test progr ...e

DETAILS OF STARTER NOTCH

2 1 4 1 2

7 PROBE WIRES6 APART

PLATEROLLINGjIRECTION

40 AL MENSION I m

Ill STARTER NOTCH IS A CENTRAL HOLE WITH SAWCUTS AND FATIGUE PRECRACKS TO A TOTAL LENGTH OF 0

(2) ALL HOLES MARKED THUS - A" ARE 18 UOO

O DIAMETER

(3) OTHER TOLERANCE INDICATIONS NOT INCLUDED

Fig. 9.2 Specimen configuration for the RAE fatigue crack groth resistance test p-i'gramm

3 St -p f f. I u, ,.,kgr(wt resi tan- tesf s i nt hesa t pravch mbe

.3Tu a ge crack groth reg -s istace- t tis tetl I 3a spral- hamer.

FA EU P -A I G T S N-- A

SIIFATA FORFAC

LEACTYP OG F COPNT AN SIEE CICE ES ORIGINSS ANDS L ENAIRONMEEST

Fig ~ ~ v y o ~ met od fo ~ fhe SA T M go ~ t E~ gth on OI G N AN D PRf O T E CTO Y T M

300 -- - r ; -r F TF- , -rt... -r F - T r

MATERIAL NEAT FATIGUE ENVIRONMENTK -252 TREArMENT

CONDITION AIR SALT SPRAY

0 -010 T 74 5 1

I

iMPs)

lO. 7|-T745 n i

iO-T7651 in ad

7010 -T7451innsai spray qpH 71

103 10 10510 6 107 108

CYCLES TO FAILURE

Fig. q.5 RAE conatant amplitude fatigue strength data contribution to the FACT progranume. Cycle frequency

15 Hz; salt spray pH 7

350 -tTp-fV 7 l rMATERIAL HEAT FATIGUE ENVIRONMENT

i ~Kt - 252 TREAT MENT -- "" l

CONDITION AIR SALTSPRAY

70-T7651 * I o

30 010-T7451 & i,

2501- o oSMa 7010T75 -710T75

200-1

150 w . a -- J_ ____ _ _. - J ____i1000 10,000 100,000

SIMULATED FLIGHTS TO FAILURE

Fig. 9.6 RAE FALSTAFF fatigue strength data contribution to the FACT programme. Cycle frequency 15 R;salt spray pH7

I - I

6 10-6-% 6

o o

°AD lee

FA o101 o _j 1 OSAN MLTD

CONSTANT AMPLITUDE 0p COS AAPLTDFATIGUE IN AIR FATIGUE IN SALT SPRAY

R =0.1; 10 H R = 01;10 H,; PH4

* 7010-T7651a 0 10-Ti5

A ~0 701-i ~ 07l-T?351475T351 A 7475-T7351

a 7050-T745I a 7050-T7451

10 ---- . _ 0 8_ ... ..

5 6 7 8 9 10 20 30 5 6 7 8 9 10 20 30

7T5b-T451 I7010-T7651 -j7050-T7451 -

701 TT,3-74511 .

10 6 711-- 15

o F / -T7351-7 -0 T 7451

7 7475-T7351 WO-T651

7

CONSTANT AMPLITUDE

R 0 .10

- FATIGUE IN AIR

---- FATIGUE IN

SALT SPRAY AT pH4

10 85 6 7 8 9 10 20 30

.1K IMPa 46i)

Fig. 9.7 Comparisons of constant amplitude fatigue and corrosion fatigue crack growth resistances of

7010-T7651, 7010-T745 1, 7475-T7351 and 7050-T7451

Oo

00

*0

00

0 0 p

o s

4

0 0

160

E <

44 C

-0 ---

4i

0

0

* -

L- -- < o ------

6 2

T ] : :~ : I'','

I T

fatigueCLE TO airKEYTOSYMOL

Cylefaqetiguez oa~ .5H in salt apaprayOT epx pie

162

10. TOE NRC CONTRIBUTION TO THE FACT PROGRAMIME

A. Baldantonl, ALCAN International, Kingston, Ontario, Canada

W. Wallace and M.D. Raizenne, National Aeronautical Establishment, Structures and MaterialsLaboratory, National Research Council, Ottawa, Canada

J.Y. Dickson, Ecole Polytechnique, Department of Metallurgical Engineering, University of Montreal,Canada

10.1 Introduction

The NRC contribution to FACT was mainly a comparison of fatigue crack growth and corrosion fatiguecrack growth properties of 7075 aluminium alloy plate in the TA51, T7351 and RRA (retrogression and reage)conditions. Stress corrosion crack growth rate tests were also carried out on the same material. Somefatigue crack growth and corrosion fatigue crack growth tests were done with 7075 aluminium alloy sheet inthe T65. T7351 and ERA conditions.

10.2 The Retrogression and Reageing (RRA) Process

The retrogression and reageing process was first described by Cina and Ranish (references 1, 2) andis a heat treatment designed for use with the 7000 series aluminium alloys. The RRA trcatment was claimedto provide in a single temper the strength and stress corrosion resistance equivalent to the best featuresof the Th and T73 empers. The heat treatment is shown schematically in figure 10.1. It is applied tomaterial in the I. condition and involves two stages of treatment. The first stage, retrogression,requires heating for short times of the order of a few seconds or minutes at temperatures in the range473 - 533 K. The second stage, reageing, is a repeat of the original Th age, which typically involvesheating for about 24 hours at 393 K.

Subsequent studies (references 3-8) have confirmed the essential features of the retrogression andreageing process. As indicated in figure 10.1, retrogression appears to involve three stages. Duringstage I the strength decreases from the initial T6 value and reaches a minimum at the start of stage II,where the strength begins to recover. A secondary hardening peak is reached at the onset of stage Ill.Continued heating through stage III causes a further loss of strength as the material effectivelyoverages. Besides changes in strength, retrogression causes a progressive increase in electricalconductivity. After relatively short regression times the material can be reaged to recover strength,sometimes to levels higher tha the initial T6 value, and electrical conductivity continues to increaseand reaches values close to those obtained by conventivoal T73 heat treatment.

Cina and Ranish claimed that heating to the minimum in the retrogression curve, followed by reageing,prodsced a material with strength equisolent to the initial T6 value together with electrical conductivityand stress corrosion resistance equivalent to thsse of T73 processed material. A serious limitation of theprocess is that the retrogression times are very short, typically 5-120 seconds depending on temperature,and thus it is difficult to obtain uniform through-thickness properties in thick section parts. Asoriginally formulated, the process is more suitable for very thin sections or as a surface modificationtreatment for thicker section parts. Wallace et al. (references 3-5, 7, 8) showed that lower temperaturesand longer retrogression times as far as the secondary hardening peak could often be used to produce moreeffective combinations of strength and stress corrosion resistance in thecet section materials.

The effects of retrogression and reageing on microstructure have been studied using transmissionelectron microscopy of thin foils (references 4-6). Although some differences in interpretation exist, itappears that stage I of the retrogression process involves partial resolutioning of G.P. zones with littleor no effect on the size or volume fraction of , or n' (MgZn,) precipitates, son figure 10.2 and table10.1. Continued retrogression -hrough stages I and III causes ao increase in size and volume fraction of

+ n' precipitates. Reageing causes a further increase in Volume fraction of n + n', but strength can berecovered only for short (to the end of stage 1) or intermediate (to the end ot stage I) retrogressiontimes. In references (5) and (6) it is shown that the size of grain boundary precipitates increasessubstantially during retrogression and approaches the size of precipitates produced by the T73 heattreatment. It has been suggested that these coarse grain boundary precipitates play as important role instress corrosion cracking by acting as trapping sites for hydrogen (reference 5). Thus hydrogen producedby hydrolysis at a crack tip and entering the metal would be encouraged to precipitate. forming moleculargas bubbles at the trapping sites and hence lowering the concentration of atomic hytrogen (presueod to bethe damaging species) in the grain boundary region ahead of the crack tip. Several workers have reportedobservations consistent with the presence of hydrogen bbbles at large grain boundary precipitates inaluminium-zinc-magnesium olloys exposed to water vapour (references 5. 9, 10).


An overview of the test programme is given in table 10.2. Tests were originally planned for both 7075and 7425 material. But testing of 7475 was discontinued because residual stresses introduced during heattreatment resulted in spurious fatigue crack growth behaviour, which Is explained in section 10.4.4. Therewere two parts to the programme:

* investigation of stress corrosion crack growth resistance of 7C75 plate as a function of heattreatment, including several TkRA conditions

* comparisons of fatigue crack growth and corrosion fatigue crack growth resista, . of 7075 sheetand plate in the T651, T7351 and optimum T6RRA conditions.

[1).1.1 Materials, heat treatments and specimen configurations

the materials used in this investigation were 7075 and 7475 alsminlum received in the Tk5l and T7351tempers respectively. The 7075 alloy was received in the form of 3.2 m thick sheet and 102 mm thickplate. The 7475 alloy was received In the form of h3.5 m thick plate.

r II l I_ 1

163

Retrogression and reageing treatments were carried out on unnotched specimens ulanks using siliconeoil baths. For the 7075 sheet and plate retrogression was carried out directly for the as-received T651materials. However, since the 7475 plate was received in the T7351 condition it was necessary to do a fullsolution treatment and age in order to obtain a T6 starting condition before retrogression and reageing.The 7475 plate was quenched into cold water at 273 K after solution treatment at 753 ± 5 K, but no stressrelieving was done before ageing at 393 K. Retrogression treatments were carried out at 493 K and 533 Kfor various times before ageing for 24 hours at 393 K.

There were three types of specimen, illustrated schematically in table 10.2. For stress corrosioncrack growth tests the specimens were of the bolt-loaded double cantilever beam (DCB) type described bySpeidel (references 11, 12). The specimens were 127 mm long, 19 mm high and 31 mm thick and orientatedwith the loading direction parallel to the short transverse (S) direction and with crack growth in thelongitudinal (L) rolling direction of the plate. This is the most sensitive orientation with respect toenvironmentally enhanced fracture.

For fatigue crack growth tests centre cracked tension (CCT) specimens were machined from the 3.2 mmthick sheet. Loading was in the longitudinal (L) direction and crack growth in the long transverse (T)direction. Compact tension (CT) specimens conforming to ASTM Standard E647-83 (B - 12.7 mm, W - 63.5 mm)were machined from fully heat treated plate specimen blanks. The CT specimens were loaded either in thelongitudinal (L) or short transverse (S) directions and crack growth was in the long transverse (T) orlongitudinal (L) directions respectively.

10.3.2 Fatigue testing conditions

Constant amplitude fatigue crack growth rates were obtained for tests in laboratory air, dry argonand flowing 3.5 % aqueous NaCi under the conditions of stress ratio (R = Smin/Smax) and cycle frequency

shown in table 10.2. CCT specimen crack lengths were measured optically and also using two FRACTOMAT KRAgauges bonded on either side of the centre crack starter. For the CT specimens crack lengths werecalculated from compliance measurements made periodically during the tests.

Since previous work (references 13-15) showed that the most pronounced effects of heat treatment onfatigue crack growth in aluminium alloys are observed at low growth rates in the threshold regime

(da/dn < 10-8

n/cycle) the present work on CT specimens concentrated on this regime. Crack growth rateswere determined for both AK-increasing and AK-decreasing conditions using a computer controlled testsystem with automatic data acquisition and analysis (reference 16). For the AK-decreasing tests atechnique described by Saxena et al. (reference 17) was used to keep the rate of change of plastic zonesize constant as the crack propagates. Thus the load shedding followed an exponential curve given by

aK(a) - AK. exp iC(a-ao)]

where the subscript "o" denotes initial values of crack length a and AK, and the constant C determines therate of decrease.

In addition, the software continually checked the load versus crack opening displacement (COD) datasets, used in the compliance technique to measure crack growth, for the occurrence of crack closure. Thecompliance data were scanned from the maximum load downwards for a 2.5 2 positive change in slope. Thispoint on the load-COD curve was taken to be the closure load, which was then used to define an effectiveAK:

6eff , Kma x - Kclosure

Detailed descriptions of the test system and methods are given in references (l6, 18).

10.4 Results

10.4.1 Effects of RRA on microstructure, mechanical properties and electrical conductivity

The microstructures, short transverse mechanical properties and electrical conductivities of the 7075alloy plate in the T65!, T6RPA and T7351 conditions are shown in figure 10.3. As found previously(reference 5) both RRA and overageing to the T7351 condition increased the size of grain boundaryprecipitates. The grain boundary precipitates had diameters - 20, 75 and 65 nm for the T651, T6RRA andT7351 conditions respectively. A general increase in size of the intragranular (matrix) precipitates isalso apparent in proceeding from the T651 to the T6RRA and T7351 conditions.

It is not the purpose of this contribution to the FACT programme to interpret these microstructuresin detail, but it is worthwhile pointing out that the T6RRA treatment appears to give a microstructurecombining the preferred features of fine matrix precipitates characteristic of the T651 temper with coarsegrain boundary precipitates characteristic of the T7351 temper. These features are believed to beresponsible for the combination of high strength and stress corrosion resistance of the T6RRA material. Asshown in figure 10.3. the yield strengths of the T651 and T6RRA materials were similar and about 8 Xgreater than that of the T7351 material, while the tensile strength of the T6RRA material was halfwaybetween the T651 and T7351 tensile strengths.

10.4.2 Stress corrosion crack growth rates

A series of heat treatments involving retrogression and retrogression and reageing, with retro-gression temperatures of 493 K and 533 K, was carried out with 7075-T651. The Vickers hardness values(VPN) and electrical conductivities (Z IACS) of these materials are listed in table 10.3. Retrogressiontreatments for times up to S minutes at 493 K and 2 minutes at 533 K were effective in providing hardnessvalues comparable to that (- 180 VPN) of the T651 starting material. Also, the retrogression times of Sminutes at 493 K and 2 minutes at 533 K resulted in electrical conductvities higher than that of7075-T651 (33-34 X IACS) and similar to values expected for *075-T7351 (38-42 Z tACS).

14

Stress corrosion crack growth rates are shown in figure 10.4 for 7075-T651, T7351 and T6RRA materials

with retrogression times of 1, 2, 4, 6 and 12 minutes at 493 K. The plateau (stress independent) crack

growth rate for 7075-T651 was ibout 8 x 10-

m/s, and the transition from stress independent to stress

dependent crack growth occurred at a stress intensity factor value of about 10 MPa m. The effect ofincreasing retrogression time at 493 K was to progressively lower the plateau crack growth rate and movethe transition to higher stress intensity factor values.

Material retrogressed for 6 minutes at 493 K had stress corrosion crack growth rates of about

2-4 x0- 10

mIs. This is slightly higher than crack growth rates in 7075-T7351 but much lower than the

plateau crack growth rate for 7075-T651. In view of this result, and also the results of the hardness andelectrical conductivity measurements listed in table 10.3, it appears that an optimum balance of strengthand stress corrosion resistance is obtained with retrogression times of 6-8 minutes at 493 K.

Thus for the second and main parr of this investigation, fatigue crack growth and corrosion fatiguecrack growth resistance tests, it was decided in the case of T6RRA material to concentrate on retro-gression times of 6-8 minutes at 493 K before reageing.

10.4.3 Fatigue and corrosion fatigue crack growth rates for centre cracked tension (CCT) specimens

The fatigue and corrosion fatigue crack growth results for CCT sheet specimens are given in figures10.5-10.7. There are three trends:

(1) For each combination of fatigue environment and cycle frequency the data fall into fairly narrowscatter bands.

(2) 7075-T7351 had the lowest crack growth rates in air. TbRRA material was intermediate, and7075-T651 had the highest crack growth rates.

(3) The overall effect of a lower cycle frequency was to shift the fatigue crack growth rates toslightly higher values. This effect is more noticeamle for fatigue in 3.5 Z aqueous NaC.

10.4.4 Fatigue and corrosion fatigue crack growth rates for compact tension (CT) specimens

As mentioned at the beginning of section 10.3, residual stresses introduced into the 7475 platematerial during heat treatment resulted in spurious fatigue crack growth hehaviour. Specifically, the7475-T6 and -ThRRA specimens required abnormally long times for initiation of fatigue precracks; theprecracks often initiated away from the machined chevron notches and grew on planes not perpendicular tothe loading direction; and the crack length values determined by computer from the compliance plots oftenvaried in an apparently random way. The load-COD plots for these specimens often showed marked non-linearities even in the higher ranges of load employed. In contrast, load-COD plots for 7475-T7351,7075-T651, -ThRRA or -T7351 were essentially linear and showed only minor iidications of curvature at verylow loads, most probably as a normal consequence of fatigue crack closure.

It is suspected that the anomalous behaviour of 7475-T6 and -ThRRA was a consequence of having to doa full solution treatment and age to obtain the T6 and T6RRA conditions from the original T7351 temper. Inparticular, it is thought that residual stresses were introduced by the cold water quench after solutiontreatment. Because of this all subsequent work was done with 7075, which had been received in the stressrelieved T651 condition and could be converted to T6RRA and T7351 without full solution treatment.

Fatigue and corrosion fatigue crack growth rates for 7075 are given in figures 10.8-10.11, which showthe following:

* figure 10.8 : fatigue crack growth in dry argon (no environmental effect)

* figure 10.9 : comparisons of fatigue and corrosion fatigue crack growth

* figure 10.10: effect of cycle frequency on corrosion fatigue crack growth

* figure 10.11: effect of stress ratio on corrosion fatigue crack growth.

Figure 10.8 shows that fatigue crack growth rates for SL orientation specimens fall into narrowscatter bands with 7075-T651 the most resistant at low values of AK and Agef f . LT orientation 7075-T651

specimens had greater resistance to fatigue crack growth than SL orientation 7075-T651 specimens at 4K andAKef f values below 8 MPa m. However, LT orientation 7075-T6RRA specimens were less resistant than SL

specimens, apparently because there was less crack closure.

Figure 10.9 shows the very large environmental effect of fetigue in salt water. There was rare crackclosure in salt water than in argon. Consequently, plotting crack growth rates against iKef f resulted in

an even greater difference between the sets of salt water and argon data. Also the apparent "knees" in the

salt water da/dn - il plots at about 10-8

s/cycle are less evident when the data are corrected for crackclosure. With respect to alloy temper, in salt water 7075-T651 was more resistant than 7075-TbRRA and7075-T7351 at low values of AK and iKeff, but less resistant at higher values.

i - _ - _ -- _

165

Figure 10.10 shows that lowering the cycle frequency from 20 Hz to 2 Hz tended to result Ir. higher

crack growth rates at higher values of AK and AKf f, and lower crack growth rates at low values of AK and

LKeff* It was expected that lower crack growth rates at low AK might be due to enhanced crack closure at

2 Hz, owing to a greater wedging effect of thicker corrosion products formed at this lower frequency.

However, correcting fo. crack closure did not change the relative positions of the data sets. The

da/dn - AK plots show knees at about 10-8

r/cycle. When corrected for crack closure the data show knees at

slightly lower crack growth rates of about 5 x 10- 9

r/cycle. Interestingly, the ranking of alloy tempersat low values of AK and AKef f changed with cycle frequency: 7075-T651 was the most resistant at 20 Hz but

the least resistant at 2 Hz.

Figure 10.11 shows a significant effect of stress ratio on fatigue crack growth rates in salt water

at 2 Hz. The range in crack growth rates plotted against AK covers about one order of magnitude.

Correcting for crack closure reduces the data spread to a factor of 3-5 in growth rates. There are knees

in the da/dn - AK plots at about 10-8 r/cycle. Below the knees the crack growth rates fall away rapidly,

indicating threshold AK values in the range 2.5 - 3.5 MPa m. Values for the three tempers tested at R -0.5 were towards the low end of this range, while at R .- 0.1 the indicated values were towards the high

end. 7075-T651 was generally the least resistant temper at both R values.

10.5 Discussion and Conclusions

Retrogression and reageing of 7075-T651 alloy results in significant increases in resistance tostress corrosion crack growth when retrogression is continued to the region of the secondary hardeningpeak. For the particular heat treatments used in this investigation the retrogression times were about 6-8minutes at 493 K. Plateau stress corrosion crack growth rates were more than an order of magnitude lowerthan that of the T651 material.

In argon the fatigue crack growth rates in SL orientation specimens were similar for all three

tempers. However, below about 10- 8

m/cycle there were indications that 7075-T651 was more resistant than7075-T6RRA and 7075-T7351. This is consistent with the results of other investigators who found that inthe threshold region the fatigue crack growth resistance of various tempers of 7075 increases in goingfrom overaged to peak aged to underaged material. These other studies were done with vacuum (reference15), laboratory air (references 14, 15) and moist air at 95 % relative humidity (reference 13), and atrelatively high frequencies in the range 25-40 Hz.

When tests were done in salt water at 20 Hz the ranking of the tempers remained the same. At thelowest crack growth rates the T651 (peak aged) material still appeared to be more resistant to fatiguecrack growth than the uveraged T7351 material and the T6RRA material. In this respect the T6RRA materialbehaves more like an overaged material than its peak aged T651 equivalent. At higher crack growth rates7075-T651 was the least resistant to fatigue crack growth. This was also observed by Suresh et al.(reference 13) for tests in moist air.

For fatigue in salt water at 2 Hz the differences in crack growth rates between the three tempers inthe near threshold regime were much less than those observed for fatigue in argon or salt water at 20 Hz.Also, the ranking of the tempers changed. 7075-T651 was the most resistant at 20 Hz but the leastresistant at 2 Hz. This indicates that the T7351 and T6RRA materials have greater resistance to corrosionfatigue crack growth.

The longer test dorations at 2 Hz would be expected to result in a greater contribution of corrosionfatigue crack growth to the overall crack growth process than at 20 Hz. However, this was observed onlyfor higher crack growth rates and higher vetues of AK and AKeff , and not fn, the lower crack growth rates

in the near threshold regime. This unusual behaviour was not caused by differences in crack closure andtherefore some other process such as crack tip blunting by anodic dissolution may be responsible. No firmconclusions on this can be reached at present, and the phenomenon is under investigation.

10.6 References

1. B. Cina and B. Ranish. "A new technique for reducing susceptibility to stress corrosion of highstrength aluminium alloys", in Aluminium Industrial Products, American Society for Metals (1974):Pittsburgh.

2. B.M. Cina, "Reducing the susceptibility of alloys, particularly aluminium alloys, to stress corrosioncracking", U.S. Patent 3,856,584, December 24 (1974).

3. W. Wallace, J.C. Beddoes and M.C. de Malherbe, "A new approach to the problem of stress corrosioncracking in 7075-T6 aluminium", Canadian Aeronautics and Space Journal, Vol. 27, pp. 222-232 (1981).

4. U.N. Cong, K. Rajan and W. Wallace, "A TEM study of microstructural changes during retrogression andreageing in 7075 aluminium". Metallurgical Transactions A, Vol. 14A, pp. 1843-1850(1983).

5. K. Rajan, W. Wallace and J.C. Beddoes, "Microstructural study of a high strength stress-corrosionresistant 7075 aluminium alloy", Journal of Materials Science, Vol. 17, pp. 2817-2824 (1982).

6. J.K. Park and A.J. Ardell, "Effect of retrogression and reageing treatments on the microstructure ofAl-7075-T651, Metallurgical Transactions A, Vol. 15A, pp. 1531-1543 (1984).

7. M.V. Islam and W. Wallace, "Retrogression and reageing of 7475 aluminium alloy", Metals Technology,Vol. 10, pp. 386-392 (1983).

166

8. M.V. Islam and W. Wallace, "Stress corrosion crack growth behaviour of 7475 T6 retrogressed andreaged aluminium alloy", Metals Technology, Vol. 11, pp. 220-222 (1984).

9. L. Christodoulou and H.M. Flower, "Hydrogen embrittlement and trapping in Al - 6 7 Zn - 3 z Mg",Acta Metallurgica, Vol. 28, pp. 481-487 (1980).

10. G.M. Scamans and C.D.S. Tuck, "The role of hydrogen in environmentally sensitive mechanical behaviourof AI-Zn-Ma alloys", in Hydrogen Effects in Metals, edited by I.M. Bernstein and A.W. Thompson, TheMetallurgical Society of AIME, pp. 482-495 (1980): Warrendale.

II. M.O. Speidel, "Stress corrosion cracking of aluminium alloys", Metallurgical Transactions A, Vol. 6A,pp. 631-651 (1975).

12. M.D. Speidel, "Current understanding of stress corrosion crack growth in aluminium alloys", in lTeTheory of Stress Corrosion Cracking in Alloys, edited by J.C. Scully, Nato Scientific AffairsDivision, pp. 289-344 (1971): Brussels.

13. S. Suresh, A.K. Vasudevan and p.E, Bretz, "Mechanisms of slow fatigue crack growth in high strengthaluminium alloys: role of microstructure and environment", Metallurgical Transactions A, Vol. I5A,pp. 369-379 (1984).

14. P. Renaud, P. Violan, J. Petit and D. Ferton, "Microstructural influence on fatigue crack growth nearthreshold in 7075 Al alloy", Scripta Metallurgica, Vol. 16, pp. 1311-1316 (1982).

15. R.D. Carter, E.W. Lee, E.A. Starke Jr. and C.J. Beevers, "The effect of micrustructure andenvironment on fatigue crack closure of 7475 aluminium alloy", Metallurgical Transactions A,Vol. 15A, pp. 555-563 (1984).

16. M.D. Raizenne, "KRAKS - an MTS Basic 770 computer program for automated fatigue crack growth ratetesting", National Aeronautical Establishment, National Research Council of Canada ReportLTR-ST-1538, February 1986.

17. A. Saxena, S.J. Hudak, Jr., J.K. Donald and D.W. Schmidt. "Computer-controlled decreasing stressintensity technique for low rate fatigue crack growth testing", Journal of Testing and Evaluation,Vol. 6, pp. 167-174 (1978).

18. A. Baldantoni, "Corrosion sous tension et fatigue-corrosion des materiaux structuraux aeronautiques",Thesis submitted in partial fulfilment of the requirements of Master o)f Applied Science, Departmentof Metallurgical Engineering, Ecole Polytechnique, University of Montreal, June 1985.

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PART IV

EVALUATION OF THE CFCTP AND FACT PROGRAMES

I. INTRODUCTION

In this final Part of the report we shall end avour to assess the total effort involved in the CFCTP

and FACT programmes. First we wish to thank the AARD Structures and Materials Panel (SMP) for enabling the

setting up of an internationally coordinated programme on corrosion fatigue. As figure I shows, this was a

formidable task that has taken more than a decade to accomplish.

2. THE CFCTP CORE PROGRAM4ME

In Parts I and II of this report it was states that the CFCTP core programme consisted of round-rbin

testing whose primary purpose was to establish whether participants could obtain confidence in one

another's fatigue testing capabiilities, especially when using a controlled atmospheric corrosion environment

(salt spray). There were originally eight participants: NADC, University of Saskatchewan, VOUCHT, AFWAINLR, DFVLR, NDRE and RAE. These were subsequently joined by two more, SIFFRL and the University of Pisa.

'The later participants obtained results significantly different from the rest. This was it least

partly due to their being supplied with new specimens which, because they had to be redri'led traminterference to press fit dimensions, turned out to have significantly inferior fatigue properties comp-red

to the first batch. The difference- are regrettable but instructive. They show how Important and necessary

it was to do the CFCTP core programme and, as first intended. to supply participants with specimens fromone batch.

An innovative combination of statistical methods was used to analyse the (FCTP care programme dat.both with respect tc fatigue lives and the locations of primary" origins at tatigue initiation and tro 't-re.

A detailed evaluation, described in Part II of this report, demonstrated the following:

(1) The original eight participants could be confident in one another's evirntrental fati4te testing

capabilities. Thus the primary purpose of the CFIP core programme was achieved.

(2) The first batch of CFCTP core programme specimens and tle mechanical and evir-mental testingconditions were highly reproducible except for the way specimens were origiTali cleaved anddried after pre-exposure to acidified salt water. The cleaning and drying prccedure was '-nrdectto be reproducible, and this amended procedure was stipulated for tie FACT programme.

(3) Environmental effects on fatigue lives were significant and cansistect. Most importantl. it was

found that environmental effects were greater at a higher stress level. This is the pposite Awhat many literature data show. This discrepancy is explalned is, section '..2.1 2f art i ,i i,

due to the fact that the majority of specimens used in environmental totigue te-tig ., simplecoupons, whereas the CFCTP specimens were designed to be realistic representations 'f a 1tia-o

critical structural joint.

It is therefore concluded that realistic specinoms are necessary Ir crrect assesstent of

environmental fatigue effects.

(4) Examination of failed specimens to determine the I otations f primary fitigue orpigic' proved t,

be essential for understanding the fatigne behaviour.

3. THE FACT SUPPLEMENTAL PROGRAM4E

As stated in Part III of this report, the intention of the FACT supplemental pr-oramme ws t, I Itwindividual participants to investigate corrosion fatigue problems it particular relevance to their ,wi

interests and yet maintain a high degree of commonality. To achieve this it was recommended that

* the same specimen configuration (1J dogboe) be used as for the cFPIP core programme

* mechanical and environmental testing conditions he identical

* efforts be made to obt.in moterials of mutual interest fiom on Iheat.

An overview of tie FACT programme is given in figure 2. There were ten participants: Vii hil. SAAB, NAys.,AFhAL, SURE, NLR, LRTH, IAG,, RAE and NRC. Six had also taken part in the CFTP core programme, niel\VOUGHT, NADC, AFWAL, NDRE, NLR and RAE. Figure 2 shows similarities and commonalities in the individual

programmes. Most participants tested l dogbone specimens under etninally identical mechanical andenvironmental conditions. The fatigue loadings were constant amplitude, as in tie "PCTP. the manvenore

spectrum FALSTAFF (references 1-3) and the gust spectrum MINITWIST (reference ) . lice environmental-- conditions generally included two or sore of those in the CFCTP. Notable exceptions were in tie SAAb and

NRC programmes.

The main interest of several participants was to c-mpare - in their hidividuai programmes - theenvironmental fatigue properties of a number of aluMinium allhtys in varitlts tempers. nowever, owing t , thecalibratory function of the CFCTP and the participants' active cooperat ion in ,btaitaing the rittssimilarities and commonalities within the iACT programme, it is possible to mahe inter-participant

comparisons of materials, protectmen sys'ems tsd fasteners as well. Ftrthermore. the c-cal testing tf-tpr-vided mart. data for -empiring fatigue eftn-ts under constant amplitud d FAI 1II 1, cding. the latterbeing a realistic :eslic load history ftr tactiocl aircraft.

-.4

176

For detailed analyses of the results, discussions and conclusions of the individual contributions tothe FACT programme we refer the reader to Part III of this report. Here we shall discuss inter-participantcomparisons of materials, protection systems and fasteners in section 3.1. and comparls)ns of environmentalfatigue effects under constant amplitude and spectrum loading in section 3.2.

3.1 Inter-Partlcipant Comparisons of Materials, Protection Systems and Fasteners

The possibilities for inter-participant comparisons of materials, protection systems and fasteners aresuemarised in table I. Comparisons of fatigue lives per fatigue testing schedule, load history and stresslevel are made in figures 3-6 and show the following:

(I) Fr constant amplitude fatigue at a higher stress level (S = 210 MPa) the fatigue lives of7075-T6 and 7075-T76 specimens were equivalent. Fatigue lives were not significantly prolonged bythe use of interference fit fasteners, a flexible primer (Koroflex) or interfay sealant.

(2) For constant amplitude fatigue at a lower stress level (Sma - 144 MPa) the rankings of

materials, protection systems and fasteners depend on environment, except that 7075-T6 specimenshad significantly shorter average fatigue lives than the rest.

For fatigue in air the fatigue lives of 7075-TRRA, 7075-T76 and 7475-T7h clad specimen. wereequivalent. The use of interference fit fasteners (7075-T76) and Interfay (7475-T761 clad) wasbeneficial to fatigue lives.

For pre-exposure + fatigue in salt spray the 7475-T761 clad specimens had longer average fatiguelives; 7075-T6RRA, 7075-T76 and 2024-T3 Alclad specimens had equivalent fatigue lives; and7075-TI specimens had shorter average fatigue lives. The use of interference fit fasteners andsealant in fastener holes (7075-T7h) was not beneficial. However, interfay sealant prolonged thefatigue lives of 7475-T761 clad specimens.

(3) For FALSTAFF fatigue at a higher stress level (S.a = 289 MPa) the average fatigue livas ,f

vpealnensf wtb interfay sealant were longer (sealant in fastener holes was most probably nothenetilalT. Yhe relatively good perform once of /O75-T6 specimens is noteworthy. The reason foe

Ihis may be that a hIgh yield srengcth helps to Postpone fatigue crack initiati-r at high stresslevels.

4) For FALSTAFF fatigue at a lower stress level (Sd, - 238 MPs) the fatigue fiscs f 000t psyei-res

were equ ivalont. The average fatigue lives of 7075-T6 specimens with the NF-5 protection systerwer shorter than the rest, including 7075-T6 specimens with the MRCA protestirn system adi Iterfay scolant. Since these 7075-Tv specimens were from the same hatch of material, it may 5rcansluded that the MiCA protestion system with foterfay sralant hnable. significantly (oncrfatigne lives than the NF- patection systen. . .owever , ln er- the .ass ncsistntbenefit from uoing on irerfay sealant. Nor can the sue f inrerforence fit fasteners or sealisin, fastener holes beneficial.

It Is clear from the foregoing observations that there are no overall trends with respect t, coteraand protection nytem ranki gs. Nesertheleus, significant improvements in enirnmental tatigue ret,. 'jcare obtainable through choice of improved materials, heat treatment and protection sVst,-v. I porti~ ir,the use af an interfay seali't can be recommended because it is sometimes very beneicial. There a-several possib Ie reasons for thin. inhibition of corrosion and fretting can postpone attigue crashinitiation at faying surfaces. And improved load transfer (via the sealant) can reduc the stressconcentrat100s at fastener holes. This has bee observed in preliminary work at the NLR using a SPAIE 6o0theenoelautis stress analysis senera.

Since there are no overall trends with respect to materials and prot-ectios syst.s, it mo beoncluded that their evaluation requires realistic load histories, stress leveis and cnvironmnts. vhis200 lusloi adds to one I . otIn 2 concerning the CFCTP core programme, namely that realisti spe imenare nenssary for corect ansessment of environmental fatigu effects.

There was, however, a,' overall trend with respect to fastener fit. The use at interference fit Hi-Loksand SLEVholts instead of press fit Hi-Lok I - net beneficial to fatigue lives. A simi-.r result wasobtained in the ACARD-coordisnted Fatigue Rated Fastener Systems programme (reference 5) for specimezs wlthmoderate to high values of secondary heding ratio ISBi). s discsusd is Appendixo I. tho Sii for

If dogbone spe imens varies from 0.2 for press fit Hi-Loks to 0.4-0.5 for interference fit SLiEVbolts andHI-Loks. Thus it is most likely that any potential improvement in fatigue life Irom using interference titfasteners was counteracted by an increase in Mii.

I" view of the foregoing, It say he qnestioned whethec the If dogbone specimen configuration issiable for the eluation of different fastener systems. Insofar as this specimen type is realistic forcertain types of aircraft structural joInts, the answer is Yes. However the present results, i.e. nosignificant differences in fatigne lives ro specimens with press and interference fit fasteners, shouldnot be extrapolated to other types of joints, part icularly those with low secondary bending ratios.

3.2 Inter-Participant Conparisons of Environmental Fatigue Effects

Inter-partici pant comparisons of nenvironmetal fatigue effect. nder constant amplitude arid manioevrespectrum (FAISTAFF) loading at different stress levels are shown in figure 7. Environmental effects weregreater at higher stresu levels. This Is the sane trend found for tire fFCTP core programme and, asmentioned in section 2. it in the opposite of what one would expect from literature data, which refermostly te simple coupos specimens.

i"

177

The reason for this discrepancy is as follows (see also section 4.2.1 of Part Ii of this report).Higher stress levels and environmental fatigue testing (pre-exposure + fatigue in salt spray) promotedfatigue initiation in the bores and at bore/faying surface corners of the fastener holes in Ij dsgbonespecimens. On the other hand, lower stress levels favoured fatigue initiation at the faying surfaces. It is

most likely that environmental effects will be greater when they promote characteristic failure modes. Thisexplains why the observed environmental effects were greater - on the average - at higher stress levels.

As before, it is concluded that this distinct difference in environmental fatigue behaviour betweensimple coupons and li dogbone specimens, which were designed to simulate an actual joint, shows thatrealistic specimens are necessary for correct assessment of environmental fatigue effects.

4. CONCLUSIONS AND RECOMM*ENDATIONS

The main objectives of the CFCTP and FACT programmes were:

" assessment of the effectiveness of state-of-the-art protection schemes for aluminium alloys with

respect to corrosion fatigue and corrosion + fatigue

* stimulation of the development of new protection products, procedures and techniques

* bringing together researchers on both sides of the Atlantic in a common testing effort that wouldresult in a better understanding ot the corrosion fatigue phenomenon and the means of mitigating it

for aerospace alloys

* enabling participating laboratories to add to their fatigue testing capabilities by using a

controlled atmospheric corrosion environment.

This report demonstrates that the first, third and fourth objectives have been achieved. It also provides

many data on a broad, international basis for achieving the second objective.

Much remains to be done to increase the understanding of aircraft corrosion fatigue and theeffectiveness of protection systems. The incentive is present in the FACT programme results: significantimprovements in corrosion fatigue resistance are obtainable.

The degree of improvement depends on specimen configuration, fatigue load history, stress level andenvironment. It is therefore essential to evaluate potential improvements in materials, protection systems

and fasteners by testing realistic specimens under representative fatigue load histories with environments

simulating mission service conditions as closely as possible.

5. REFEhNCES

I. G.M. van Dijk and J.B. de ionge, "introduction to a fighter aircraft loading standard for fatigue

evaluation "FA.STAFF"", Paper 3.61, Problems with Fatigue in Aircraft, Proceedings of the Eighth ICAFSymposium, compiled and edited by J. Pranger and F. Berger, Swiss Federal Aircraft Fstablishment F+W)

1975: Emmen, Switzerland.

2. "Description of a Fighter Aircraft Loading STAndard For Fatigue evaluation," Combined Report of theF+W, LBP, NLR and lABC, March 197h.

i. J.B. de Jonge, "Additional infor-at Ion aboor FALSTAFF". NlR Technical Report TR 79056 U, June 1979,

4. H. l.owak, J.. de Jange, J. Franz and D. Schiltz, "MINITWIST. A shortened version of TWIST", CombinedReport of the LBF and NIR, NLR Miscellaneous Publication MP 29018 1', May 1979.

5. H.H. van der Linden, "Fatigue rated fastener systems", AGARD Report No. 721, November 1985.

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184

FALSTAFF, S... = 238 MPa FATIGUE IN AIR

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187

APPENDIX I

LOAD TRANSFER AND SECONDARY BENDING IN THE Ii DOGBONE SPECIMEN

I. INTRODUCTION

This Appendix describes the load transfer and secondary bending characteristics of Ii dogbonespecimens similar to those used In the CFCT? core programme and reconended for the FACT supplementalprogramme. The CFCTP and FACT specimen configuration is illustrated in figure 1. The specimen configurationwas designed to simulate the load transfer and secondary bending characteristics of runouts of stiffenersattached to the outer skin of an airframe structure. The design goals were a load transfer of 40 % andsecondary bending ratio of 0.5 (reference I). These parameters are defined in figure 2.

The NLR and the University of Pisa conducted a programme to determine the actual values of loadtransfer (LT) and secondary bending ratio (SBR) in ii dogbone specimens (references 2,3). This programmewas based on specimen requirements for the AGARD-coordinated Fatigue Rated Fastener Systems (FRFS)programme (reference 2). The specimen configuration was identIcal to th-t in figure I except for thealuminium alloy sheet thickness and fastener fit, as follows:

CFCTP AND FACT LT AND SR PROGRAMME

ALUMINIUM ALLOY 3.2 mm 5 mmSHEET THICKNESS

FASTENER TYPE Hi-Lok Hi-Luk Hi-Tigue

NOMINAL FIT slight press : - 0.019 clearance : 0.020OF FASTENERS * interference : - 0.077 interference : - 0.025

• Dimensions in millimetres. + - clearance, - interference.

Despite the differences in specimen configuration the results of the load transfer and secondarybending ratio programme are relevant to the behaviour of the 11 dogbone specimens used in the CFCTP andFACT Programmes.

2. OVERVIEW OF THE LOAD TRANSFER AND SECONDARY BENDING RATIO PROGRAMME

An overview of the load transfer and secondary bending ratio (LT and SBR) programme Is given in tableI. The number, positions and dimensions of the strain gauges on the fatigue specimen -1 are important. Adetailed discussion of these aspects is given in reference (2). An example of strain gaugeing the fatiguespecimen -1 Is given in figure 3a. The strain gauges and wire leads at the faying surface were accommodatedby shallow recesses milled in the half plate -2, figure 3b.

The strain gauges were bonded to the fatigue specimens after priming and before assembly intoli dogbones. Assembly was done using the appropriate fasteners and with polysulphide sealant at the layingsurfaces and in the fastener holes.

The specimens were fatigue cycled under constant amplitude loading with maximum stress Sma x = 250 MPaand a stress ratio R - Sm in/Sma of 0. Fatigue cycling was interrupted at fixed intervals, e.g. cycles I,

5, 100, 1000, 5000 and 10,000, In order to measure strains in a "static" loading test with S... - 250 MPaand Smin - - 67 MPa.

3. RESULrS

The results of the LT and SBR programme are compiled in references (2,3). Figure 4 presentscharacteristic values of load transfer and secondary bending ratio at Sa x - 250 MPa. The following trendscan be observed:

(I) Load transfer is almost independent of fastener fit, with a typical value of 27 %. This islower than the design goal of 40 2.

(2) Secondary bending ratio depends strongly on fastener fit, reaching a maximum of about 0.47 for

an Interference fit of - 0.070 mm. Again the values are lower than the design goal of 0.5.

4. ESTIMATION OF LOAD TRANSFER AND SECONDARY BENDING RATIO IN THE CFCTP AND FACT SPECIMENS

Most 11 dogbone specimens for the CFCTP and FACT programmes were assembled using Hi-Loks and aslight press fit resulting in a nominal interference of - 0,019 mm. The NADC, AFWAL and RAE contributionsto the FACT programme also included specimens with higher interference fit fasteners, see sections 4, 5 and9 of Part Ill of this report. Estimates of the load transfer and secondary bending characteristics of thethree specimen types have been made using the LT and SBR programme data, specifically the correlationsbetween fastener fit, load transfer and secondary bending ratio. These estimates are as follows:

I 1w

Sa 250 MPaFASTENER NOMI!I,,L FIT OFTYPE FASTNERS * LOAD SECONDARY BENDING

TRANSFER RATIO

CFCTP AND FACT PROGRAMMES Hi-Lok slight press -0.019 24 % 0.20

NADC AND RAE CONTRIBUIONS TO FACT Hi-Lok interference -0.077 30 2 0.51

AFWAL CONTRIBUTION TO FACT SLEEVbolt interference -0.064 29 X 0.44

*Dimensions in millimetres. -=interference.

5. REFERENCES

i. D. Schltz and JUj Gerharz, "Schs.Engfestigkeit von FOgungen mit Sonderbefestigungselementen".Fraunhofer-Institut fijr Betriebsfestigkeit Technische Mitteilungen TM 69/73. 1973.

2. H.H. van der Linden, "Fatigue rated fastener systems", AGARD Repnrt No. 721, November 1985.

3. N.N. van der Linden, L. Lazzerl and A. Lanciotti, "Fatigue rated fastener systems in Ij dogbonespecimens", NLR Technical Report TN 86082 U, August 1986.

IS9

TABLE 1: OVERVIEW OF THE LOAD TRANSFER AND SECONDARY BENDING RATIO PROGRAMME

MATERIAL a 5 mm thick 7075-T76 aluminium alloy sheet

CLEARANCE OR INTERFERENCE FIT

SPECIMEN a i LaksOR H-Tipmw

300mm

FASiENER HOLE QUALITY * Reamed with or without prior cold work

PROTECTION SYSTEM 0 Chromate containing epoxy primer + polysulphide sealant in fastener

holes and at interfays

SPECIMEN * Strain gauges on fatigue specimen -1

INSTRUMENTATION

LOADING CONDITIONS 0 Constant amplitude fatigue cycling with intermittent measurements

of strains

REFERENCE 2 3

NUMBER OF

STRAIN GAUGES 18 22PER SPECIMEN

TEST PROGRAMME FASTENER cold worked cold worked

HOLE reamed + reamed +

QUALITY reamed reamed

FASTENER Hi-Lok Hi-Lok Hi-Tigue Hi-Tigue

FASTENER clearance low high highFIT interference interference interferonce

)\OL FAA EELI SPOCEMI

INK. 55.052 1. 2

3S SPACEON

--------- ---- ----

SECIO C-C LGUESNSIONSIM

30.0 20.00.

2BS.d=05.

Fig 1AFLr 3)e SPT oeporaea eomne ATsupeetlpormeACmER

2) HALFPLATE

LOA TRNFR*TOTAL LOAD P *- BYPASS LOAD B

1) FATCSIGUESPECIMEN

LOADTRANSFER- T 0100%P, + P.

2) HALfPLATE

I/ IATIG/iE SPECIMEN

ISECONDARY BENDING=

WHERE ea AND e b ARE THE AXIAL ANDI

BENDING STRAINS RESPECTIVELY

Fig. 2 Definition of load transfer and secondary bending for the 1i dogbone specimen

LOCATIONS OF STRAIN GAUGES1

ON FATIGUE SPECIMEN t) 1 50

'2 3 .56 6i 32?

b0 8 n DEEP RECESSES ON HALF I[ 17 20

PLATE Vl FAY/I/I SURF/,.? VIE A7 2

___ 0MEuNSIOSI

Fig. 3 Example of strain gaugeing a ii dogbone specimen for measurement of load transfer

and secondary bending ratio

LOAD TRANSFER (%)0 10 20 30 40 50 s

SECONDANDAR SENNNGINGI

F 4 r ef fitd r a n s f e .e in e o d e d e b ~ c e i t c e o i d g o e n e i e n ~

AIiFYTII 11

"TA! I I I CAL ISF1 HP

Th a t he~ st 11 ds se t i I5 s te FIL r rite d -f S' sicpi,, i rcdI i s t oi-rii di l

"Isrcv o tfie stt I-t 1, li 11,t 1 -A p redi, I*r as i g tlee CFCTF core lr.ae risce fatin . ic

nia pr--,ar I iaigne- L'- 'L tic se fit fciet hr 1 et, at igie Ii fe, aalta were t ira t eheced t -r

c-iaiits l ti Ies5 i ITie ir a, 11s si s la t e C-1) I i-Lc W Ith these eand iti ii. i s s-iff ic-i est ast-rce-fi II t t 1,l r LeItLi,,ll 1 ' aci~jLsis -w tiltp 1, Fac tor atia Iyis of var i ance . This cs

1 I oed F-, 'i:,* -- ,ing -sce gitIi cist d i' f frerere tester ur .. ee's. -,s iti:lt ipc ri 1:,Ce tcct.

bs Lcs hhe 1 -1-c Le~ 1 t -, 1 1e rn test was aprpIie d rsly dies analysisiar-cIc~ t t -icts I- mlt ipepi ricgl te't en'r hil lned whether -c tint -I, s

U-en "c1 i1t 'i~Ii i t i - t ric P' "I r 1 -cise the Ii < i sgriii ieast diff- eciicc test .c ai Icwcsiiiit.. I-' 'Iis - whcir -sayissi oisatae did sat indieste -ign L't

r T cj tlini t.ns ,,dtohe~ or adequ-te -irplI ce ) sed -'-,ItcL i ii till iitgi Ii L

ser ii tit ,, t lin tt 11 l! Ll the X2tet 'I -edepesdooi , Yaltes erre' ted atst reiire-,at tes1t, niec1e: ,'" ilp-pri~t- I-r these tchts it is sefliciestto I" is 'C.1

t ,ljI ;,Ct i t- tr' id's sen Ile II Ihas teSts were Also aced (ais lipi,,riats, to chech iee

ti ," !, ae r 'te'- 1'--ce itji'' 11-s en prismrs irticee ori-giss icr etch f -l rf ceiu1teta 'i ei ge ira see red tsing ssldnIles, whIticih are gi-ve hiere toer rsiercses:

-~~~A 1___,;iiIL FS i IIIl' SI:HFD IFS

-- ~~ ~ ~~~ ~~~ t <i 7 <__ -__ ___

ii~~~ l Ili i i

ili t ti - . Ce 1c

''t-si - tf

kb Leer a h oui ft q nP(' 1 from the appropr ate salon ol,'ie the

tO tlgnal teX distribu tin(nlddi otsadr et rs~tion ot'd mae opiprcimatn

dau f zi istriuto provide thet these logaith of eac datu isosed

4~~ Thee r seer etos of es ting frauesgeo of aroiances. For thei CaxiTP corelprihood tw

et e eusd, bath ate whc- redsuseendeoen.()

7i) Ifrtlett.sdtest whic is e sand owhesi.I2> ,rj~ the sampeoSanesslrge

repeo ohsny ruplcoe varae is t ie s than n 4. tbe2 hs hw htL daaalcxm

C he o~~~~~~~isr b iectroideo th's et st hc h yohsstat the oriantes cf fc hau pisltle usre

'qo.Thise Iresedoeabymeosimaeting thehmgeetyo variances. For each ofT thcismlsore folleiimm the

oeptat aal peurtes t ind tal3, which is asdwe tesm lost~ islf-gespaaoy.h wetssae st

scio.The statistic uned in Bartiett's teat is

-I-IL wt ideyross offren

or F =-=with and -degrees oh freedom.

bos's tesc chige tl dcenlnIrar''r ,f the st,,t lric F a fol lows:

P with aed s degrees ot eeoiFo /

%, t, that for large simples Bartlett's test aed in1's test arc coeslstunt with each otnier.

The calculated values s2 and F, ore comparod With appropriate naliws of Xa and F- en tables of the X2

-u distrit tice ( included I,, most stacdard tests , 1 statistics) . The ,ppropriate valses of he a:.d F are

listed under the chosen significance tlesel -a and fir -sd a , degrees of freedom rnspncrieelv. If X2

and F. are less thon r equal to the respect ise 2 a,d F so) ss, the hspcchns is that the oar ianc; .12 )f

tie , yopiiiaiio- are equal is cot toected. If Xa and F. ice grocter thA: the respect ise an-d F atc

tne hapotts is is rejected.

[snles o tie is. of hart et's test and ion's test far the iP(TP core irg romne data ore given inIIc F.ile I 'ea10sos ot tie fat ig'ie iso;, were -ad tor all caic'ilat iaeasic both testsasne

ANALYSIS ,;F VARIK f

Avas gs l s'ci'ie - astatlstical tecil-i tor coopsring titte or more sets at esperirertal data

t. ,ercioe th e e ttlIct vr can cos In,,tern s esycLo: e n tal svacri ablIesi ', 4se characcc'e ri s t ico f a prodac t o rp -, '.. In a' a Iy , Is ho; bsed on !;epatat Lois the total oariation pcct eet in the- do to sets into parts, each

ot whitlia ase sriabiltt oitrih-tahbe to a syeclfic scarce.

uhrineW wasoe)1_i 14m visrii'tce was ssed fir the TyrHf corn yr -grae. lv a th-te-ay arolosis of.'inc wht1 i cva1- te- the s I m o iace Ls neIvc t s ofI t hree !oc to rs . Lt.o saotrces of va ri at ioi -'to

.ar111C ' dlie toI eocI h sam ac ILt-". i cterac t ieg Iact )Ir and resided., iesacnc- tt err r . The patti; of

-i rtaiic .- i-alcrci l-,-~c the -me: inalvsis of varlotiel fr sci:Lfisait dlfterevees betweeni thesiL, i- mporiog tiri sea:. Thin di l;;tie t' tnd In tha.t ihe sean. if F pepsiaties tram

aih k-c 1 le are bht,,i-d are eqic!]

Toe procedure for testing this hypothesis is IIlustratcd scematicIyIV in table 5 for a fairk simple

hrk, ector nspriment . The three maiii factors are assigned to columns (c), rows (r and 4roups g) . In

tiue cun-putatioris tile sums of tile squares rid mean squares are determined for each main factor, the two-way

icteractions, the three-way interaction and the residual error term. The ratio of each mean sqcare to the

residual ram square prosides - -ues of the statistic P ., which is then compared with appropriate F values

ron. the F distribution table. When F is greater than F the infl ieoce of a factor or cohmination of

actor; cn the doto is considered significant. When P i, less than or equal to F onc differences in neacs

are indicated to be due t, chance or experiment. I error only.

Certain issumpticis ore necessary when analysis of variance is used. These are:

(I) The data represent random sooples fros normally distributed populations.

) The variances of these populations ore equal.

Failure t, meet these assumptions may affect the validity of the analysis. However, the F distribution is

%ery robost, i.e. "forgiving", with respect to violation of the assumptions, so that moderate violations

hosid not oitect the outcome of the analysis. For the CFCTP core programme the data were found to be

Io-oraI or to approcimate iog-normal distributions, and there were only a few slight-to-moderate

violatio os if tile criteria for homogeneity of variances (see sections 3.2.1 and 3.2.2 ut Part II of this

reort) . These resnlts were considered sufficient for continuing the statistical treatment of the fatigue

Ilife data provided that tile logrithms cf the fatigue lives were used.

Tabe h Fives a schematic of the three-way analysis of variance for the CFCTP core programme. The

inpt data are much more exteesive than the schematic three factor experiment plan in table 5. This is why

the analysis was dore using a computer program called "ANOVA", which is part of the well-known Statist-al

Vfcksge flr the Social Sciences (reference 5).

a. "1h TIUNING" WiTH THE LEAST SIGNIFICANT DIFFEREiNcE TFST

shown in figure 1, significant effects indicated by analysis of variase .,-re Investigated in more

detail ("fine tuning") using the least significant difference test (references (,,7). Prom table Lhe

significot erfects Indicated by analysis of variance were

a laboratory

* stress

* encirlmeot

* stress: environment.

However, it was not necessary to analyse the effect of stress level in more detail. Since there were -nly

two stress levels it is obvious that the significant difference Is between them.

The least significant difference test locates the source or sources 01 the significant difference in

the data. This is done by comparing all possible comhirations of two means In the k samples in order todrtermive which of the h (k-1) comparisons are significant and which are not. The test is based on thestaiitic t, which can he expressed as

5t -)X

resma n i n

where X ;Lud n. are tie means of two samples of sices n and oe respectively, and MSresidual is the

residual mean square ubtained from analysis of variance (see table 5). The procedsrs for the least

significant difference test is given in table 7 for the usual case of equal sample sizes and in modifiedtorm ror unequal sample sizes. Tabulated values of the t distribition for various significance levels anddegrees of freedom are included in most standard texts on statistics. The criteria for indication of a

significant difference hetooen two means are

- J- a

l t I • t~l Siire, sidmal

hcomples if tile use if tiue least signilfluant dirlererce test to locate the 50, rce or sources of a

igoi!,-at eteot iLudicated lhy ..uu is ,f variance -ire given in table 8. Nite that omisslon of data for

rea:embled specimens resulted out ,uuilc In unequ.iI almiple sizes, het al o changed tile values of MS res idutal

. , uuh".ined iin a fls i'I voriacct. In ther words, one caiiorut simpiy omit data in the "fine

tulrlg" st,.ge. A cirplete re-na lysis ,1 carlance has t., be dune as well.

h. "FINE TUNING" WITH DUNCAN'S NEW MULTIPLE RANGE TEST

As shown in figure 1, Duncan's new multiple range test (references 8,9) was used to inuestigate in

more detail the experimental variabs, or their interactioss, that were eot tud to be significant by

analysis of variance. From table 6 these are

" laboratory: stress

* laboratory: environment

* laboratory: stress: environment.

However, it should be noted that Duncan's test can be used whether or not tle analysis of variance

indicates a significant effect. Like the least significant difference test, Duncan's new mul iple range

test locates the source or sources of the significant difference in the data by comparing all possible

combinations of two means in the k samples in order to determine which of the k (k-1) comparisons are

significant and which are not. The test is based ol the range of the k means., .e. tie difference between

the smallest and largest means of the samples involved in a comparison. The difference between ane tworanked means is significant if it exceeds a shortest signilicant range (SSR).

The procedure fur Dunean's test is best illustrated using actual examples from the CFCTP core

programme. Table 9 gives an example for the usual case of equal sample sizes. As shown in the table. tie

procedure consists of the following steps:

(1) hank tie means and calculate the standard error of the mean s- from the residual mean sx-rex.Piresidual (obtained from analysis ol variance) and the sample scze n.

(2) Choose a significance level a and determine the significant studentized ranges ? for appropriate

values of p, the number of means involved in a comparison, and Iresidual degrees of freedom.

Tables of z values are available e.g. in references (8,9).

(3) Calculate the shortest signIficant ranges SSR from the products tf sZ and the appropriate zvalues.

(4 T lest the differences between means in the following order: largest mrxus the smialest, largestminus the second smallest, and so or, ending with the second ma llest mius the smallest. Gith

one exception each dincerence is declared significant i: it exceeds the correspacdieg sso.otherwise ir is declared insigulfic-t. The exception is that nu ditfereoe between t.' eons cac

be declared signif icact if they are both contained in a sob-set of mea,.s which han

non-sugnificant range. Thus as soon as a ncn-sigrlficant ditterence betwseen two means ix fund,

the remaining differences between these uioins and all the intervening ones are insigniticTt and

need not be tested against the SSR. However, this rtstilxg is shown f-r completeness in table Q,

Table I gives an example of Duncan's test fur unequal sample sizes. The procedure is much the same isfor equal sample sizes except tiat s MT esdual is used instead it x an the difference between two

means u, and n is multiplied by where n and u -re the sumple sizes for -ch mean. Ausax

mentioned in section 5, it should be noted that ,mission oi data for ressembled specimens resulted not

only in unequal sample sizes, out als changed the values of 11Sre du and ridual obttined tram

.,,xlysis of variance.

i. LIPSON AND SHETH METHID FOR ADEQUATE SA.MPLE SIZE

Scatter in the CFCTP core programne fatigue life dato was used to) check tor odqtuaor of samp , si e

(iour sprcimens per test condition per participant). The metbod used is due to ,i yups and ohech (ref se

IS) and involves selecting an) acceptable error Levei, usually 5 7 and I0 7, and tindiog the required sample

size for a particular confidence level. The sample size check has two purp. sex:

(l) To find the combin.ation of error and cooftidence levels tIr which the actua l sample nize was

su ficient.

(2) Io give am Indication ut diflerences it: data scatter between p.rticipatt ard fatigue text

eond it ions.

Table I! llustr,,tes the lipssr, t.nld Sher, r -rlod by cIce at: actiul tYample from the tIK.1' , r

e

programme. ihe table is largely self-explonatory. O1 the basis o ,1 cc normal distrihut ic fr the

populat ion tie percent coeficient if variati In is calculated. An error evel is selected and tit percent

error divided by the percent coefficient of vaeatltn is used r, grapht:ally detrmlle the tearest inte.er

sample size for a given, confidence level. Tihe curvec it: the graph ,;-, derived trest the t distrbutjotaccording to the following expression:

U Ehiffl f0I5a'Z COEFFICIENT OF VAIATIOD -

where a Is tine signif icane level and n and ; are the appropr ate sampl, siees and d egrets at 1 r,-dom, - tt-l).

To indicate dlifferemoes in data scatter the req:ied sample sines ,- a giuen cubln at ct'rr -tttd

confidence levels were determined for the complete set I CVFlTP' core programme !,itis ue life data, assh,wnIn table 12. the shade

0 regions demote exceedan(e Id tile actt. -ample sil, an, ., larger requireo sanple

size reflects greater scatter In the data.

8. X2 TEST OF INDEPENPENCE, YATES' CORRECTED X' TEST AND FiSEK'S EXACT TEST

As mentioned in the ivtrductoc to this Appendix and shown i, figare 1, the X2

test cf i'idependeuce,Yates corrected 4

2 test or Fisher's exact test were used, as appropriate, to analy'se the primry atigut

origin data with respect to the inriuencn of stress level and environment. Ic jIdition, tee tests wereused (as appropriate) to check whether there were significant correlations between fatigue lis arixlocations o f primary fatigue origins for each of the eight combination~s of latigue stress ievels and

testing schedules.

8.1 A2 Test of Independence

The X2

test of independence (reference 1) involves testing the hypothesis that two variables orcharacteristics of a sample are independent of each other. Data for this test ore arranged in a table whichshows one characteristic and its r categories down the left side of the table, on the other characteristicand its c categories dcross the top. Thir table is known as a contingency table. It has r rows andc columns that form cells in the body of the table. Each cell contains the number of sample nnunersobserved to have each particular combination of the characteristics being examined.

8..l Analysis of primary fatigue origin data

Construction of a contingency table and the procedure for the X2

test to analyse the priary fatigueorigin data with respect to stress level and environment will be illustrated using table 13, which gives anexample from the CFCTP core programme. The test compares the observed frequencies of occurrence f in eachcell with the theoretically expected frequency I if the hypothesis of independence is true. The oxpectedfrequency tar a cell is obtained from the product of the total of the row and total of the column in whichthe cell appears, divided by the total number of observations. The sum of th, expected frequencies shouldequal the total number of observations.

Application of the X2 test is reliable only if every expected frequency is at least five, If thisrequirement is not satisfied the results of two or more categories must be combined to raise the expectedfrequency to the necessary level. The initial contingency table in table 13 does not contain enough B/N,C/O and D/P primary fatigue origins to give expected frequencies of five in each cell. Therefore the B/N,L/O and DIP categories were combined with the E/ category in a modified contingency table.

The procedure is then as follows:

(n- ft)O

(1) Compute the deviation X2 =

(2) Choose a significance level a (e.g. 5 1, I%).

(3) Determine the solution c of the equation PFx2

c) - 1-0 from the appropriate value of X' in thetable of the X

2 distribution (included in most standard texts on statistics). The appropriate

value of X2 is listed under the chosen value of , and for (r-ll(c-l) degrees of freedom.

(4) If 6 c, do not reject the hypothesis. If c2 > c, reject the hypothesis,

0 0

For the example it table 13 the hypothesis is rejected, i.e. it is concluded that the locations ofprimary fatigue origins depend on the environments (fatigue testing schedules).

8.2 Yates' Corrected X2 Test

A slight modification of the Y2

test is usually recommended for contingency tables with r-2 and c-2(one degree of freedom). This modification is known as Yates' correction for continuity (reference I). Itis used to correct for the fact that the X

2 distribution is continuous whereas the observed frequencies are

discrete.

The only change is that the formula S2 (If - ft)a/ft is modified to

nn _ it -0. t

ft

where I f- ft is the absolute value of (f1 - ft. is always smaller then x12. This means that the

hypothesis of independence is more readily accepted, I.e. Sates' corrected t test is more conservative.

8,2.1 Correlation of fatigue lives and primary fatigue origins

Table 14 gives an example of using Yates' corrected r2

test to check an association between fatiguelives and primary fatigue origins. The fatigue life data were arranged in ascending order together with thecorresponding primary fatigue origins. The median value of fatigue life was used to separate the data intotwo columns for the contingency table. The median value was used Instead of the mean because the median Isless affected by data scatter.

The hypothesis to be tested is that the locations of primary fatigue origins do not depend on fatiguelife. The initial contingency table in table 14 does not contain enough F/R and G/iS primary fatigue originsto give expected frequencies of five In each cell. Therefore the F/R and G/S categories were combined in amodified contingency table.

I 'iI

The results in table 14 indicate that the hypothesis should be accepted, .e. it is concluded that tor

this fatigue test condition (fatigue in air at Sma = 21t MPo) the locations of primary fatigue rigins donot depend on the fatigue lives.

8.3 Fisher's Exact Test

Fisher'a exact test (reference 12) is used for contingency tables with r=2 and c=2 when the taoalsample size is 20 or when the sample size Is between 10 and 40 and the small ct expected frequency isless than five. This Is because Yates' corrected X

2 test is iraccurate ior small numbers. In Fisher's test

a probability is calculated from the values in the contingency table and is compared to the actual value ofa chosen significance level a. For a 2 X 2 contingency table cortainlng four values a, b, r, d; marginaltotals n . n , n , n and a grand total N, thus:

I 2 3 4

a b n

c d n

n n N

The probability P Is given by

n X n X n ! X n!1 2 3 5

= N! X a! X b! X c! T d!

An illustration of Fisher's exact test is given in table 5. The initial contingency table is modifiedby combining the E/Q and C/O categories and the F/R and G/S catefories. The probability F s calculatedfrom the modified contingency table.

The hypothesis to be tested is that the locations of primary faticue origins do not depend or faticorlife. The hypothesis is accepted if the calculated probability is gr,ater than ,,. The result in table 15indicates that the hypothesis should be rejected, i.e. it is concluded that for this fatigue test condition

(fatigue in salt spray at Smax = 144 MPa) the locations of prinar fotigue origins depeid on the fotigue

lives.

9. REFERENCES

1. G.M. Smith, A Simplified Guide to Sartistics, Twelfth Printing, Rinehart and Company, Inc., pp. 80'16(1958): New York.

2. E. Kreyszlg, Advanced Engineering Mathematics, Third Edition, John iley and Sons, Inc., pp. 779-781

(1972): New York.

3. C. Lipson and N.J. Sheth, Statistical Desig and Analysis of Engineering Experiments, McC raw-Hil I ok

Company, Inc., p. 458 (1973): New York.

4. J.C.R. Li, Statistical Inference i, Edwards Brothers, Inc., pp. 552-5Sk (1964): Ann Arbor, Michigan.

5. N.H. Nie, C.H. Hull, J.G. Jenkins, K. Steinbrenner and D.H. Sent, Sttistical Package for the Ma~alSciences, Second Edition, Version 8.0, McGraw-Hill Book Company, Inc. (1975): New York.

6. J.C.R. Li, Statistical Inference I, Edwards Brothers, Inc., pp. 265-270 (1964): Ann Arbor Michigan.

7. R.G.D. Steel and J.8. Tartie, Principles and Procedures of Statistics, McGraw-Hill S-ck Company, Inc..pp. 106-107 (1960): New York.

8. J.C.R. Li, Statistical Inference i, Edwards Brothers, Inc., pp. 270-273 (1964): Ann Arbor Michilan.

9. R.G.D. Steel and J.H. Torrie, Principles and Procedures of Statistics, McGraw-Hill Book Conpan. In'..pp. 107-109 (1960): New York.

I. C. Lipson and N.J. Sheth, Statistical Design and Asoiysis of Engineerin Experimert!;. Mcrw-iili Ho,kCompany, Inc., pp. 265-270 (1973): New York.

It. W.J. Dixon and F.J. Massey, Jr., Introduction to Statistical Analysis. Forth Edition. Mc(raw-HillBook Company, Inc., p. 278 (1983): New York.

12. R. Langley, Practical Statistics, Dover Publications, Inc., pp. 292-313 (11;/ ): New York.

I9

TABLE 1: EXAMPLE OF GRAPHICAL PROCEDURE FOR TEST OF NORMALITY

PRE-EXPOSURE . FATIGUE IN SALT SPRAY AT S_ - 144 MPa

FATIGUE LIFE . E..A..RATES FATIGUE LIFE MEDIAT RANKS FATIGUE LIFE MEDIANR RANKS FATIGE LIFFEIO ,RATKS(CYCLES) (81 (CYCLES) 87 (CYCLES) 7%)

FATIGUF LIFE DATA 0 I 73 39572 24.48 67 798 51.23 93692 75.99AACE1522.608 .22 4.1,20 28 96 67,91.0 51 71 104.82,7 78 46

ACSIGEEE 28.139 6.68 4(1557 1(7.4 02 54.38 106,209 80 9..ASCENDING ORDERAND ASSIGNED 31.2206 13 43 44.550 76 24 ,2 61 5D 121.927 85S

33,693 26 58 76,478 51 6 8 13 '9.620 R, 8,ME INRN S11.162 14 I. 4F.2'1 a8 '*80 6 61 125. 5 88 36

36,553 79 05 5 ,574 43 8158 04 1(3,841 03 31

31.012 27 77 o'Ol 442 801 720 j7 0 144.144 95'38512 24 01 65.121 48.76 90 32 73.1 145,20 Q8 ?

8881- 2 5 -0,19 -. 2 ' V,7'0 > O1 ' 74,A 9)

889e, IEORA -, ! IRMERRA885- -AITWI -l.-A

98

90

7o060,-

• (;.A['II { I~l 5030'

2 0-

3-. r -L - -L L_

5 100 100000 O10,000 108000 TO0,300

L CYCLES TO FAILURE

2001

TABLE 21 EXAMPLE OF . TEST FOR GOODNESS OF FIT TO CHECK FOR NORMLALITY

X TEST PROCEDURE FOR THE CFCTP CORE PROGRAMMIE

ASSUME F1.1 IS TOE NORMAL DlSTlIBUTZON. DIVIDE F111 INTO K - 4 INTERVALS SUCH TOOT EACH INTERVAL INCLUDES 1/4 OF TOEPOPU LATION. THIS IS A CONVENIENT DIVISION BASED ON TOE SAMPLE SIZE A - 4.0 FOR EACH OF TOE EIGHT COMBINATIONS OFFAT IGUE STRESS LEVELS AND TESTING SCHEDULES. TOIS DIVISION ALSO REARS TOOT TOE THEORETICAL FREQUENCY f. IS 1/4. OFTOE SAMPLE, I E., t - 10.

ARRANGE TOE SAMPLE IN ASCENDING ORDER OF FATIGUE LIVES AND CALCULATE

- TE EA xTOESE ADD TOE MAXIRMUM IUIKOOD ESTIMATES FOR TOEA I SAME (UNKNOWN)1 PARALMETERS FOR P71?. I E r - 2

i-

-TOE STANDARD DEV'IAUION I

- THE STANDARDIZED NORMAL VARIATE r- FOR EACH DATUM

COM4PARE TOE VALUES OF . FOR TOE SAMPLE WITH TOE VALUES BOUNDING TOE K INTERVALS OF Fl'.) THIS ENABLES DIVIDING' THESAMPLE INTO TOE K INTERVALS AND CIVES TOE NUMBER OF SAMPFLE VALUES f. IN EACH) INTERVAL.

FATIGUE IN SALT SPRAY AT S 210 MP,

TEST FOR NORMAL DISTRIBUTION TEST FOR LOU -NORMAL DISTRIBTION

FATIGUE LIFE A OR( L 0? FAIICUE L DIVFEA (x) FAT ICE LIFE A TA FATIUELIFE 2/ ' FT(CYCLES TCYCLES) E A (LOU CYCLES) (LO CY CLE S O

4.13 93 I T 111E 0 0 RB 1 2 00 A 056 0 2S 5E I 13A6 0.17 3 1 1 5 A LE 0 02A,33 1 11 5' 4 1I 103 0 - 13 A 0A V VAAA2 1 11 77 - 1 30809 - 1 70 0

6002 1I.O'0 -0 02l 34' - 2 A 0? 0.136 - 120. 07 0 B0 A08 I

AL.011 12,61A V 06 3.86. 1 07 4.10 A 2I

.4 '0 91 1 41ES AR 873 Yd I1 0' L 4EIH O 1S I A E FO THE

A lR-5 0 10 A1 B NN -. 100 'O100 0317.6 TH STA0 0 21 1 0"01 0 9 VA 131

- TjI N TAN8ARDI0ED OPARAL-VAA AT1 0 .57.03 0 ___2__ 14.0670 0.'.' 3 100 0 015 A 166 0 13

V O 06 10E A 0 O S.54E 3 9 -C VN R A L I VA9.10 0-6.89 1 1 0 3 900 0.3 '.013 1 1 L'50 IR8S5? 1 17 3 981 4 0 A209 1 20

10.137 0,1 18.9?0 1.2 A OV 6 0.26 A270 210.2O 19 T 3 , E 0 R -29 20 29 1 D5R .. ?110.820T8 2SA1T0 1 32 AT 03 0 '0 1 A 31 1 ..11.026 13 24.22.0 6 1 91 .4'2 - A1 111.105 0,22 2 12 A O8A 0 0 06 11 0 05I.360 009 12728 O 1 055 00 1 4 120 2 00

12 I 30 . 5 ,353 A 053 A - 0 10?

THEORETICAL OBSERVED (P "" OF'CA'

BOUNDING T EA A U TI O BSEROET O -U5K. INTAL FR EQUENC , FREQUENCY 'AREQ UC FEQUENCY

OF Fl)AOFFA

, -O A 0 o 1 0.100U - -0 AR 10 1.5 I 100-0 8 0 09 10 1 200 5 7 ' -O68 0 00 10 A .00 ,00 060 10 5 2,5 0 0 00 0.6A 10 12 1A.l30 68- 0 0 9 a100 0 6 0 0 010

TOTALS 40 1 .0 2,75,200 TOTOLS 0 010 {o18 0100

P 1,08 o - S K A IDE , I 1 DEGREE OF FREEDOM x' - 3 841. FOB% Ar-I - 4.2- 1 DECREE OF FREEDOM - 81SICES 20 0 1 TEPOPULATION IN NOT NRML SINCO 000 <3 1 TE POPULATION IS NORMAL o

21)1

TABLE 3: BARTLETT AND BOX TEST PROCEDURES FOR TESTING THE HOMOGENEITY OF VARIANCES

SAMPLE SAMPLE SIZE SUM OF SQUARES DEGREES OF FREEDOM I SAMPLE VAR AICE

NUMBER n SS - LS- - 5s - Sogo$

1 's, loN2 g

-- s o I

'2 SS2 1 "21 "

SUMs

POOLED - S E___ ____E__ ) 2I~OIFFERENCE [ - - -D

K- 2.3026 2 D ' - /3 u -k -I

BAf.TLETT TEST

- - WITH ,O DEREES OF FREE;()M '2

CHOOSE A SSENIFICAN,'E LEVFI, I 0F 1 S 0- - - - K. -

2'

COMPARE WITH THE ' LISTED UNDER F W ITH 1 AND 2EGREFS OF FREEDOM

a - % N AND FCOR FI DECEES OF FREEDOM

CHOOSE A SIGNrFIcANCE LEVE. a E C 5 R)

COMPARE F WITH THE F LISTED UNDER a - 5 1 AND

FOR 'l AND "2 DECREES OF FRFEDOM

5

202

TABLE 4: EX.APLES OF BARTLETT'S TEST AND BOX'S lEST FOR HOMOGENEITY OF VARIANCES

BARTLETT rEST TO CHECK FOR DIFFERENCES BET'1EEN FATIGUE TEST CONDITIONS

COMPARISON OF POPULATION VARIANCES FOR FATIGUE IN AIR AND F-TT.UE IN SALT SPRAY AT S - 210 MPa

SAMPLE SIZE SUM OF SQUARES DEGREES OF FREEDOM SAMPLE 7..I.CE1

SAMPLE NUMBER -" s0'

i Eatigu.e 1 air 00 1.292 39 0026 0.033 - 480 - 57 7062 E.:ie in KalE spra 00 1.269 9 0 026 0 033 .1.8 - 58 012

k-.I

sum - - 0 052 - 115 '28

POOLED 2 561 78 0 013 O 033 -1..08. - 115 727

DIFFERENCE - - 0 - 0 019 - U2 - -0 009

- 2,3026 D2

- - 0.021 L - kl - O I13 .1 k - I -

X- - - 0 021 WITH I DECREE OF FREEOM021. WT1.1

FOR 5 - % AND I DEGREE OF FREEDOM 1- 3 81 SINCE - 021 ,T 3,RoI THE POPUITION VAEIANCES ARE EL)'CCI

DUOX TEST TO CHECK FOR INTER-IAORA*ToOy DIFERENCCES

COMPARISON OF POPULATION VARIANCES FOR PRU-PAP-LUR . NATEUE IN SALT SPRAi AT S 1.. E.,

SAMPLE SIZE SUM OF SQUARES DEGREES OF FREEDOM SAMPLE VARIANCE

SAMPLE NUMBER I .SS E- X n1x~ - I . S IKK K

I NADC 4 0091 3 0.333 0 030 -1.517 - 4,0'12 SASKATEKEWAN 4 0 250 3 0 333 0 083 -1 079 - 237

VOUGHT 1 0 085 3 0 333 0.02 -1.509 - 4 .6014 AFWAL 4 0 060 1 0 31, 0,02 - '.77 - 5 01

N LD 4 0 261 3 0 131 0 081 -! 001 3182DFVLR 4 O,157 3 0 133 0052 -1 1281 -38KW

7 NORE 4 0333 3 1 333 0 111 -O 51 - 2 16,aRAE 4 O 5 3 0 333 0 052 -i 297 1 3.661SIFFRL 4 ).302 3 0.33 0 101 -0 933 - 7 993

10 PISA 1 0001 3 U 333 0 006 -2.701 - 6 723

k - 10

sOm - 1 -3330 - o980

POOLED - 1 711 300033 00 U - 37 116

DIFFERECE - - D - 329, - D2 - 3 6?3

D1

.K- 3026 D - 1 -- -0 11 k2 "- 9 ' ___

- I 2 L - K - 831 F -- - 0,836 WITH 9 AND 739 DEGREES OF FREEDOM

FOR a - N N NOD 9 AND 139 DEGREES OF FREEDOM F - I 990 SINCE 0 816 < 1 980 TOE POPIATION VARIANCESRE EQVAL,

2'03

TABLE 5- PROCEDURE FOR THREE-WAY ANALYSIS 0iF VARIANCE

005.751 F0 T___FI NAI -1

_____T, I

*AR I5 FTlNF -1 - 1. - -

<IAA~l FA - .T- - T FS'iT - .

I'T

204

TABLE 6: SChEMATIC OF THREE-WAY ANALYSIS OF VARIANCE FOR THE CFCTF CORE PROGRA TIE

INPUT DATA

FATIGUE LIFE TO FAILURE (3G CYCES)

CO COLUMNS (IABORATORIES)

(ENVIRONMENTS) NADC SASK 0U020T AFVAI. 8 10 DSL S DR E PAE 116P8 I SA

4 22 3 83 4085 438 335 4399 4 40 13111 403 34504.08 4o 10412]4.513 4 395 4 404 .0 . 160 U7 '8

441 4 339 4 12 4 531 4 7 4 412 4 28 4.30 59

4449 44 31 1 384 4 561 4 509 4 536 412 4:2 1919 3 92

3 699 4 263 4 4 0 . 1298 3 192 4 .30 4 16 1 . 1 ,

21'~01l1. 3823902 4 305 14196 414 411 2.2 68 0''t A19 0 004 3922 Il 1394 339 4084 3

4216 4 15 308 4 1 51 3 928 1 5,, 3 43 8' 106

4070 4101 3955 131 4 034 4 06 411 1 1' 2' 4

1":, -1:1.2 1 055 4 2- 36951 3 831 4 092 .. 19540 384; .321.0:ay ] 19o .5 3o 3900 42238 4006 4 353 .. 04.., 81 109

3879 49 13 38I9 4 391 3?35 1 211 6 429 2 140 00 980

223 3 33 3575 3 8' 711 7 52 0. 1 ill, 1 8 ' A

ieeosu 1' 3V 3329 3892 3641 4001 4 198 .084 3 32.

3663 39 3.']~ 10 4A5 33 1 318044 990ly 2 [3910 3.939 38B80 4 209 3 833 39' q,0093 405. '

fatguein r 5 168 30379 5 284 5918 5212 5 259 9 0,? . 6 )311303223548295038445 All

,1; .6o 2 5 62 5 24 5 1 84 9 09 010 1 2 - 5 1g0;1

30I3 5088 9. 3 5 , 5 124 5 , I 00

51330 2063 5022 1 929 6. p 883 ' 3 20 0 : 1

1 J 90 s13 5133 3011 1 'lo 5 0 9303 0246 ] I 51 .53.3-. '-.1>92O0 583 663 5381 4895 4 914 4 8I l 656 .7 69 ,.23

4 69 5 164 5 5 213 I 670 4 9 73 5 0 8 5 3 ."A -535

-----1 11 4 4 +1

4 752 4 972 4934I5I45Io596 9.39. 506 9 86r s03 4 502 .73 902'4<8 . , C'64 4353 9331 4

4531188 1 5n 832 52:,9 51 ~6-81-o .1

S~~~~~~~~~TATllrlCAL ?ACKG O gS:.Ls,{. E

9iALYSIS OF VARIANCE RESULTS (95 % CONFIDENCE I E a - )

608 OF DEGREES OF MEAN SQEADES MEAN SQUARE RAT0IOF D1S3818U310 1219EECTS OF

SOURCE OF VARIATION SQUARES FREEDOM S3 3 A E.PERMENTAAI 0,FR2683E44SS8S-5 - S -3858- -F VALUE

Raboratory 9 362 9 0 218 5 163 1 993

FACTORS 588.s 59 494 I 59.4, 1572 813 3 83 ...

Mnv1ro99n135 1 312 3 104 1,5 055 2 65

1*bo..tory trA- 0 557 1 0 062 1 637 1 93 n

INTERACTING labort.ory evirol. n I 427 20 0 051 1 393 I 5.

FACTORS S..M.M .-MAro. nt 0 302 3 0 101 2 662 2.65

.bo.rator.y stres env9ror ent 1513 23 0 056 1638 '54

retidual 9038 240 98

2Ii

" In

2 CI

-- I,. 14 H

iCI I Z

, + ci

TLE- S: EXAMPILES OF LEAST SIGNIFICAN:T DIFFERFENCE CENST FOR LOCATING THlE 50111CI OR SOURCES F A N

EFFECT OF STRESS CERSUS ENSIRONMELNT (FACIGUE TESTING SCHEDULE) INIIICASFI) M A-NALYSIS IF.AK!A:YT

F __:__IF I _ _

N7"

lb; XP1>'li' OF LJ1xN. V' 1i, MV1:1.IPLE KRAIKL lf-Ir U KL flAKLF JIIA AND iIl I llil:ERSS -1v IRNNEN (Is I L SiINC; . CAiSlf.~i ! NTKSKL IONS

21R

TABLE 10: EXAMPLE OF DUNCAN'S NEW MULTIPLE RANGE TEST FOR UNEQUAL SAMPLE SIZES AND FOR ONF OF TIlE

LABORATORY VERSUS STRESS VERSUS ENVIRONMENT (FATIGUE TESTING SCHEDULE) INTERACTIONS

UNEQU SAMPAI51 SAS S IONA [IFA DAA 11 ',IO pA M I T I T

AI TAT T A A TS F; I-I A A AA1 d I -

1TR __S 11.1. ____211] _________- __, _P _

A&AA, AlA APAAA:' A IATA rEAAtINA AlAN IN IN :. NS l NA OI AA I

AJ A A

-PIII- 'AN - A ATA AA5,A AIIINIITNT LAX: I TANT TI:NI -:A1F I I A

........ .......... ........ITsAl~sAAA-AAsA,,AA,,I IA I AA"

?, --

Ii

* S A AT AAAI A .AAIoIr. IA . tI 5 AS A. AA AAip e os5 re I fati5 AA Ie 5 1,s l pr v : )

AATI OU@ A.tl~ 151 IA AIAXAAuIIAAIA.IIAXAAA

AAa m n nf~In gl AIAAIAAIIIA I .sn

TABL.E I I<3 I[ME IPSO AND) SliETl METHOD OF SAMIPLE SIZE 1AVOI INAT IGN

RESULTS FK,29 SIFFRI, PUR FRSI-,E IN AIR

s f FATIGUE ,1M; .. ".E AMI :NFL: SAMPE 5 1 -I'l ARE - TADA D 4 1 1 -T I A:7

I0< I S-

I~ FEE! I

-! 1 Fi Si { > ?/ !.7 ;;i 1:

LOE Fi ENT OR UARIATION

COFDEC

RU I17- 'nll ,1 I: .22L.'I 1,

* I F I'A 9 I P2 2

ml: 2 3231 LD ,MI<010 320 070 O~ F -IN 1/h I/l

2 .1-- ~pi s I _

22 ' ' I A IE 2 RE E A PI S FI .O AN . 6l ~ IDE 1F,1?

I>~~~ .... .7-I 1 5 -

F~~~~~ 12' 2 R

210

TABLE 13: EXAMPLE OF x5 TEST OF INDEPENDENCE FOR ANALYSING THE PRIMARY FATIGUE ORIGIN DATA

(ORIGINAL EIGHT CFCTP FARTICIPANTS A3D SIFFRL)

EDIMARY FATIGUE ORIINS VERSUS M1,IROIRENT S _ 144 KF.

COIIWNS. 0 10001G6 T.TIN4 C*OLSRO S. I_______ _______

(LOCATIONS OP00** RIMARr0S0*....FATICCOE ORIGIMS) 0.ttg,.. I. aIr .. E IR*.

Sr..It OD,.y ROw TOTALS. R

C5500 CTION 0F INITIAL.COSTINCENCO T-BL _____________ __1______1,__

.O IVrG Asc 25 L, lo 15 .

c/oMP 0016 1ECS 0*0( 10S EIOOO 0 4

ROWS.

N010IF1'ATIO.% (IF11 TIIE ft TOI1*p. 00 J001

U- lur, rooN. ', 0( .1( .

TH-FTICA 000(IONC TEI TlEOAL F., 70M 0If*6 (-f 1

0-' [( y f-I

15% IS

601 OF 0 61( 111 26, S 0

* 00016I.0OI0S or 11. - 0(. 1 I

000 COOOOSISON 4100

AND 14 6 I00 I 1 I 20 I 1 22 2I 4 10OF( -00,111., (40 I.A

000*000t 03 1690M 11111 ' 0

14 1. 11

14 -I 1 --- y-EDO - 9(10.0 1 -)1 r

60 E000 *100 V I CONFIDE.NCE DM0 T AR TVIS 10 A 10 SO FICOT AS-ICITION 860066* PEINARNFAIC F*10 I(0(1.100 CNOEOVIROON.TS (0,00016 00*110S000W 6

TABLE 14: L4LMLE OF YATES' CORRECTLY TEST FOR CHIECKING; TIE ASSOCGIAIIN OF FAIl ICLE LIVESA:)'RM Y

FAFIGUE ORIGINS (ORIGINAL C IGII CECT F PARTICII FANlY)

FATIU.E It' AIR AT S 21- JILLI

LO)CATIONS WFLOATIONS OF LOCAMINS 1F C AILSF

FATE GEE LIFE PRIMARY FAT lACE LIFE' PRIMARY FTICCE LI FE PRIMARY FATILLkE LIFE OFCARALOCLIES) FATIGCE ICYCLES) FATIICOR C'yLS, FATICLE : CYLES FA .L

FE Iti~t ORGII IE ERICIN INSIS COY;

* CORRELATED AITHI FATI (CC 'l, R" 10LIVES ARRANSEES IV

* ASCEYSDLOC ORDER tLOC, I

A~l F

5''M1

,P, AT IA. LI-s 0

CM1 '; 1LTAL[ I

1 INL , FAOL h ,ki

Tr ' ATAL C' - L

'LA ~ RIK"T LOC'C L .. CLOFOL0LII LOL.2oA CLSI "P L

rABLE I S LAMIL! OF F ISIUER'S EXALTr TEST FOR CkLEIMOG THE ASSOC! Al IIO OF TAl 11 G1T 1. 1.1 FS NVD PI (AS FA 1 (L F,

OR[ S(xlR CNAL. 1051 CFLTP FART CIPAAM 5)

FAT ICU E I I SAlIT S FRA) AT S I V1SP

LOCATONS l LOATION IA AJUTIA W A0T '

FATIGUE (.FE ('lTR TFATiG FE1iil P RIK0R% FATIGUIE L F ',A l''F F RMR

,C A I 71'XlII,, FTIGU INS I FY

-ARl I'.IIUFS1 A1iAS.F t, F I

FATIMAUL FATRl ( ES 52' -

'AAt I1. FF F iiF - - - - - -7 iFATGU 1'A> 124 'F

*~~~ "IIIT, FF III I- a

'IPI U DEA VAAl I.IF IFFAI____FE-_!

R'S'WI, r .' 11.

.'iM'ARI'IS~TII - sm p - U I III N IA II I ,I A E U~CI:I II''S V

TICIF S F!UIAI, FEWE IAIU IGE AT IA T PIIA FIl

X2 TEST OF INDEPENDENCE

ORYATES CORRECTED X2

TESTOR

FISHERIS EXACT TEST

____________0 ORIGINS AND STRESS LEVELS

TEST FOR NORMALITY 0 ORIGINS AND ENVIRONMENTS

ARITHMETIC AND LOGARITHMIC 0 FATIGUE LIVES AND ORIGINSNORMAL PROBABILITY PLOTS PEI TEST CONDITION

k' TEST FOR GOODNESS OF FIT

QUALIFICATION E

FOR MAIN YO AINESTATISTICALANALYSIS BOTETTCHKFR

PFATIGUE TEST CONDITION

FAIU TES COAYNNTERATION

OAI -PP OXIrRSS: HOMO- STOP~

STATISTI -A - - -tes - - -borat --or-y r

ANALYSIS OF VARIANCEeI LIPON ND at MEHO

* 3 -WAY INTERACTIONS

-stress. envirnmet. laboratory

"FINE TUNING"

LEAST SIGNIFICANT G1IFERENCE TESTl OUNCANS NEW MULTIPLE RANGETES

Fig. I Survey of statistical methods for analysing She CFCTP fatigue life and primary fatiguae origin data

REPORT DOCUMENTATION PAGE

I. Recipient's Reference 2. Originator's Reference 3. Further Reference 7 4. Security Classificafionof Document

AGARD-R-713 ISBN 92-835-0495-X UNCLASSIFIEDI

5. Originator Advisory Group for Aerospace Research and DevelopmentNorth Atlantic Treaty Organization7 rue Ancelle, 92200 Neuilly sur Seine, France

6. TitleTHE FATIGUE IN AIRCRAFT CORROSION TESTING (FACT)PROGRAMME

7. Presented at

8. Author(s)/Editor(s) D 9. DateR.J.H.Wanhill, J.J De Luccia anrd N.T.Russo February 1 989

10. Author's/Editor's Address 11. Pages

See Flyleaf. 2 14

12. Distribution Statement This document is distributed in accordance with AGARI)

policies and regulations, which are outlined on theOutside Back Covers of all AGARD publications.

13. Keywords/Descriptors

Corrosion fatigue AircraftAluminum alloys Fatigue testsJoints (junctions) Cyclic loadsCorrosion prevention

14. Abstract 7In accordance with the mission of AGARD the Structures and Materials Panel (SMP) has alwayskept an open eye for the possibilities to sponsor collaborative programmes of research. AGARDis unique in its ability to realise the cooperation of laboratories in up to sixteen nations. In thisway AGARD distinguishes itself from other international scientific and technical organisations.

In the 1970s the SMP decided to embark on collaborative research activities in the area offatigue. One of the first activities was the Corrosion Fatigue Cooperative Testing Programme(CFCTP). the precursor to the Fatigue in Aircraft Corrosion Testing (FACT) programme. Bothprogrammes are described in this report.

Failure by fatigue and degradation by corrosion continue to be major considerations inaircraft design. Environmental effects influence both initiation and propagation of fatigue cracks.and dynamic loading may cause more rapid deterioration of corrosion protection systems.Therefore the conjoint action of dynamic loading and environmental attack, i.e. corrosionfatigue, requires special attention.

Many corrosion fatigue tests have been done on aluminium alloys. However, few included criticalstructural details like joints, under realistic cyclic load histories and in service-like cnvironmcnts.Even fewer used practical corrosion protection systems. These aspects are specifically addressedby the C-FCT-P antDFACT programmesThe results provide a significant contribution to theunderstanding of aircraft corrosion fatigue and should encourage further investigation in thisdifficult and challenging area of aerospace technology.

>44

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Documents

PFWE MOPI KII - apps.dtic.mil · 2.2 Protection System a,] Specimen Assembly 7 2.3 Mechnial Testing Conditions (Static Prestressing and Fatigue) 7 2.4 Environmental Conditions (Pre-exposure,