*ISO 9001

Dr. S. R. SinghDeputy DirectorMST Division

ct.&or

National Metallurgical Laboratory(Council of Scientific & Industrial Research)

Jamshedpur 831 007, India

BY COURIERRef.: NML/MST/EML/l82

15thApril, 2005

To:

Sqll. Ldr. s. nnhunuru~nllDClmty Chief of Quality AssunanceFor Commanding Officcr4BRDAir Forcc Stntion, ChakcriKallpur-208008.

Subiect: Failure Mode Analvsis/Metallure.ical Examination of Viper Aeroene.ine Jet. Pipe BellowAssemblv Ex Aircraft U-719. .

Report No.: NML/MST/IAF/1.13/58/2005. April 2005.

Dear Sqn. Ldr. Balamurugan,Plense refer lo yom letter No.4 BlH)14552/1/DI/Tech.dilled Jill .IIIIIIIIII'Y2005,

regarding the above-referred investigation.The investigation is completed and linal report isenclosedherewithfo~your informationand necessaryaction at your end.

The invoice of this investigationwill be sent to AOC, Air Force Station, Tambram,Chennai, under intimation to you, shortly by our office. If you need any clarification, pleasedo not hesitateto contact me.

------

Yours sincerely

-::::.Q.<; <~<=~ ~ J

(S. R. Singh)

/\ Enc!.:ReportNo. NML/MST/IAF/I.13/58/2005,April 2005.\~v

Fax: (0657)2270527; Telephone: (0657)2270027 (0), (0657)2431556 (R).E-mail: srsingh(cl).nmlindia.org

N"ML tyrc-k.t'~

-:2-dOS- ( (;)~:>

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REPORT No: NMLIMST/IAF/1.13/58/2005Circulation

April, 2005

Restricted

FAILURE MODE ANAL YSIS/MET ALLURGICAL

EXAMINATION OF VIPER AEROENGINE JET PIPE BELLOW

ASSEMBLY Ex AIRCRAFTU-719.

Sponsored by

4 Base Repair DepotAir Force Station, Chal\.eri

Kanpur - 208008..-------

I*.,,,,, ~

Ma terials Science & Technology Division

National Metallurgical Laboratory

(Council of Scientific and Industrial Research)

Jalnshedpur - 831007

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FAILURE MOUE ANALYSIS/METALLIIIH;ICAL EXAMINATION 01. "II'En AEnOEN(;(NE.I 1:1 I'II'E UELLUWASSF.MHLY Ex AIRCRAFT U-719

lM~],/II\FILJJi~8/2()()~. AL1!'iI2()()5

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<-un-' 'U>L .'.LU 4q, ... NATIONAL METALLURGICAL LABORATORY(Council of Scientific & Industrial Research)

JAMSHEDPUR

PROJECT COMPLETION REPORT

l'roJect Title: FAILURE MODE Project No. :ANALYSls/METALLURGIC)l.L EXAMINATIONOF VIPER

(Ieport No.: (NML/MST/IAF/1.13/58/2005)AEROENGINE JET PIPE BELLOW ASSEMBLY ExAIRCRAFTU.719

Unte of Project InitIation: 03.01.2005

Date of Completion: 15.04.2005

Project Team Members: Class:

Dr. M. Ghosh (PL), Dr. S.R SIngh (CoPL) Public document (Free/Priced)

Mr. S.K. Das, Mr. S. Soren and Dr. RN. Ghosh Restricted circulation (X)Only to clientSecret

Customer / Client's name and address:Classification:

In-houseSqn. Ldr. S. BalamurJlgan (,runt-in-aidDy. Chiefof()uality Assurance Sponsored (X)For Commanding Omcer Consultancy4 Base Repair Depot CollaborativeAir Force Station, ChakeriKanpur -208008

Date of Report: 15.04.2005

Area: Material Characterization. Specify Type:

Sub Area: Failure analysisOngoing Area: (X)New Area Initiated:

Key Words: Jet pipe bellow assembly, Kiran aircraft, austenitic stainless steel, nansgranular fracture,interranular fracture, fatigue crack, fatigue, abrasion. i

Abstract: The viper aeroengine jet pipe bellow assembly ex aircrull U-719, failed owing to bellow burst.The objective of root cause failure analysis is to find out whether it is caused by either material itself or byexcessive stresses. Circumferential cracking is observed at number of locations on the component. Samplinghos bccn done lI'om the two such crackcd locations i. c onc Ihull corrugoted (austcnitic stainless stccl, type8309(0) and othcr from non-corrugalcd (austcnitic stainlcss stcel. typc S329(0) rcgion. The hardness value,optical and scanning electron microscopy exuminations rcvcal, that the crack initiation occurs byintergranular decohesion owing to overheating induced intergranular precipitation of brittle phases. Usuallyfatigue crack moves from the location of tensile to compressive type of stressed location i.e from outer toinner surface. In the present situation, crack initiates from inner surface and propagates toward outer surfaceand circumferential direction i.e from compression to tensile side by fatigue mechanism. During serviceexposure, perhaps due to sudden rise in temperature, at thl' inner surface compressive stress changes totensile stress. Absence of dimple rapture indicates fililure took place in the brittle mode, which is caused byoverheating during service exposure.

Details of IPRs (PI tick): NA If no IPR taken, reasons: N/\

Patent

CopyrightTrade Marks

Report Issuance Authority: Dr. S. R. Singh Signature: S.R. :--, . ',-

NATIONAL MET ALLU ItG ICAL LADOItA TO It\' ,JAMSII EDI'UItFAILUREMODEANALYSIS/METALLlJRGICALEXAMINATIONOF VIPER AEROENGINEJET PIPE BELLOW

ASSEMBLYEx AIRCRAFf U-719REPORTNo. NMLlMST/IAFII.13/58/2005,April 2005

OBJECTIVE.

The investigation consists of examining Viper acrocngine jd pipe bellow assembly,

whichwas faileddue to bellow burst, to find out the causeof failurewhetherit is owingto

material itself or due to excessive stresses.

BACKGROUND

At one of the operating bases of the IAF, it is observed that during TRS of aircralt,

alter Oying yellow band area on the starboard side of the rear fuselage is found discoloured

due to heavy gas leak. The strip examination of jet pipe revealed the rupture of expansion

bellow assembly. But no damage is observed on the other portion of the assembly. Therefore

a circumferential cut-piece of bellow of jet pipe from viper aeroengine of Ex ain.:rull U-719

has been sent to NML for metallurgical failure analysis vide letter No.4

BRD/4552/I/DIffech dated 3rd January 2005.The damagedportionof the componentwas

received at NML on 28/02/05 for the investigation.

SCOPE OF WORK

Based on the informatiol1 provided for the failure analysis, it is decided to do the

following.

I. Visual examination, photographic recording and measurements of different failed

locations of the ruptured bellow of jet pipe.

2. Fractography of fracture surfaces to find out the fracture mode.

3. Microstructural & hardness evaluation of two samples (one from corrugated region and

other from non-corrugatcd region) to find out any unusual features, if any.

4. EDScOllIPositionalanalysisorhase melalulIll predpilales in two samplcs.

5. Finuoullhe root causeof lililureof the bellow ofjcl pipe and suggestrClllcdiull1Icasures.

COMPONENT, MATERIAL & SERVICE EXPOSURE

The bellow of jet pipe (See/Ref No. 236 STIV-16875) is manufactured by Rolls

.Roycc, UK, from an austcnitic stainless steel. It has maximum working temperature of 200°C

with overhaul life of 2300 hrs but duration of its exploitation is not known. It is located at the

front end of the jet pipe and after exhaust cone of aeroengine. The received cut-piece of

expansion bellow assembly has corrugatcd zone bonded by non-corrugated zone on both

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3

NATIONAL METALLURG ICAL LADORATORY, JAMSII EIWLJRI'AILlJREMODE ANALYSIS/M.:TAU,IIIU;ICAI.EXAMINATIONOi' VII'.:R AERO.:N(iJNEJET 1)11'.:n.:U,OW

ASSEMBLYEx AIRCRAFTU-719REPORTNo. NMLlMST/IAF/1.I3/58/2005, April 2005

sides. These two zones are welded together. It seems that two or more types of sheet metal are

used in fabricating the assembly.

INTRODUCI'ION

The austenitic stainless steel has nickel and chromium as the main two alloying

elements. Nickel increases the amount of austenite present at the solution treatment

temperature. With the presence of 0.1wt% carbon, the ISCr-SNi stainlcss stcel bccomcs full

austenite at and above 900°C with Ms temperature just below the room temperature. For this

kind of steel, austenite would partially transforms to ":1artensite either during refrigeration or

during cold working. At low nickel content, the austenite may also transforms to marten.site.

The structure of the steel varies depending upon the chromium content. At low Cr content, it

acts as austenite stabilizer. With more than ISwt% Cr, ferrite forming tendency of chromium

begins and at this stage the alloy requires increasing amount or nickel to eliminate delta

ferrite. Ti containing stainless steel exhibits strain-induced precipitation during hot working,

which results in strengthening and low ductility. With respect to the constitution of the

austenitic stainless steel, M23C6precipitate plays a vital role and they form below SOO°e.Heat

treatment below 900°C is called sensitization, which promotes M23C6precipitation and Cr-

impoverished region. Mn acts as austenite stabilizer for this alloy and along with nitrogen it

can partially replaces Ni content of the steel. For medium and high temperature application,

Mn concentration is often increased to eliminate delta ferrite. Yet this type of steel contaips

added Si to promote its temperature stability, which on the other hand produces delta ferrite.

In the temperature range of 700-950°C, austenitic stainless steel can form brittle

intermetallic compound a-phase. The tendency of sigma phase formation increases with

chromium content. Mo, Ti and Si accentuate its formation. The precipitation of intergranular

sigma phase lowers ductility and toughness after long time exposure at high temperature

leadingto intergranulardecohesion. The delta ferrite may also transform to sigma phase and

austenite due to higher amount of chromium solubility in delta ferrite with respect toaustenite.

Thc exposure of the componcntat 200"C for 2300 Ius in gencral, docs not lead to any

structural detoriation of the parent alloy. However the precipitation of brittle second phases

may occur due to over temperature exposure, leading to cracking of the component

prematurely.The variation of stress in service condition, also afTectsthe component life. The

4

.'

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NATIONAL METALUJlU;1< 'AL LAUOltATOltY. JAI\ISllIml'UltFAILUltEMOUEANALYSIS/METALUJlU;ICALEXAMINATION(W VII'ER A.:ltOEN(;INEJKr I'll'.: n.:U.OW

ASS.:MIJI.YEx AmCItAFT U-719REPORTNo. NML/MST/IAF/I.13/5R/2005, Arril 2005.

vibration loading may promote Hltigue failure. Sudden rise in the stress value (over load) may

also result in failure, which takes place by micro-void coalescence (dimple rapture).

- EXPEIUMRNTAL RESULTS & DISCUSSIONS

From the failed component, two locations are chosen (marked by arrow in Fig.l) and

characterised. The location of failure is observed visually to find out its appearance and to

measure its dimension. The fracture surfaces are examined in secondary electron mode in

JEOL JSM 840A scanni'ng electron microscope equipped with Kevex energy dispersive

spectrometer to reveal the fracture mode. Metallography samples are prepared from the

thicknesssection as well as the surface section of the component.These samples are etched in

glycergia to reveal the microstructure and subsequently examined in optical microscope and

SEM for determining the phase dislrihillion and grain si:t.l'. Ilnnlm'ss of Ihl' IIInll'rinl is nisI!

lI1easureu in Vickers scale lIsing J0 Kgf load. The details of the investigation arc prcscnted in

the following subsections.

1. VISUAL EXAMINATION

The as received failed jet pipe bellow assembly is shown in Fig.l. Circumferential

cracking has been observed at number of locations. Besides the crack-I and 2, crack-3 is

found to be located 160mm and 180mm away from crack-2 and crack-I with the length of

140mm. On the opposite face of the component, one failed region has been found out (not

shown in figure) having the length of 160mm. Dent marks (length of dented region 300mm)

near crack (25mm length) are also observed. Out of the different cracked regions, considering

the symmetry of locations, two zones have been investigated(marked 1 and 2). Visually, the

fracture surfaces are darkened due to affect of heat. The cracked regions IA and IB of crack-I

have dull grainy appearance. The length of the crack-I is 210mm. At the middle of the

sample-I, the wall thickness is -0.48111111.which rcduccd to -0.24mll1 ncar fracturcd cnd. This

indicates a 50% reduction in thickness which may be caused by localiscJ nccking. For the---------

cracked regions 2A and 2B of crack-2, the fracture surfaces are folded with shinning

appearance. At the same crack-2 another crack formation takes place with the length of

28mm. Apparently, shinning appearance of these regions infer, that alter initial crack

formation owing to rubbing contact the features of failed region is lost. The crack length of

crack 2 is 41mm. The thicknessof this area remains uniform (-1.18mm) over the entire crack

5

NATIONAL METALLtIIU;ICAL I,AUOI{ATOIW. .IAMSIIEIH'IJI{FAILUREMOUEANALYSIS/METALLURGICALEXAMINATIONOFVIPER AEROENGINEJET I'II'E BELLOW

ASSEMBLYEx AmCRAFT U-719REPORTNo. NMLlMST/IAF/1. I3/58/2005. April 2005

is formed near the opposite surface of crack-I and 2 having the length oflength. Crack-4

35mm.

'" "",..; ".. ,.,

Figs. I(a-f): Photographs of as-received component with different types of fractureLocation (FL) and sampling locations (SL): a) failed component, b) crack-I(sample -I), c) crack-3, d) crack-4, e) crack-2 and f) dent mark. Crack-I and 3 areon corrugated part while crack-2 and 4 arc on non-corrugated part or the component.

---------(.

NA1'IONAL METALLUItGICAL LABOI{ATOH.Y, .JAMSIIIWI'UI{I'AILlIIU:Mom: ANALYSIS/METALLIIIHIICALEXAMINATIONOJ' VIPlm AF.IWENGlNt:JET PIPE nt:LLOW

ASSt:MIILYEx AmcHAFT 11-719

REPORTNu. NML/MST/IAFII.13/58/2005. April 2005CHEMICAL COMPOSITION2. ------

The chemical composition of major alloying elements present in the material is analysed by

EDS spectroscopy. The quantified data are collated in table I. Sample I is sheet metal from corrugated

portion while sample 2 is taken from outer non-corrugated region.

Table I: EDS microanalysis of bellow of jet pipe.

It indicates, that corrugated and non-corrugated regions are made of two different

types of austenitic stainless steel. The chemical composition of sample 1 has close

resemblance with type 309 austenitic stainless steel (UNS designatiun: S30900). In wruught

form the mechanical properties of the specified alloy is 0.2% yield sterngth-205MPa,

UTS-515MPa and elongation-30(Vo.Ilowever, the sample I exhibits slightly higher amount

of chromium with respect to the austenitic stainlesssteel 309.

The concentration pf alloying elements in the sample 2 indicates that the alloy

composition is close to type 329 austenitic stainless steel (UNS designation: S32900). In the

wrought form the tensile properties of the alloy is 0.2% yield sterngth-550MPa,

UTS-724MPa and elongation-25%. However, the sample 2 shows higher amount ofMn with

respect to austenitic stainless steel 329.

3. MICROSTRUCTURAL ANALYSIS

The metallographic prepared sections are examined in optical microscope. The

micrographs of surface and thickness sections of sample IA are shown in Figs. 2a & 2b

respectively while that of sample IB are shown in Figs. 3a & 3b. The optical microstructures

of lA and IB exhibits more or less polyhedral equiaxed grains and the alloy is single phase.

However, the sample IB shows lower average grain size (14Ilm) as compared to that of

sample 1A (45Ilm). Moreover, the thickness section of sample IB shows preferred orientation

of grains while that of sample IA did not reveal any preferred orientation. As the samples IA

& IB are drawn from the two ends of same crack, therefore such variation in grain size and

texture can only be accollnted ir there is local ised overheating at sample 11\.The overheating

7

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Sample No. Elements in wt. pet.

Si Ti Cr Mn Ni Mo FeI 0.2 0.5 26.8 2.0 15.4 0.3 Ual-- --2 1.6 0.2 29.3 4.70 5.2 1.9 Ual

NATIONAL MET ALLURG ICAL LABORATORY, JAMSII EUPlJ RFAILlJREMOUEANAL.YSIS/MKI'AI.I.IIIWICAI.EXAMINATION0... VII'.:I{ A.:nc)t:N(;IN.: J.:-I' I'll'.: IH:U.OW

ASSI':MBI.YEx AmCnAI'T lJ-719REPORTNo. NML/MST/Ii\F/ 1.13/51\/2005. i\rril 2005

caused the recrystallization and grain growth leading to the loss of original preferred

orientation and increased grain size respectively in sample IA.

The grain boundary in sample I (Fig.2 and 3) is decorated with the precipitates. These

precipitates show substantial quantity of Cr & Ti in it. During normal operatioll, the exposure

of the component at 200"C did not lead to precipitation of X and (J phases at thc austcnitic

grain boundaries. Such type of precipitation is possible only if it is overheated in temperature

range -700-950°C. Fine distribution of sccond phasc is also occurrcd within thc grains. This

type of precipitatiGnis perhaps Cr-carbide,Ti-carbide and different carbonitrides though this

type of carbides may also occur at the grain boundaries. Inclusions in the formof stringersare

also found. From all microstnll.:tural characteristics it can be inltmed, that the component is

exposed to localised overheating during its exploitation and the changes may promote

brittleness to the material.

0.0344 mm 0.0344 mm

ri!~s.2(n& h):Oplicnllllicrop.rnphsorthc snnlplc 11\showing nliclOstlilclulillfeatures in (a) surface section, and (b) thickness section.

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8

NATIONAL METALLURGICAL LABORATORY, JAMSIIEDI'URFAILUREMODE ANALYSIS/METALUIRGICALEXAMINATIONOFVIPER AEROENGINEJET PIPE BELLOW

ASSF.MBLYEx AmcRAFT 11-719J!J~KLN()L NMJ,':Mt'I!MELLLJ!~B!200~, t\J)Jjl2()(}~

0.0344 mm 0.0344 mm

Figs. 3(a & b): Opticalmicrograrhs of the sample IB showing microstructuralfeatures in (a) surface section, and (b) thickness section.

0.0344 mm 0.0344 mm

Figs.4(a & b): Optical micrographsof the sample 2A showing microstructuralfeatures in (a) sur/ace section,and (b) thickness section.

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9

",

NATIONAL METALLlIl{GICAL LABOI{ATORY, JAMSIIEIH'lmFAILURE MOUE ANALYSIS/METALLUI{GICAL EXAMINATION OF VIPER AEROENGINE JET PII'E BELLOW

ASSEMBLY Ex AIRCRAFT U-719REPORT NO. NML/MST/IAF/I.13/58/2005, April 2005

0.0344 mm 0.0344 mm

Figs. 5(a & b): Optical micrographs or the sample 211showing microstructuralfeatures i.n (a) surlilCC section, and (h) thickncss st'ction.

Sample 2A and 28 show austenite grains containing annealing twins (Fig.4 and 5). The twins

are not revealed in sample-I. This indicates that precipitation on the twin boundaries in case

of the former is scanty. The volumc fraction or the sccond phasc is also grcatcr in casc or the

later. This in turn indicates the exposure of this location at much higher temperature with

respect to the previous one (sample-I). The average grain.size of 2A and 2B is 16J.lmand

30J.lmrespectively, which means though one end (2A) retains its original grainsize, other end

(28) shows the growth of the same perhaps due to localised temperature overshoot.

---

Figs. 6: SEM photographsof the thicknesssectionof the samples a) IA and b) IB showingintergranular fracture(IF) and slip band.

10

NATIONAL METALLURGICAL LABORATOH.Y, JAMSllEUl'liRFAILURE MODE ANALYSiS/METALLURGICAL EXAMINATION OF VIPER AEROENGINE JET PIPE BELLOW

ASSF:MBLY Ex AIRCI~AFT U-719REPORT No. NMLlMST/IAF/1.13/58/2005, April 2005

The precipitation of the second phase is clearly shown in Fig. 6. The volume fraction

of second phase is smaller within grain interior (intcrgranular) in comparison to grain

boundary. The size of them at grain body was very small (<1J.lm). However, quantitative

analysis of the precipitates at grain boundary reveals the presence of large amount of Cr, Ni

and Mo along with Si, Ti, Mn and Fe (bal).

Table 2: EDS microanalysis of grain boundary precipitate

The concentration of chemical species indicates the presence of brittle cr phase to which

M23C6type of precipitates may be associated. The embrittlement by the second phases at

grain boundary leads to intergranular failure, which is observed in the micrograph. Slip bands

are also found within the grain body indicating the initiation characteristics of fatigue failure.

4. IIAIWNI':SS MI':ASUHEMI':NTS

Vickers hardness of all the samples is measured on metallographic prepared sections

at 10 Kgf load. The hardness data is collated in table 3. The hardness of sample IB is higher

than the sample IA. This is consistent with microstructural observation and the reduction in

hardness is caused by overheating induced recrystallisation and grain growth. The regions of

sample 2 have very large variation in hardness.

Table 32: Vickers lumlncss of jet plpc hcllow Itsscmhly.

5. SEM FRACTOGRAPIIY

The SEM observations of the fracture surfaces are shown in Figs. 7(a, b, c & d). Both

the samples IA and IB from the crack I are examined. Considering the fractographs of the

two samples, the failure occurred prcdominantly owing to fatiguc, whcrc fracture is

transgranular in nature. However, at the vicinity of initiation, the fracture is brittle

-------

intergranular in nature. It. indicates, that the grain boundary embrittlement is mainly

II

r~\ :~~- '-'; ..i "., I .

: . ~ 2 V'\)~ - (f» i'" ..2..(f1Q~- I. . J . L, . It... "'-- .

SampleNo. Elcmcntsin wt. pet.

Si Ti Cr Mn Ni Mo Fe

1 0.3 0.2 26.5 1.6 17.5 0.7 Bal2 0.4 0.2 39.8 0.3 14.3 1.3 Bal

SIIIIIJllc I. n. IllInlncssI

1/\ 201. '--'-'---'--'-'-'" , , ..--. .... ---.

III 214f--

2/\ ISH213 263

NATIONAL METALLUH.GICAI, LABOItATOltV, JAMSllEUl'liltFAILUREMODEANALYSIS/METALLlII~GICALEXAMINATIONOFVIPER AEROENGINEJET PIPE BELLOW

ASSEMBLYEx AIRCRAFTU-719REPORTNo. NML/MST/IAF/1.13/58/2005.April 2005

responsible for the initiation of fatigue cracks which propagate in circumferential direction

over a period of time in normal vibrational loading. The crack initiation took place from the

inner surface of sheet thickness and propagate towards the outer surface of the sheet as well as- .

in circumfercntialdirection. This typc of cilibrittlcmcntmay bc causctl by scnsitisationwhich

usually occurs at and above 700°C. Therefore, localised overheating experienced by the

corrugated region is sufficient enough to cause the sensitisation leading to grain boundary

embrittlement induced cracking.

After the intergranular cracking due to overheating induced sensitisation at the

initiation site, the crack propagates in transgranular mode. In this mode the vibrational loading

has caused the growth of fatigue cracks in stepwise crack opening .manifested by fatigue

striations. Besides this, mallY secolldary cracks allJl1~ crack propagatioll directioll me nlso

observed. No overload ZUlU; (Iilst li'Helurc 01' dililple rupturc) Oil thc liuellllc slllliH:e Is

observed because cracked ends are still connected.

Microvoid coalescence i.e dimple rapture is also found having shallow depth

indicating low ductility. This feature is perhaps not the outcome of service exposure rather

mechanicaltearing during handling i.e overload generatingan artefact. The fracture surface in

this region is not darkened becausc it is not cxposctlhcat source.

The cracking in non-corrugatcd region is also examinetl. The fracture surfaces of

samples 2A and 2B reveal the abrasion marks on its surface. This abrasion mark may be

formed allcr initial cracking Hndsubscqucnt rubbing contact. Thereforc, the original fracture

morphology is lost and nothing can be conc\udetl. Ilowever; in some localisctl region

intergranularcracking is also observed.The wide variation in hardness is observed at the two

locations i.e. 2A and 2B. At one end the low hardness indicates that the material is ductile

enough and on the other end high hardness value indicates low ductility. Inhomogeneity of

material is also responsible fix the failure. The rubbing marks rUIl more or less parallel to. ------

open edges of the sample along its longitudinal direction.

12

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NATIONAL METALLlmGICAL LAUOI{ATOIW,JAMSIIEUI'UI{FAILURE MOl>!<: ANAL.YSIS/METALLURGICAL EXAMINATION OF VII'EI~ AEltOEN(tlN.: JKI' I'IPE Ih:LLOW

ASS.:MnLYEx AmcRAFT U-719REPORTNo. NML/MST/IAF/I.13/58/2005. Avril 2005

"- -~"'

Figs. 7(a, b, c & d): SEM fractographs of sample 1A show~ng (a) intergranularfracture at the crack initiation site, (b) transgranular fracture imprinted with

fatigue striations (FS), (Q)fatigue crack initiation location and secondary crack(SC) and (d) beach marks (13M).

13 ----------

NATIONAL METALLURG ICAL LABORATORY, JA MSIIIWI'U RFAILURE MODE ANALYSiS/METALLURGICAL EXAMINATION OF VIPER AEROENGINE JET PIPE BELLOW

ASSEMBLY Ex AIRCRAFT U-719REPORT No. NMLlMST/IAF/I.13/5H/2005. April 2005

~

----. ..

...'~1

Figs. 8(a & b): SEM fraclographs of sampic IB showing (a) fatiguc striations (FS) andsecondary cracks (SC), and (b) ductile dimple rupture.

Figs. 9: SEM fractographof fracture surfaceof sample 2A showing abrasion marks (AM).

14

NATIONAL METAI.UJlWICAI, I.AIJOI{ATOnV, JAMSIIEIWlJl{FAILUREMODEANALYSIS/METALLllltGICALEXAMINATIONOFVIPElt AEROENGINEJET PIPE BELLOW

ASSEMBLYEx AIRCRAFTU-719REPORTNo. NML/MST/IAF/1.13/58/2005. Aoril2005

.:.

~r ,:

'j~!~

Figs. lO(a & b): SEM fractographs of sample 28 showing abrasion mark (AM) andtearing.

CONCLUSIONS

1. The circumferential cracking on the component is observed at corrugated region as

well as not corrugated region. These two regions are made of two different types

of austenitic stainlcss stcel closclysimilar to 830900 (corrugated region) and

832900 (non-corrugated region).

2. Microstructural and hardness measurcments on the different regions of crack

indicated overheating induced intergranular precipitation that may induce

intergranular decohesion.

.,J. The cracking is initiated by intergral1l~lar decohcsion at the inner surlhce and

propagated by flltiglle mechanism tuwards ollter sllrlace.

4. As the crack inititation is caused by localised overheating, therefore, it is not a

material problem rather the cracking is associated with the operational problem

during service exploitation.

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15

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~ --=--~=