Static and fatigue analyses of welded steel

structures – some aspects towards lightweight

design

Mansoor Khurshid

Doctoral Thesis

Stockholm, Sweden 2017

Division of Lightweight Structures

Department of Aeronautical and Vehicle Engineering

School of Engineering Sciences

KTH-Royal Institute of Technology

TRITA AVE 2017:04 KTH School of Engineering Sciences

ISSN: 1651-7660 SE - 100 44 Stockholm

ISBN: 978-91-7729-270-8 Sweden

Academic dissertation which with permission of Kungliga Tekniska Högskolan in Stockholm is

presented for public review and doctoral examination on April 07th

2017 at 09:00 in F3,

Lindstedtsvägen 26, KTH, Stockholm.

© Mansoor Khurshid, 2017

I

Table of Contents Preface ...................................................................................................................................................... i

Abstract .................................................................................................................................................. iii

Sammanfattning...................................................................................................................................... iv

Dissertation .............................................................................................................................................. v

Publications not included in the thesis ................................................................................................... vi

Conference proceedings: .................................................................................................................... vi

Peer reviewed Journals: ...................................................................................................................... vi

Division of work among the Authors .................................................................................................... vii

Paper A .............................................................................................................................................. vii

Paper B .............................................................................................................................................. vii

Paper C .............................................................................................................................................. vii

Paper D .............................................................................................................................................. vii

Paper E .............................................................................................................................................. vii

Paper F ............................................................................................................................................... vii

Paper G .............................................................................................................................................. vii

Paper H ............................................................................................................................................. viii

1 Introduction ..................................................................................................................................... 1

1.1 Background ............................................................................................................................. 1

1.2 Research aim ........................................................................................................................... 2

1.3 Research approach ................................................................................................................... 3

1.3.1 Static strength assessment-testing, selection of base and filler materials ........................ 3

1.3.2 Static strength assessment-Standard methods and numerical models ............................. 3

1.3.3 Fatigue strength assessment-testing and life estimation .................................................. 3

1.3.4 Residual stresses in improved welds-Behavior under constant and variable amplitude

loading ..........................................................................................................................................3

1.3.5 Residual stresses in improved welds-Effect of process parameters and numerical

simulations....................................................................................................................................... 3

2 Lightweight design concept ............................................................................................................. 5

3 Material Selection ............................................................................................................................ 7

3.1 Introduction static strength and high strength steels ............................................................... 7

3.2 Static testing ............................................................................................................................ 9

3.3 Influence of lack of penetration and strength of filler material ............................................... 9

3.4 Eurocode3, AWS D1.1, testing and finite element analysis .................................................. 10

3.5 Failure criteria for estimating static strength using finite element analysis .......................... 10

4 Fatigue Design Methods ................................................................................................................ 12

II

4.1 Introduction fatigue ............................................................................................................... 12

4.2 Fatigue testing ....................................................................................................................... 13

4.3 Fatigue strength assessment methods .................................................................................... 14

4.3.1 Nominal stress method .................................................................................................. 14

4.3.2 Structural hotspot stress method .................................................................................... 14

4.3.3 Effective notch stress method ........................................................................................ 14

4.4 Multiaxial fatigue strength assessment methods ................................................................... 15

4.5 Multiaxial fatigue strength assessment of inclined Butt welded joints failing at the weld root

................................................................................................................................................16

4.6 Fatigue strength assessment of fillet welded tube-to-plate joints failing at the weld root..... 16

5 Increased performance by post weld improvement techniques ..................................................... 19

5.1 Introduction High frequency mechanical impact treatment .................................................. 19

5.2 Residual stress measurements- X-ray diffraction technique ................................................. 20

5.3 Behavior of residuals stresses in HFMI treated welded joints .............................................. 21

5.4 Effect of HFMI process parameters on induced residual stresses ......................................... 22

5.5 Numerical estimation of residual stress state induced in HFMI treated welded joints and

plates ................................................................................................................................................22

6 Conclusions ................................................................................................................................... 24

7 Contribution to the field ................................................................................................................ 25

8 Summary of the appended papers .................................................................................................. 26

9 Future work ................................................................................................................................... 29

10 References ................................................................................................................................. 30

i

Preface The work in thesis has been carried out at the Department of Aeronautical and Vehicle Engineering,

Division of Lightweight Structures, KTH-Royal Institute of Technology, Sweden from September

2012 to December 2016. The work done in this thesis is part of European project,”Improving the

fatigue life of high strength steel welded structures by postweld treatments and specific filler material

” (FATWELDHSS) Volvo Construction Equipment’s project MUD, and Swedish R&D project

,”Innovative concepts for lightweight design and fabrication of welded high strength steel structures”

(INNODEFAB).

Some of the work has been presented and discussed within commission XIII and commission XV in

the International Institute of Welding during the Annual Assembly in Helsinki, Finland in 2015 and

Melbourne, Australia in 2016. Part of the work has been awarded finalist paper award at the ASME

International conference on pressure vessel and pipelines in Paris in 2013. The work in this thesis is

funded by Volvo Construction Equipment, Cargotec Sweden AB Bromma Conquip, VINNOVA, and

European fund for coal and steel. Stresstech Oy, and Sonats are acknowledged for residual stress

measurements and SSAB is acknowledged for manufacturing and testing test specimens in paper B.

First and foremost, I would like to express my gratitude to my supervisor Dr Zuheir Barsoum and co

supervisor Dr Imad Barsoum for their continued support, encouragement, and patience throughout the

work. Their trust in me and guidance as scientists, mentors, friends, and elder brothers is greatly

appreciated. Dr Thomas Däuwel at Volvo Construction Equipment Germany, Associate Professor

Micheal Stoschka and Assistant Professor Martin Leitner at Leoben University Austria, Professor

Gary Marquis at Aalto university Finland, Dr Eeva Mikola at VTT Finland, Professor Cetin Morris

Sonsino at Lbf Germany, Assistant Professor Halid Can Yildirim, Professor Muhammad Al Imrani

and Poja Shams Hakimi at Chalmers Tekniska Högskolan Sweden, Lasse Souminen (Finland) and

Per Lundin (Sweden) from Stresstech Oy, Professor Jack Samuelsson KTH, Dr Christof Schneider

and Dr Joakim Hedegård at Swerea Kimab, Dr Ayjwat Awais Bhatti at Inspecta, Ahmad Mahmoudi

and Dr Matthew Doré at TWI UK are highly acknowledged for their discussions during the projects

and sharing authorship in numerous publications. I would also like to thank my colleagues within the

division of lightweight structures for providing a nice working environment.

I am grateful to Dr Elsiddig Emulkashfi Post-doctoral candidate at Oxford University The UK, and Dr

Aftab Ahmad research scientist at ABB and Mathilda Karlsson at KTH for their scientific and friendly

discussions during my studies. Dr Mehmood Alam Khan Malagori, for scientific discussions and

taking care of me when I got sick during 2015. Izhaar ul Hassan, for helping me out with

accommodation and initial guidance and all the support in Sweden. Without you guys life is Sweden

would have not been so easy.

Special and hearty thanks to Katrin Olausson and Emma Drake for being great friends, and teaching

me Swedish language. Thank you for all the discussions, fikas, lunches, and dinners during these

years. Without you guys, I would have not been able to speak the Swedish I speak today and felt

integrated in Swedish society.

To Juan Francisco Rodriguiz ,Sherif Fouda, Akbar Khan, Dr Navid Ahmed, Ali Reza Ahmadi, Anis

Nasir, Dr Iqbal Hussein, Anna Larsson, Anna Ludwall, Simon Hallgren, Daniel Lundel, Herman Rask,

Amil Lafeh, Maria Santillana, Yoann Prigent, Awais Mehmood, Seyhmus Bedirhanoglu, and Oscar

Liedhem. Thank you for your friendship and quality time.

ii

Last but not the least, I am forever grateful to my parents Dr. Khurshid Ahmed and Saira Bibi, my

brother Dr.Yasir Khurshid and his wife Nosheen Yasir, my sisters Kashmala Yasir and Dr.Urooj

Khurshid and her husband Dr.Mohsin Sami, my niece Zartasha Yasir, my uncle Samiullah Khan and

aunt Seema Sami for their long lasting and unconditional love, support and prayers.

Stockholm, January 2017

Mansoor Khurshid

iii

Abstract This thesis work is concerned with the static and fatigue strength design of welded steel structures.

Welded joints in steels of grade S355, S690QT, S600MC, S700MC, and S960 are focused. Several

topics related to the design of welded joints are addressed such as; influence of filler material strength

and weld metal penetration on the static strength, static testing, fatigue testing, multi-axial root fatigue

strength estimations by nominal and local stress based approaches, multiaxial fatigue strength of joints

subjected to high shear to normal stress ratios, effect of improvement techniques on the fatigue life of

welded joints, influence of residual stresses on the fatigue strength, and residual stress estimations and

measurements and their behavior during fatigue loading in improved welds, the effect of material

models on the estimated residual stress states in simple plates induced by high frequency mechanical

impact (HFMI) treatment, and influence of HFMI treatment process parameters on the induced

residual stress state.

The objectives of this thesis comprise of overcoming the challenges in designing lightweight welded

structures such as material selection, choice of fatigue design methods, and increased performance by

using improvement techniques. Material selection of welded joints is dependent on the filler and base

material strengths. Partially and fully penetrated cruciform and butt welded joints were designed in

under-matching, matching, and over-matching filler materials. Base material steel grades were

S600MC, S700MC, and S960. Current design rules are developed for welds in steel up to yield

strength of 700MPa. Therefore, design rules in Eurocode3, AWS d1.1, and BSK 07 were verified and

recommendations for developing design rules for designing welded joints in S960 were concluded.

Numerical methodology for estimating static strength of welded joints by simulating heat affected

zone was also developed.

Another objective of the thesis work was to overcome the challenges in selection of fatigue design

methods. The available design curves in standards are developed for uniaxial stress states, however, in

real life the welds in mechanical structures are subjected to complex multiaxial stress states.

Furthermore; weld toe failures are frequently investigated, weld root failures are seldom investigated.

Therefore, in this work the multiaxial fatigue strength of welded joints failing at the weld root was

assessed using experiments and various nominal and local stress based approaches. Butt welded joints

with different weld seam inclinations with respect to applied uniaxial loading were designed to assess

the root fatigue strength in higher multiaxial stress ratio regime. The fatigue strength of multi-pass

tube-to-plate welded joints subjected to internal pressure only and combined internal pressure and

torsion in and 90° out of phase loading was also investigated. Test data generated in this thesis was

evaluated together with the test data collected from literature.

Last objective of the thesis included investigation of the increased performance in fatigue strength by

post weld treatment methods such as HFMI. The behavior of residual stresses induced due to HFMI

treatment during fatigue loading is studied. Numerical residual stress estimations and residual stress

relaxation models are developed and the effect of various HFMI treatment process parameters and

steel grade on the induced residual stress state is investigated. Specimens studied were non load

carrying longitudinal attachments and simple plates. Residual stresses in both test specimens were

measured using X-ray diffraction technique

iv

Sammanfattning Denna doktorsavhandling berör statiskt-och utmatning av svetsade konstruktioner. Där fokus på

svetsförband S355, S690QT, S600MC, S700MC och S960 är studerade. Flera ämnen relaterade till

konstruktion av fogningsmetoder behandlas såsom: Påverkan av tillsatsmaterialets hållfasthet och

svetspenetrering på statiskt belastning, statiskt provning och utmatningsprovning, uppskatningar av

utmattningshållfasthet för fleraxliga laster i svetsroten baserad på nominala och lokala

spänningsmetoder, fleraxligt utmatningshållfasthet i svetsfogden på grund av skjuvning och

normalspänningar, effekten av förbräningsmetoder på uttmatninghållfasthet, inverkan utav

restspäningar på utmaninghållfasthet, uppskatning av restspänningar och mätningar i förbättrade

svetsfogar och dess inverkan på uttmaningsbelastning, effekten av materialmodeller och uppskattning

av restspäningar i enkla plattor behandlade med high frequency mechanical impact (HFMI) och

inverkan av HFMI behandlingsprocessparametrar på inducerade restspänningar.

Målet för denna avhandling har varit att överinna utmaningarna vid dimensioneringen utav svetsade

lättkonstruktiner såsom materialval, samt valet av utmattningsdimensioneringen för på såsätt öka

prestandan genom att använda olika förberäningsmetoder. Materialvalet av svetsförband är beroende

av fyllmedlet och basmaterialets hållfast, Delvis eller fullt pentreradade korsformade-och

stumsvetsade svetsförband är dimisionerat med under-matching(anpassning), matching (anpassning),

and over-matching(anpassning) med hänsyn till tillsatsmaterial och Basmaterial i stålgrad S600MC,

S700MC och S960. Nuvarande konstruktionkrav är utvecklade för svetsar för stål upp till 700Mpa i

sträckgräns. Därmed verifierades riktlinjer i Eurocode3, AWS D1.1, och BSK 07 och

rekommendationer för att utveckla konstruktionskrav för svetsförband i S960 förkastas. Det har också

utvecklats en numerisk algoritm som utvärderar den värmepåverkande svetsade ytan för att beräkna

fram statisk belastning.

Ett annat syfte med avhandlingen var att övervinna utmaningarna i form av utmattningshållfasthet

dimensionerande. De tillgängliga rekommendations kurvor som anges i handböcker har utvecklas för

enaxlade spänningstillstånd, men i verkliga verket utsätts svetsarna i mekaniska konstruktioner för

komplexa fleraxliga spänningstillstånd. Vidare så sker mer undersökningen utav svetsroten i

jämförelse med svetshalsen. Och därmed har i detta arbete fokus lagts på fleraxigt

uttmatningenbelastning för brott i svetsroten med hjälp av experiment och olika nominella och lokala

spänningsbaserade metoder. Stumsvetsförband med olika lutningar på svetsfog med avseende på

tillämpad enaxligt belastning var utformade för att utvärdera utmattningshållfasthet i roten i högre

multiaxiell spänningsvidd. Utmattningshållfasthet mot multi-pass rör-till-platt för svetsfogar som

utsätts för endast inre tryck eller kombination inre tryck och vridning i och 90 ° ur fas laddning har

också undersökts. Provningsresultat i denna avhandling utvärderades tillsammans med testdata som

samlats in från olika avhandlingar.

Sista delen i avhandlingen där har undersökning av ökad prestanda i utmattningshållfasthet per post

svets behandlingsmetoder såsom HFMI studerats. Man har även studerats beteendet hos

restspänningar som inducerats på grund av HFMI behandling under utmattningsbelastning.

Uppskattningar utav Numeriska restspänningar och restspänningsrelaxa modeller har utvecklas och

effekten av olika HFMI behandlingsprocessparametrar och stålsort på den inducerade

restspänningstillståndet har undersökts. Proverstavar som studerades var icke bärande längsgående

styvningar och enkla plattor. Restspänningar i provstavarna har mätts med hjälp av röntgendiffraktion

teknik.

v

Dissertation This doctoral thesis consists of an introduction to the area of research and the following appended

papers:

Paper A

M. Khurshid, Z. Barsoum , N.A. Mumtaz. Ultimate strength and failure modes for fillet welds in high

strength steels. Materials & Design, Volume 40, September 2012, Pages 36-42.

Paper B

M. Khurshid, Z. Barsoum, I. Barsoum. Load carrying capacities of butt welded joints in high strength

steels. ASME Journal of Engineering Materials and Technology, October 2015, Vol. 137 / 041003-

1.doi: 10.1115/1.4030687

Paper C

M. Khurshid, Z. Barsoum, I. Barsoum, T. Däuwel. Multiaxial weld root fatigue of butt welded joints

subjected to uniaxial loading. Fatigue & Fracture of engineering materials & structures. Volume 39,

Issue 10, October 2016 Pages 1281–1298.

Paper D

M. Khurshid, Z. Barsoum , T. Däuwel, I. Barsoum. Multiaxial weld root fatigue of fillet welded tube

to plate joints subjected to non-proportional multiaxial stress state. International Journal of Fatigue

(submitted)

Paper E

M. Khurshid, Z. Barsoum, G. Marquis. Behavior of compressive residual stresses in HSS welds

induced by HFMI treatment. ASME journal of pressure vessel technology, August 2014, Vol. 136 /

041404-1

Paper F

Lasse Suominen , Mansoor Khurshid, Jari Parantainen. Residual stresses in welded components

following post-weld treatment methods. Procedia Engineering 66 (2013) 181 – 191

Paper G

M. Khurshid, M. Leitner, Z. Barsoum, C. Schneider. Residual stress state induced by High Frequency

Mechanical Impact Treatment in different steel grades -numerical and experimental study.

International Journal of Mechanical Sciences (accepted)

Paper H

M. Leitner, M. Khurshid, Z. Barsoum. Stability of High Frequency Mechanical Impact (HFMI) post-

treatment induced residual stress states under cyclic loading of welded steel joints. Journal of

engineering structures (submitted)

vi

Publications not included in the thesis

Conference proceedings:

M. Khurshid, Z. Barsoum, G. Marquis. Behavior of compressive residual stresses in HSS welds

induced by HFMI treatment. ASME PVP July Paris. France, 2013 (Awarded finalist paper prize in

Phd Student paper competition)

E.Mikkola, M. Doré, M. Khurshid. Fatigue strength of HFMI treated structures under high R-ratio and

variable amplitude loading. Proceedia Engineering 66 (2013) 161-170

M. Khurshid, Z. Barsoum. Static behavior of fillet welded cruciform joints in S600MC high strength

steel. Proceedings of 2nd Swedish conference on design and fabrication of welded structures, October

9-10, 2013, Borlänge, Sweden.

Peer reviewed Journals:

Z. Barsoum, M. Khurshid, I. Barsoum. Fatigue strength evaluation of friction stir welded aluminium

joints using the nominal and notch stress concepts. Materials & Design, Volume 41, October 2012,

Pages 231-238

E. Mikkola, M. Dore, G. Marquis, M. Khurshid. Fatigue assessment of high-frequency mechanical

impact (HFMI)-treated welded joints subjected to high mean stresses and spectrum loading. Fatigue

Fract Engng Mater Struct 00, 1–14.(2015) doi: 10.1111/ffe.12296

A.A. Bhatti, Z. Barsoum, M. Khurshid. Development of a finite element simulation framework for the

prediction of residual stresses in large welded structures. Computers and Structures 133 (2014) 1-11.

vii

Division of work among the Authors

Paper A

Khurshid and Barsoum setup the criteria for designing the specimen, selecting filler and base

materials, wrote the manuscript and addressed the review comments. Khurshid also conducted Finite

element simulations. Mumtaz contributed in conducting the experiments, helped in comparing the

standards and provided comments on the paper.

Paper B

Khurshid selected base and filler materials, designed test specimens, conducted the experiments, set up

finite element model and wrote the manuscript. Z.Barsoum contributed in writing part of the paper and

added valuable scientific discussion and comments. I.Barsoum contributed in developing finite

element models, comparing standards, writing part of the paper, and providing comments to the

manuscript.

Paper C

M.Khurshid selected base material, designed test specimen, conducted experiments, identified the

scientific gap, collected data from literature, compared different standards and methods, developed

finite element models, and wrote the manuscript. Z.Barsoum provided scientific comments and

contributed in writing the manuscript. I.Barsoum provided scientific comments and contributed in

writing the manuscript. T.Däuwel contributed in designing the test specimens, analyzing the test

results and provided valuable comments to the manuscript.

Paper D

M.Khurshid selected the loading, contributed in conducting the experiments, identified the scientific

gap, collected data from literature, compared different standards and methods, developed finite

element models, and wrote the manuscript. Z.Barsoum provided scientific comments and contributed

in writing the manuscript. T.Däuwel contributed in designing the test specimens, analyzing the test

results with different methods and provided valuable comments to the manuscript. I.Barsoum provided

scientific comments and contributed in writing the manuscript.

Paper E

Khurshid planned fatigue testing and residual stress measurements, designed fixture for measuring

residual stresses, developed finite element models, wrote the manuscript. Barsoum provided scientific

comments, helped in fatigue test setup, defining the load levels, designed the load block and

contributed in writing the manuscript. Marquis provided scientific comments, designed the load block

and contributed in writing the manuscript.

Paper F

Khurshid supported Souminen in writing and finalizing the manuscript. Khurshid contributed in

writing part of the manuscript, and analyzing residual stress measurements in HFMI treated welded

joints. Parantainen contributed in conducting residual stress measurements and provided comments to

the manuscript.

Paper G

Khurshid selected materials for investigation, designed the plates, selected HFMI treatment

parameters, developed finite element models, analyzed the results and wrote the manuscript. Leitner

contributed in identifying the scientific gap, establishing material models, and writing the manuscript.

viii

Barsoum contributed in writing part of the manuscript and provided scientific discussions. Schneider

contributed in planning residual stress measurements, measuring the indentation depth, and provided

valuable comments to the manuscript.

Paper H

Leitner planned and conducted the experiments, conducted finite element simulations, and Khurshid

supported Leitner in writing and finalizing the manuscript. Khurshid contributed in identifying the

scientific gap for parametric study of the residual stress state due to HFMI, analyzed the existing

analytical models. Barsoum provided valuable scientific discussions and comments to the paper.

1

1 Introduction

1.1 Background

Structural integrity is important in many industrial sectors where welding is a primary technique for

joining. In numerous structures welds are identified as critical sections and prone to mechanical

failures. Mechanical failures involve extremely complex interaction of load, time, material,

manufacturing processes, and environment. Environment includes temperature, corrosion, and

operation in H2S environment. Loads may be monotonic, steady, variable, uniaxial or multi-axial. Also

the duration of loads can last from a fraction of second to years e.g. loading in firing a hand gun and

loading in bridges. Two of the thirteen common failure modes identified in metals are ductile failure

and fatigue failure 1. A particular welded structure is usually designed against these failures to ensure

its integrity. Ductile failure utilizes excess plastic deformation which is used in designing structures in

vehicle industry e.g. cranes, spreaders, and Haulers etc. It is also used for designing cabin of the

drivers in construction equipment etc to protect drivers in an unexpected roll over collisions. The other

common failure mode is due to fatigue which is found to be responsible for 50 to 90 percent of all

mechanical failures. Welded joints in mechanical structures which are subjected to these kinds of

failures are shown in figure 1.

Figure 1: Paper C, Welds in mechanical structures

It is estimated that about one-fifth of the EU’s CO2 emissions through road transport is the main

cause of global warming. A reduction of 10% in vehicle weight can result in 6% fuel economy

improvement.2 Therefore, one of the goals of aerospace, vehicle, construction equipment, and oil and

gas etc. industries is to develop structures with lower weight, better performance, high payload

capacity, more fuel efficient, and less CO2 emissions rates. Welded steel is a common material utilized

in the construction of the products of these industries e.g. eighty percent of the main structure and

components in a Volvo Wheel Loader is welded steel3. One way to achieve the above mentioned goals

is to utilize more high strength steel e.g. steels of yield strength higher than 355MPa. Other examples

of mechanical structures mostly made of high strength steel are spreaders and cranes.

To successfully implement HSS in mechanical structures it is important that different parts are joined

together without defects in the weld and therefore achieving sufficient static and fatigue strengths. The

speed of developing new steels is much higher than the speed of developing new filler materials of

higher strength. As the yield strength of steel reaches 960MPa, it becomes difficult to find filler

materials of similar strength which is one of the hurdles in utilizing higher strength steels as base

materials. Another problem with welding higher strength steels is the development of heat affected

2

zone (HAZ) during welding. Due to melting of the steel, microstructural changes occur in the vicinity

of the weld which results in the development of HAZ. For steels of higher yield strength especially

greater than 500MPa this zone is very obvious and can result in lower hardness than the base material

and weld metal. This zone might limit the global strength of the joint and the strength of base material

will not be utilized to full capacity. In this way the target of higher pay load capacity might not be

achieved. However; this problem of lack of similar strength filler material can be solved by designing

proper size of the weld. Similarly, the influence of HAZ can be minimized by optimizing weld

processes. Other than this weld toe and weld root notches, lack of fusion and undercuts are very

common in welded joints. These defects can give rise to higher stress concentrations and act as

potential sites for crack initiation. Residual stresses are also developed in the welds due to rapid

heating and cooling. The combination of residual stresses and applied load can affect the fatigue life.

Higher stress concentrations and tensile residual stresses decrease the fatigue life while low stress

concentrations and compressive residual stresses increase the fatigue life. Various improvement

techniques are used to achieve higher fatigue lives in HSS welds. Post weld heat treatment, TIG

dressing, Hammer peening, bur grinding and High frequency mechanical impact treatment (HFMI) are

few to mention. Post weld heat treatment reduces residual stresses, bur grinding improves the weld toe

transition radius, and Tig dressing, hammer peeing, and HFMI improve the weld toe transition and

reduces notch stress concentration and induces compressive residual stresses. Among them HFMI is a

promising method but the effect of HFMI process parameters on the fatigue life and state of residual

stresses is unclear. Similarly, the behavior of residual stresses under variable loading in HFMI treated

welds is unclear and needs further investigations.

The type of loading also has an effect on the fatigue strength of welded joints. It is important to define

correct capacity of the weld. Design curves in standard available today are developed for simple

uniaxial stress states. Welds in real life structures as shown in figure 1 are subjected to complex multi-

axial stress states. In most of the cases these stress states decrease the fatigue life. To design welds

against these complex stress states choice of the appropriate analysis method and definition of the

loading is important. Furthermore; knowledge of the fatigue behavior of welds under higher multiaxial

stress states is unknown.

Better understanding of the static strength, multiaxial fatigue strength, and the effect of improvement

methods on the fatigue strength will make possible manufacturing more components with HSS and

thus minimizing weight and increasing structural durability.

1.2 Research aim

The research work in this doctoral thesis aims to improve the design of lightweight welded joints in

steels. The following specific research questions in the area of static strength, multi-axial fatigue

strength, and influence of improvement techniques are addressed:

Influence of filler material strength and weld size on the static strength of welds in HSS

Verification and modification of the design rules in standards for static strength design

Development of continuum damage mechanics based numerical models for estimating

ductile failure in welds

Effect of multiaxial stress ratio on the root fatigue strength and generating experimental

results for increased statistics of joints subjected to complex loading

Influence of residual stresses under complex loading on the fatigue strength

Assessing the capabilities of existing multiaxial fatigue models for the estimation of fatigue

strength of welded joints failing at the root subjected to proportional and non-proportional

multiaxial stress states

3

Behavior of compressive residual stresses under constant and variable amplitude loading in

improved welds

Development of numerical models for the estimation of compressive residual stress state due

to HFMI

Establishing a link between HFMI process parameters and the magnitude of induced

compressive residual stresses

1.3 Research approach

The research can be divided into the following items:

1.3.1 Static strength assessment-testing, selection of base and filler materials

Fillet welded cruciform and butt welded dumble shaped specimens were manufactured to assess the

static strength of welds in steels of grade S600MC, S700MC, and S960. Tensile testing was performed

in fully penetrated and partially penetrated welds in under-matching, matching and overmatching filler

materials. Hardness testing and etching of the joints was performed to quantify HAZ and changes in

material properties due to welding process.

1.3.2 Static strength assessment-Standard methods and numerical models

Design methodologies for static strength design in BSK07, Eurodcode3, and AWS D1.1 were verified

for joints in steels of grade S600MC, and S700MC. There validity for design of welds in S960 steel

was also studied and correlation factors were recommended. The load carrying capacities were

estimated numerically using finite element analysis (FEA) by simulating HAZ. The properties of weld

metal, base material and HAZ were based on hardness tests and etching.

1.3.3 Fatigue strength assessment-testing and life estimation

Multiaxial fatigue strength of inclined butt welds subjected to proportional multiaxial stress state due

to uniaxial loading was studied. The fatigue strength was studied in low and high multiaxial stress

ratio regimes. Tube-to-plate welded test pieces were manufactured to study the fatigue strength under

complex loading. The test pieces were tested in internal pressure only, and combined internal pressure

and torsion in and out of phase loading. The combined loading produced non proportional stress state.

Weld root failure was studied. The fatigue life estimation capabilities of interaction equation in

Eurocode3, and modified Gough Pollard equation, and interaction equation in DNV standard in

nominal stress system and fatigue life estimation capabilities of local stress based assessment methods

in local stress system such as von Mises strength hypothesis, principal stress hypothesis, critical plane

based modified Wöhler curve method, and integral hypothesis effective equivalent stress hypothesis

were compared.

1.3.4 Residual stresses in improved welds-Behavior under constant and variable

amplitude loading

Compressive residual stresses in high frequency mechanical impact treated welds were measured

using X-ray diffraction technique. Their behavior was studied in non-load carrying longitudinal

attachments subjected to constant and variable amplitude loading. Behavior of both surface and depth

residual stresses were studied.

1.3.5 Residual stresses in improved welds-Effect of process parameters and numerical

simulations

The effect of process parameters of HFMI treatment on the magnitude of compressive residual stresses

was studied on plate specimens. Pneumatic impact treatment and ultrasonic impact treatment

methodologies were investigated. Numerical models were developed to estimate compressive residual

4

stress states induced by both methodologies. X-ray diffraction technique was used to measure surface

residual stresses. X-ray diffraction technique along with electro polishing was used to measure

residual stresses in the thickness direction.

5

2 Lightweight design concept Lightweight design concept is defined as “a subject aiming to develop and widen the use of

lightweight structures and lightweight materials in order to achieve increased performance for a wide

range of structural applications. Performance is conceptualized as a general quantity aiming at both

functionality and use in terms of e.g. reduced fuel consumption, environmental impact and life cycle

cost. Lightweight structure is a generic but also application near research area based on material

science, structural mechanics, processing and design.”4. The construction of lightweight structures

involves overcoming various challenges. Main challenges are as follow:

Material selection for design

Structural performance

Manufacturing and joining processes

Life cycle costs

Environmental impact

The aim of light weight concept is to develop light and stiff structures but material properties put

limits on the design. Usually the stiffness to weight ratio is an important parameter for the choice of

material. In welded joints the selection of filler and base materials puts limits on the static strength of

the structure and thus it influences the pay load capacities. Appropriate material selection criteria are

important for developing welded structures. It is a known fact that materials have properties but no

shape. They are shaped into different forms using various manufacturing and joining processes. When

materials are joined into different shapes e.g. hollow welded beams or welded stiffeners on plates etc.

the structural performance has to be ensured. The structural performance is ensured against failures

due to elastic instability i.e. buckling resistance and failures due to repetitive loads i.e. fatigue

resistance. In welded joints fatigue is the major cause of failure and hence the structural performance

is affected significantly by this kind of failure. The selection of suitable design methods will increase

the accuracy of estimating the structural performance and thus enhance the development of lightweight

structures.

In this thesis the lightweight construction challenging aspects such as material selection for design and

structural performance have been addressed. In figure 2, it can be seen that the structural durability is

affected by the interaction of loadings, the material, design, and manufacturing processes which

influences cost of the mechanical product. Two of the well-known failures which affect structural

durability are failures against static and fatigue loading. Design of welded joints against static strength

motivates the selection of base and filler materials for welded joints and sets up criteria for

dimensioning the weld. Design against fatigue loading is a wide area and it is affect by various factors

such as selection of fatigue design methods, influence of residual stresses, type of loading, welding

processes, and improvement methods for increasing the fatigue resistance. Fatigue resistance models

against multiaxial loading and the influence of improvement techniques on the fatigue strength are

focused.

6

Figure 2:Structural durability

7

3 Material Selection

3.1 Introduction static strength and high strength steels

Static strength is important for utilizing full capacity of the base material. This becomes more

important when high strength steels are used in the construction of mechanical structures and

achieving higher pay load capacities are one of the targets. The development in the strength of steel is

far ahead than the development of strength of filler materials. In5 a detailed description about steel

development is provided. In figure 3, several different steel grades classified by their metallurgical

designation are plotted. Steels are usually designated as low strength steels, high strength steel (HSS)

and advanced high strength steel (AHSS). The principal difference between conventional HSS and

AHSS is their microstructure. Conventional HSS are single-phase ferritic steels with a potential for

some pearlite in C-Mn steels. AHSS are primarily steels with a microstructure containing a phase

other than ferrite, pearlite, or cementite e.g. martensite, bainite, austenite, or retained austenite in

quantities sufficient to produce unique mechanical properties. Some types of AHSS have a higher

strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other

types have ultra-high yield and tensile strengths and show a bake hardening behavior. Steels with yield

strength levels in excess of 550 MPa are generally referred to as AHSS. These steels are also

sometimes called “ultrahigh-strength steels” for tensile strengths exceeding 780 MPa. AHSS with

tensile strength of at least 1000 MPa are often called “Giga Pascal steel” (1000 MPa =1GPa).

Austenitic Stainless Steel as shown in figure 3 has excellent strength combined with excellent

ductility. These properties make them appropriate choices for many vehicles but they are expensive

choices for many components, and their joining is a challenge. The Third Generation AHSS seeks to

offer comparable or improved capabilities at significantly lower cost 5. Usually steels can be classified

in several different ways. One is a metallurgical designation. Common designations include low

strength steels (interstitial-free and mild steels); conventional HSS (Carbon-Manganese, bake

hardenable and high-strength, low-alloy steels); and the new AHSS (dual phase, transformation-

induced plasticity, twinning-induced plasticity, ferritic-bainitic, complex phase and martensitic steels).

Additional higher strength steels for the automotive market include hot-formed, post-forming heat

treated steels, and steels designed for unique applications that include improved edge stretch and

stretch bending. A second classification method important to part designers is the strength of the steel.

This classification system has a problem because of the ongoing development of steels e.g. DP or

TRIP steels have strength grades that encompass two or more strength ranges. A third classification

method presents various mechanical properties or forming parameters of different steels, such as total

elongation, work hardening exponent, or hole expansion ratio. The boundary line for HSS and ultra-

high strength steel (UHSS) is not defined but generally steels with tensile strengths greater than

700MPa are regarded UHSS 2. In this thesis only HSS are investigated. Figure 4 shows the

development in strength of filler materials and HSS. The comparison is only based on the yield

strengths of the available steels and filler materials. It can be seen that filler materials of similar yield

strength are rarely available as the yield strength of base material reaches 960MPa.This make steels of

yield strength greater than 700MPa to be welded with under-matching filler i.e. yield strength of filler

material is lower than the yield strength of base material. Therefore, the static strength of the joint is

limited to the strength of filler material. Using steel of yield strength greater than 700MPa in such

situation might not be an appropriate choice if the weld is not properly designed.

8

Figure 3: Tensile strength as a function of elongation and classification of steel2

Figure 4: Development of HSS and filler materials

Various design methodologies for welded joints subjected to predominant static loading are given in

Eurocode36 and AWS D1.1

7. These are valid for welds in steel up to yield strength 700MPa. Darrel

etal8 investigated the ultimate strength of partially penetrated groove welds in two different mild steels

and five weld metal penetration ratios (20-50%). They developed a method and proposed equations for

predicting the ultimate tensile strength of welded joint. Lesik et al9 developed expressions for

predicting ultimate load and deformation capacities in fillet welded connections. Satoh et al10

studied

the influence of under-matching filler on the strength of weld in HT 80 structural steel. They found

that the strength of the weld reaches the strength of base metal as the width of soft layer i.e. the under-

matching weld is decreased. A reasonable under-matching strength was found not less than 90% of the

base metal. In11–15

, the validity of design rules in Eurocode3 for HSS are investigated. Under-matching

filler is observed to increase the ductility of the joint and increase in the strength of filler material

increases the global strength of the joint. Collin etal12

proposed that average strength of base and filler

material should be used in estimating the ultimate strength of welded joint with overmatching filler

material. In this thesis the static strength of fillet welded cruciform specimens in S600MC is assessed

in paper A. Paper B addresses the static strength of butt welds in S700MC and S960 steels. In both

paper A and paper B the influence of lack of weld metal penetration, strength of filler material and

HAZ on the static strength is studied. Load carrying capacities are estimated numerically using limited

plastic strains as a failure criterion. The design methodologies in Eurocode3, AWS D1.1and results

from testing and finite element analyses are compared. Recommendations for correlation factors for

Eurocode3 for the design of welded joints in S960 are made in paper B. In paper C, numerical

methodology based on failure strain and stress triaxiality has been used to design test specimen with

9

sufficient static strength. Based on the numerical estimations, appropriate base and filler materials

were selected for manufacturing the test specimen.

3.2 Static testing

For welded joints static strength is assessed by performing tensile testing. Applied loading can be

uniaxial force, bending, or torsion. These tests are usually performed at different strain rates. The test

specimens are clamped at one end and pulled at the other end until complete rupture. The results are

plotted as load deformation curve. This curve quantifies both load carrying and deformation capacities

of the joints. The deformation of the joint is measured using linear extensometer and strain gauges.

Strain gauges were mounted at the reference points for the hotspot of the weld. Paper A, describes

tests on fillet welded cruciform joints, while paper B describes tests performed on butt welded dumble

shaped test specimens. The test rig used in paper A was SCHENK HYDROPULS PSB 250 located at

KTH, while the test rig used for experiments in paper B was Schenk-Trebel machine with a capacity

of 600kN located at SSAB. The test setups are shown in figure 5.

Figure 5: Testing rigs used for static testing: (a) Paper A (b) Paper B

3.3 Influence of lack of penetration and strength of filler material

The most important geometrical parameters of welds are the notch radius (weld toe and root), the flank

angle at the weld toe and the depth of penetration at the weld root7. The most important mechanical

parameters of the welded joint are strength of base metal, strength of filler material, and strength of

HAZ. Along with these parameters the static strength is influenced by the width of weld metal, and

HAZ. Welding process has influence on the strength of HAZ. This zone is created due to

microstructural changes occurring because of welding process in the base material near by the weld

metal. In high strength steel welds this affect is more prominent than the mild steel welded joints.

Usually a decrease in strength is observed in HAZ as compared to the base material strength. Various

investigations11,16–20

, have studied the effect of heat input on the strength of HAZ. The effect of lower

strength of HAZ on static strength is also studied in these investigations. It is observed that the overall

effect of soft zone in the HAZ on global strength depend on the relative thickness of the soft

interlayer, ratio of the sample width to sheet thickness, strength decrease in the soft zone, and local

orientation of the soft zone with respect to the applied loading. In paper A and paper B narrow HAZ

with reduced strength in comparison to the base material were observed.

Among the geometrical parameters weld metal penetration is an important parameter which has

significant influence on both static and fatigue strengths of the joint. In paper A and paper B it is

10

observed that the static strength of the joint increases with an increase in weld metal penetration and it

decreases with a decrease in weld metal penetration. This effect is more significant in joints with

under-matching filler materials, moderate in joints with matching filler material, and comparatively

less in joints with overmatching filler material. It is also observed that with the increase of the yield

strength of the steel the static strength increases provided similar weld metal penetration is achieved

and similar strength filler material is used in the joint.

3.4 Eurocode3, AWS D1.1, testing and finite element analysis

In paper A, the design methodologies in BSK07, Eurocode3, and AWS D1.1 are evaluated. In paper B,

design methodologies in Eurocode3 and AWS D1.1 are evaluated. Load carrying capacities estimated

by these methodologies, finite element analysis (FEA) and testing are compared. The methodologies

in standards and codes were observed conservative and valid for evaluating welded joints in S600MC,

S700MC, and S960. They were able to estimate load carrying capacities under-matching, matching,

and overmatching welded joints. All the estimates by FEA and design methodologies were in good

agreement with the experiments. Figure 6, shows results from paper B.

Figure 6: Paper B (a) S700MC Fully penetrated joint (b) Partially penetrated joint (c) S960 fully penetrated joint (d)

S960 Partially penetrated joint

The correlation factors in Eurocode3 were also evaluated. In paper B, the use of correlation factor of

1.0 was found appropriate for under-matching welds in S700MC. Correlation factor of 1.0 was also

found appropriate for welds in S960 which ensured 20% safety to the maximum load carrying capacity

of the joint.

3.5 Failure criteria for estimating static strength using finite element analysis

Limited plastic strain criterion and ductile failure criterion are usually used in evaluating static

strength. Limited plastic strain criterion gives reasonable estimates of the load carrying capacity of the

joint but it fails to estimate the deformation capacities. Various reaserchers11

have used this criterion in

estimating the load carrying capacity. In both paper A and paper B, this criterion has been adopted

since load carrying capacity of the joints was mainly focused. Ductile failure criterion is based on the

11

relationship of failure strain and stress triaxiality. This criterion estimates both load carrying and

deformation capacities reasonably well. This has also been frequently used21

. In paper C, this criterion

was implemented to design test specimen against static loading.

12

4 Fatigue Design Methods

4.1 Introduction fatigue

Design methodologies play a vital role in the construction of lightweight structures. The word fatigue

comes from Latin word fatigare which means “to tire”. Fatigue is the most common type of failure in

engineering structures. About 50-90% of failures are due to fatigue1. Most of these failures are

unexpected and are caused by repetitive load cycles. The load is usually not large enough to cause

static failure. This phenomenon was observed in the beginning of 19th century. W.Albert, a German

mining engineer presented the first known fatigue tests in 1837. In 1860, August Wöhler did

investigations on railway axels and lead to characterization of fatigue data in the form of stress

amplitude-life curves (S-N)1. Welded joints in mechanical structures are potential sites for crack

initiations. This is due to the presence of welding defects in these joints. Therefore, designing of

welded joints against fatigue loading is very important.

Poor design of welds in mechanical structure can lead to unexpected accidents. One such example was

the collapse of the offshore platform Alexander Kielland. In this platform fatigue cracks originated

from a poorly designed fillet weld which lead to its catastrophic collapse in 14 minutes. Fortunately;

these extreme failures are not that common but non extreme failures can have an economic impact e.g.

repair costs increases. Therefore; an appropriate fatigue design and assessment of welded joints is of

high importance.

In Eurocode322

and International Institute of Welding recommendations (IIW)23

various design SN

curves for different weld details are given. These are assigned different FAT classes at a life of two

million cycles and are developed for uniaxial constant amplitude loading. In real life structures such as

in figure 1 and in24,25

, the welds are subjected to complex multiaxial stress states. These stress states

are either proportional where principal stress direction is constant or non-proportional where the

principal stress direction is changing with respect to time. These stress states are shown in figure 7.

Figure 7: (a) Proportional stress state (b) Stress path proportional stress state (c) Non proportional stress state (d)

Stress path non proportional stress state

13

Complex multiaxial stress states are usually converted into an equivalent stress and used along with

design SN curves to define fatigue resistance. For a more durable and lightweight design precise

knowledge of complex loading and defining appropriate capacity of the weld is important. Over

estimation of loads and under estimation of the capacity of weld lead to heavy components which

increases fuel consumptions and cost. However under estimation of loads and over estimation of the

weld capacity lead to higher failure rates and costly maintenance campaigns. Various fatigue models

for evaluating the fatigue strength of welds subjected to complex multiaxial stress states have been

developed26,27

. Most of the data assessed by these models has been for weld toe failures. In this part of

the thesis multiaxial fatigue strength of butt and fillet welds failing at the weld root subjected to

proportional and non-proportional stress states is investigated in paper C and paper D.

4.2 Fatigue testing

The fatigue testing for welded joints is usually performed by applying constant amplitude uniaxial

loading. The applied stress range and corresponding life in cycles is documented and plotted in SN

diagrams. The failure criterion is either complete rupture or a crack length of half the plate thickness.

In paper C, the applied loading was uniaxial which produced proportional multiaxial stress state at the

weld root. This stress state has been obtained by the constraint in the weld with respect to the applied

loading. The test specimens were partially penetrated double sided butt welded plates and stress ratio

was R = 0.1. In paper D, fillet welded tube-to-plate specimens were tested. Applied loading was

internal pressure and combined internal pressure and torsion in and 90° out phase. Failure criterion

was oil leakage from the test specimen. Stress ratio for internal pressure was R = 0.01 and stress ratio

for torsion was R = -1. Lower limit for pressure was set to 5 bar to avoid any fluctuations in the

applied load. Set up for test rig in paper C is shown in figure 2 (a) and set up for test rig in paper D is

shown in figure 8. In paper D, strain gauges were mounted in transverse and longitudinal directions at

the middle of the specimens to measure hoop and axial strains. The machine used for applying

combined loading was a torsion servo-hydraulic test machine with torsional load capacity of ±25 kNm.

Figure 8: Paper D, test rig applied torsion and internal pressure

14

4.3 Fatigue strength assessment methods

Stress based fatigue strength assessment methods are used for estimating the fatigue strength of

welded joints in high cycle fatigue regime. In these methods stresses due to applied loading are

determined at the critical locations using empirical formulae or finite element methods. This calculated

stress is then used with a corresponding design SN curve and the fatigue life is estimated. The fatigue

strength assessment of welded joints is done using the nominal stress method, structural hotspot stress

method, effective notch stress method, or linear elastic fracture mechanics (LEFM). As the complexity

of welded structure increases, the accuracy of these methods decreases. Both for simple and complex

structures the accuracy of advanced methods such as effective notch stress method and LEFM are

higher than the nominal and structural hotspot stress methods. This is shown in figure 9.

Figure 9: Accuracy of stress based assessment methods3

4.3.1 Nominal stress method

The nominal stress method is the most simple and widely used method for fatigue assessment. In IIW

recommendations23

, and Eurocode328

different design SN curves are provided for different weld

configurations. Nominal stress is used along with these design curves and the fatigue life is estimated.

For simple weld configurations computing nominal stress is easy but when the structure or loading is

more complex defining nominal stress is not possible. In such cases nominal stress method is not

applicable and other methods need to be used.

4.3.2 Structural hotspot stress method

When nominal stress cannot be defined, structural hotspot stress method is used for estimating the

fatigue life. This approach was developed for offshore industry. In this approach the stress raising

effects due to design of the joint member are considered. Any stress raising effects from the weld

geometry are excluded. The hotspot stress is determined using linear extrapolation of the stresses in

front of the weld toe line. The locations of the critical points for hot spot are dependent on the loading

and plate thickness of the component. This method is included in the IIW recommendations and

Eurocode3. It is only applicable to weld toe failures29

.

4.3.3 Effective notch stress method

Effective notch stress method is a local method. Effective notch stress includes the stress raising

effects of the local weld geometry. This method is based on the theory of notch plasticity by Neuber

and it is applicable for weld toe and weld root failures. The weld toe or weld root are rounded by a

15

fictitious radius to get effective notch stress. The fictitious radius is computed using Eq.4.3.3.1. which

takes into account the statistical nature of the notch at the weld toe or root. The real radius is assumed

0 and s is proposed 2.5 for welded joints assuming plain strain conditions at the root of sharp notches

combined with vonMises multiaxial strength criterion for ductile materials. The microstructural

support factor for mild steels is 0.4mm29

.

*

*

(4.3.3.1)

sup

f r

r

s x

real notch radius

s stress multiaxiality and strength criterion factor

microstructural port factor

The fatigue life is determined using vonMises (vMH) or prinicipal stress hypothesis (PSH) depending

on the load cases and amount of multiaxiality. FAT are defined for normal and shear stresses in both

vMH and PSH30

. These are given in table1.

Table 1: FAT and m for normal and shear stresses in vMH and PSH for effective notch stress method

Loading Radius [mm] vMH FAT [MPa] PSH FAT [MPa] Slope m

Normal stress only 1 200 225 3

Shear stress only 1 280 160 5

The element size is recommended to be 1/4th or 1/6

th of the weld toe or weld root radius. This also

depends on the type of element used. Second order elements are recommended to use. The choice of

notch radius and corresponding FAT is dependent on the plate thickness. Recommended FAT for

different plate thicknesses are summarized by Bruder et al31

.

In effective notch stress method an assumption is that plastic strains are low and linear elastic

simulations can be used for estimating notch stresses. However; there can be situations where plastic

strains can increase at the weld toe or at the weld root. In such cases, the fatigue strength is governed

by the fatigue properties of the base material. Notch shape factor Kw is determined to account for this

effect.

Nw

HS

N

HS

K

Notch stress

Structural hotspot stress

In IIW recommendations it is recommended to use Kw ≥ 1.6. If Kw < 1.6 the resulting notch stress

should be determined by multiplying the structural stress with a factor of 1.6. The SN curve for

effective notch stress in low cycle fatigue applications will have a slope of m=5 and FAT value of

Kw*160MPa. 160 MPa is FAT for base material.

4.4 Multiaxial fatigue strength assessment methods

The details in real structures are usually subjected to multiple external forces which give rise to multi-

axial stress state. The resulting stresses are normal and shear stresses. In most of the cases multi-axial

problem is reduced to a uniaxial fatigue case, in some occasions this simplification is inappropriate.

Stress based models are widely used to estimate the fatigue strength because most of the components

are required to perform near or below the fatigue threshold. Different approaches have been defined in

16

IIW recommendations and standards over time to do fatigue analysis of welded joints, among them

some local approaches have gained popularity regarding accuracy. Recently people have been

applying stress based models along with the approaches proposed in IIW recommendations and

standards like Eurocode 3 for the fatigue analysis of welded joints. Various models for multi-axial

fatigue analysis have been proposed in 26,27,32–34

. In the literature 23,28,32–36

about 18 different models

have been described. It is observed that all the successful damage parameters have a general form

given in Eq.(4.4.1) below, where contributions from both shear and normal stresses is considered.

(4.4.1)nk

The fatigue strength of welded joints in high cycle fatigue regime is assessed using stress based

approaches. These approaches are used in nominal and local stress systems. They include von Mises

stress hypothesis, principal stress hypothesis, critical plane based methodologies, Integral hypothesis

and modified Gough Pollard interaction equations. Their details are provided in paper C and paper D.

4.5 Multiaxial fatigue strength assessment of inclined Butt welded joints failing at the

weld root

The multiaxial fatigue strength of welded joints failing at the weld toe has been investigated in various

studies but the fatigue strength of welded joints failing at the weld root is seldom being

investigated35,37–40

. In these studies the fatigue strength of inclined butt welds from 0° to 45° with

respect to the applied loading is investigated. The normal to shear stress ratio is up to 1.0. The fatigue

strength of inclined butt welds greater than 45° inclination with respect to applied loading is not being

investigated. A comparison of nominal and local stress based methodologies is also not been done. In

paper C, the multiaxial fatigue strength of inclined butt welds from 0° to 70° inclination is

investigated. This paper extends the experimental data base from low to high multiaxial stress ratio

regime. Low multiaxial stress ratio regime is dominated by normal stress component while high

multiaxial stress ratio regime is dominated by shear stress component. Furthermore; the fatigue

strength estimation capabilities of stress based nominal and local methodologies for failures at the

weld root are also compared. The butt welds failed in the weld root and the stress state at the weld root

was proportional. The nominal stress based methodologies included the Gough Pollard interaction

equation (GPE), interaction equation in DNV (IDNV), and interaction equation in Eurocode3 (IEC3).

The local methodologies included vMH, PSH, and critical plane based modified Wöhler curve method

(MWCM). It was observed that the fatigue strength of the joints increases until weld seam inclination

45°. After this a decrease in fatigue strength is seen. GPE, IDNV, and IEC3 are able to estimate the

fatigue strength is low and high multiaxial fatigue ratio regimes. vMH is over conservative in low

multiaxial stress ratio regime. PSH is non conservative in high multiaxial stress ratio regime. MWCM

is able to estimate the fatigue strength well in both low and high multiaxial stress ratio regimes. In IIW

recommendation23

, it is suggested to use principal stress as a damage parameter when the direction of

principal stress is within ±60°. In paper C, it was verified that the use of principal stress as a damage

parameter is valid when the direction of principal stress is within ±60° from the weld normal stress.

4.6 Fatigue strength assessment of fillet welded tube-to-plate joints failing at the weld

root

The fatigue strength of fillet welded tube-to-plate joints is investigated by various researchers26,41–45

.

Most of these studies are done for weld toe failures35,39

. Very few studies address failures at the weld

root44–46

. In these studies the joints are tested in pure axial/bending, pure torsion, combined in phase

and 90° out of phase axial/bending and torsion loading. In paper D, the fatigue strength of fillet

welded tube-to-plate joints was assessed. The joints were tested in pure internal pressure, pure torsion,

17

and combined in phase and 90° out of phase internal pressure and torsion loading. The fillet welds

were partially penetrated and the failure location was at the weld root. The stress state generated at the

weld root by combined loading was non proportional. Test data from the literature for fillet welded

tube-to-plate joints failing at the weld root was collected and assessed together with the experimental

data generated in paper D. The fatigue strength was assessed in local stress concept. The fatigue

strength estimation capabilities of vMH, PSH, MWCM, and effective equivalent stress hypothesis

(EESH) were compared. No significant effect of the out of phase loading on the fatigue strength was

observed for the tube-to-plate joints investigated in paper D. However; for the data collected from

literature a decrease in fatigue strength is observed due to out of phase loading. Overall a decrease in

fatigue strength is observed when the joints are subjected to combined loading. The fatigue strength

estimation capability of modified Gough Pollard interaction equation (GPE) in local stress system was

also investigated in paper D. GPE was able to evaluate the fatigue strength of joints subjected to pure

normal, pure shear, and combined normal and shear stresses. The suggested comparison values for

non-proportional and proportional stress states in IIW recommendations were also verified. The

fatigue strength estimation capabilities of vMH, PSH, MWCM, and EESH are shown in figure 10.

They were conservative in estimating the fatigue strength of joints subjected to combine in phase

loading. Only MWCM has shown 5 times over estimation in fatigue life for one of the specimens in

this study. For the out of phase data of Frendo et al, it can be seen that PSH overestimates fatigue

lives by a factor of 3-35. Over estimation is higher for the data which are tested with higher shear

stress combinations. Out of phase test data lives in this study are also over estimated. vMH

overestimated the lives by a factor of 4-25. The fatigue life estimation by MWCM and EESH are

better. Their fatigue life overestimation factors are in a range of 1 to 5. For the out of phase data in this

study except PSH, the fatigue life estimations factors by vMH, EESH, and MWCM are near 1.0. vMH,

however can be expected to over-estimate the fatigue strength in this case. Overall EESH and MWCM

are appropriate for estimating the fatigue life in the case of stress histories with varying principal stress

direction with respect to time.

18

Figure 10: Paper D, Fatigue strength estimation capabilities of vMH, PSH, EESH, MWCM for tube-to-plate welded

joints subjected to non-proportional stress states

19

5 Increased performance by post weld improvement techniques

5.1 Introduction High frequency mechanical impact treatment

Welded structures are subjected to fatigue loading and are prone to failure due to notches and defects.

Residual stresses also play a vital role in the fatigue strength of a welded joint. Tensile residual

stresses are detrimental while compressive residual stresses are beneficial for the fatigue strength47

. A

combination of tensile residual stresses and weld toe notches reduces the fatigue strength of a

particular weld. Various improvement techniques have been developed for modifying the weld toe

geometry or residual stresses or both and thus improving the fatigue strength. Some of these

techniques are applied during the welding process e.g. controlling the weld profile and using low

temperature transformation electrodes. Other techniques are applied after welding as post weld

treatment techniques. Post weld treatment techniques are classified as weld profile modification

techniques and residual stress modification techniques. The aim of weld profile modification

techniques is to remove defects at the weld toe which might lead to crack initiation and reduce the

stress concentration factor at the weld toe. These techniques include grinding, and TIG or plasma

dressing. The aim of residual stress modification techniques is to modify detrimental tensile residual

stresses at the weld toe into beneficial compressive residual stresses. These include hammer and

needle peening. Another improvement method which modifies both the weld toe geometry and the

residual stress state at the weld toe is high frequency mechanical impact treatment (HFMI). This

technology was developed at the Northern Scientific and Technological Foundation in Russia in

association with Paton Welding Institute in Ukraine. Ultrasonic impact treatment (UIT), ultrasonic

peening (UP), ultrasonic peening treatment (UPT), high frequency impact treatment (HiFiT),

pneumatic impact treatment (PIT) and ultrasonic needle peening (UNP) are few of the HFMI devices

to mention. In HFMI, high strength steel cylindrical indenters are accelerated against a component or

structure with high frequency. The cylindrical indenters come in different diameters. The indenters can

be single or multiple depending on the manufacturer of the device and the purpose of use. Typical

range of pin radii of the indenters of HFMI device used is 1.5-8mm. Different power sources e.g.,

ultrasonic piezoelectric elements, ultrasonic magnetostrictive elements or compressed air are used. The

impacted material is highly plastically deformed causing changes in the material microstructure, the

local geometry and the residual stress state in the region of impact. In comparison to other peening

methods this operation is more user-friendly and the spacing between alternate impacts on the work

piece is very small resulting in a finer surface finish.

Various researchers48

have investigated the effect of HFMI on the fatigue strength of welded joints.

HFMI is observed to increase the fatigue strength of welded joints as the strength of base material is

increased. This is not the case for as welded joints. In as welded joints, the fatigue strength does not

improve with the increase in the strength of base material, see figure 11. This capability of increasing

the fatigue strength of welded joints by HFMI treatment leads to increase in the utilization of high

strength steel in the construction of lightweight mechanical structures.

20

Figure 11: Fatigue strength with respect to yield strength of steel plate and welded joints

HFMI is observed to increase the fatigue life of welded joints in steels in the medium to high cycle

fatigue regime. However, the increase in fatigue life due to HFMI in low cycle fatigue regime is not

significant. It is due to the fact that in low cycle fatigue the strain hardened material may recover and

the increase in fatigue strength might be lost. Recently design guidelines for the assessment of fatigue

strength are published by the international institute of welding49

. In this part of the thesis, the behavior

of residual stresses during constant and variable amplitude loading is studied in paper E and paper F.

Compressive residual stress state induced in steels of grade S355, S700, and S960 by HFMI is studied

experimentally and numerically in paper G. The effect of HFMI process parameters on the residual

stress state and relaxation of residual stresses is studied experimentally and numerically in paper H.

5.2 Residual stress measurements- X-ray diffraction technique

Residual stresses in metals are measured using destructive, semi destructive and non-destructive

methods. Destructive methods include measurement of elastic strains released using strain gauges by

sectioning, drilling, or milling the sample surface. Non-destructive methods include X-ray and neutron

diffraction methods. X-ray and neutron diffraction methods are used for measuring residual stresses in

crystalline structures. The distance between crystallographic planes is used as a strain gauge3. The

deformations cause changes in lattice spacing and their difference from stress free surface is measured.

This difference is proportional to the magnitude of stresses. The diffraction angle, 2θ, is measured

experimentally and then the lattice spacing is calculated from the diffraction angle, and the known X-

ray wavelength using Bragg's Law. Once the d-spacing values are known, they can be plotted versus

sin2ψ, (ψ is the tilt angle). These plots are linear, non-linear or oscillatory depending on the

homogeneous and inhomogeneous nature of the material under investigation. X-ray diffraction can

measure stresses in the surface i.e. a depth of 0.0025mm. In depth measurements are done using

electro polishing by removing material from the surface. Neutron diffraction can measure residual

stresses up to a depth of 20mm. However, neutron diffraction is expensive and less frequently

available in comparison to X-ray diffraction.

In paper E, X-ray diffraction is used to measure residual stresses in HFMI treated welds. Residual

stresses are also measured after fatigue loading to study relaxation of the stresses. These stresses are

measured near the weld toe where fatigue failure was expected. A special setup was designed in order

to make sure residual stresses are measured at the same location every time the specimen is subjected

to fatigue loading. This setup is shown in figure 12. Here the movement of the specimen is restricted

in three directions. The apparatus used was Xstress3000 with a robot. The robot was programmed to

21

measure residual stresses each time at the same location. In paper F, X-ray diffraction along with

electro polishing was used to measure residual stresses in depth. Residual stresses were measured

before and after fatigue testing. In this paper X-ray diffraction is also used for measuring residual

stresses in TIG and LTT specimens but that is out of the scope of this thesis. In paper G, X-ray

diffraction along with electro polishing is used to measure residual stresses in surface and depth in

HFMI treated plates. In paper H, residual stresses are measured using X-ray diffraction along with

electro polishing in surface and depth in HFMI treated plates and welded joints. The results from

measurements are used for the verification of developed numerical models.

Figure 12: Residual stress measurement setup in paper E

5.3 Behavior of residuals stresses in HFMI treated welded joints

It is generally observed that tensile residual stresses are present at the weld toes. These stresses are

detrimental and reduce the fatigue strength of welded joints47

. However, it is observed that these

stresses relax especially when subjected to variable amplitude loading with overloads 50

. This

relaxation has beneficial effect on the fatigue strength of welded joints and it is responsible for

increased fatigue life when joints are subjected to variable amplitude loading i.e Gassner curves lie

above Wöhler curves. Residual stresses can also be compressive in nature. These stresses are

beneficial and increase the fatigue strength of welded joints. Compressive residual stresses are

effective in decreasing the locally applied stresses. They keep the crack tip closed until a certain

magnitude is reached and increases the crack propagation life. HFMI is one of the improvement

techniques used for modifying the local detrimental tensile residual stress state into beneficial

compressive residual stress state at the weld toe. Numerous studies have been done to understand the

behavior of induced compressive residual stresses under constant and variable amplitude loading.

I.Weich et al51

studied HFMI treated butt welded joints and longitudinal stiffeners in S355J2 and S690

QL.They observed that compressive residual stresses are present up to a depth of 1-1.5 mm. They

studied the behavior of surface residual stresses before and after fatigue loading in welded joints in

both steel grades. They showed that high tensile fatigue loads lead to slight relaxation of residual

stresses in low strength steels. For high strength steel no relaxation was observed in the initial fatigue

life. It is also concluded that compressive residual stresses are responsible for the increase in fatigue

life in HFMI treated joints in comparison to grinded joints. In paper E, four non load carrying

22

longitudinal stiffeners were studied. These specimens were treated in different HFMI treatment

conditions. Two specimens were tested in constant amplitude loading and two specimens were tested

in variable amplitude loading. Residual stresses were measured before, after and during fatigue

loading. For constant amplitude loading, no residual stress relaxation was observed in the initial

fatigue life. Stress relaxation was seen after the joints were subjected to certain number of load cycles.

For variable amplitude loaded specimens, stress relaxation was seen early in life in comparison to

constant amplitude tested specimens. Less compressive residual stresses were measured in one of the

specimens which could be due to over peening. In paper F, surface residual stresses were measured in

S420 MC and S700 MC specimens. Higher compressive residual stresses were measured in S700 MC

welded joints which represent the effect of steel grade on the magnitude of induced residual stresses.

The effect of strength of the steel on the magnitude of induced residual stress state is also studied in

paper G. It was observed that the magnitude of residual stresses increases with the increase in the

strength of the steel. In paper F, residual stresses were also measured in depth direction before and

after fatigue loading. Measurements were performed on four specimens 8mm thick and S700 MC as

base material. Residual stresses relaxed in three specimens while they were stable in one specimen. In

paper F, G, and H compressive residual stresses were measured until a depth of 1.5-2mm.

5.4 Effect of HFMI process parameters on induced residual stresses

The input parameters of the HFMI process can affect the induced residual stress state. The input

parameters affect the treatment intensity and thus the residual stress state and indentation depth. These

parameters are also used to under peen, moderately peen, or over peen the surface of a metal. Two

HFMI processes were investigated. Ultrasonic impact treatment process was investigated in paper G.

The input parameter for setting up the treatment intensity is the amplitude of the sonotrode. The plate

specimens were treated with three different vibration amplitudes of the sonotrode 20µm, 40µm, and

60µm. No significant effect was observed in the indentation depth for S355 and S960 material,

however, for S700 material the indentation depth on average increased with the increase in treatment

intensity. This was also reflected in residual stress measurements. The position of the maximum peak

of the compressive residual stress state for S700MC shifted to higher depth for higher intensity

treatment. The effect of higher intensity treatment was also studied in 52

for welded joints in S355

grade steel. On average they observed an increase in the indentation depth with the increase in

treatment intensity. In this study the welded joint is treated more than one time while in paper G the

plates are treated only one time. The indentation depth is an input parameter for conducting

displacement controlled HFMI simulations53

. Simulations with higher indentation depth have shown

that the maximum stress peak of the induced residual stress state is shifted to higher depth54

, similar

phenomenon is observed in paper G for S700MC material. In paper H, the effect of input parameters

of the pneumatic impact treatment on the induced compressive residual stress state is studied.

Parameters such as frequency and pin tip radii are considered. In 53

, the effect of number of treatment

passes is studied on the indentation depth. It is observed that the indentation depth increases with the

increase in the number of treatment passes. Paper H shows that the nominal post-treatment frequency

and the pin tip radius affects the induced residual stress state on plate specimens. The magnitude of

induced residual stresses increased with a rise in treatment frequency and pin tip radius.

5.5 Numerical estimation of residual stress state induced in HFMI treated welded

joints and plates

In paper G and paper H, residual stress state induced by HFMI in plate and welded joints is studied

numerically. The choice of material model has a significant influence on the estimated residual stress

state53

. During HFMI treatment the surface of the material is subjected to high plastic strains, high

strain rates about 400-500/s53,55,56

, and cyclic loading. Various materials respond different to HFMI

23

treatment. Some materials cyclically soften, some cyclically hardens, and others have no effect. These

factors affect the eventual induced compressive residual stress state. Therefore; the accuracy of the

estimated residual stress state by finite element analyses increases when a material model takes into

account these effects. Compressive residual stress state induced in HFMI treated welded joints and

simple plates has been studied by various authors52,54–60

. In most of these investigations residual stress

state in steels of grade S355 is studied but the residual stress state induced in HFMI treated higher

strength steels than S355 is not studied. In paper G, the residual stress state in steels of grade S355,

S700, S960 is studied. The specimens studied were simple plate specimens. Finite element analyses

was done with isotropic, strain rate dependent isotropic, combined (isotropic and kinematic)

Chaboche, combined strain rate dependent material hardening models. It was observed that the

residual stress state estimated with combined material models was in better agreement with the

experimentally measured residual stress state. Simulation models with isotropic and strain rate

dependent isotropic material models were over estimating the induced compressive residual stress

state. The difference was significantly higher for S700MC grade steel. Numerical models for S960

were developed with only isotropic and strain rate dependent isotropic material models due to lack of

availability of cyclic material data. In paper H, the stability of residual stresses induced in HFMI

treated welds in S355 is studied numerically. A simulation chain consisting of welding simulations,

HFMI simulation, and cyclic fatigue simulations is established. It is observed that the residual stress

relaxation estimations by finite element analyses are in good agreement with the experimental

measurements.

24

6 Conclusions Proper design of welded joints is an important aspect in manufacturing lightweight welded structures.

High strength steels are usually used to decrease the weight of these structures. Choice of right

materials and geometric parameters for welded joint plays a vital role in utilizing full strength of the

high strength steel. This ensures appropriate static and fatigue strengths and increases pay load

capacities and operational life time. This thesis can be concluded with the following points.

In material selection section of the thesis the existing methodologies for static strength design

in Eurocode3, AWS D1.1., and BSK07 are verified for steel up to yield strength of 700MPa.

A finite element methodology is established to estimate the load carrying capacity of welded

joints. Correlation factors for designing welds against static strength are recommended for

designing welded joints in S960 grade steel. It is concluded that the filler material strength

should be at least matching otherwise for joints with under-matching filler materials the weld

throat size and weld width has to be appropriate to utilize full strength of the base material.

Thus appropriate filler and base materials can be selected for a welded joint.

In the fatigue design methods section, more advanced local methods such as modified Wöhler

curve method and effective equivalent stress hypothesis are observed better for estimating root

fatigue strength in non-proportional stress state. Comparison values in IIW recommendations

are verified and the use of principal stress hypothesis when principal stress direction is within

±60° is justified. Furthermore modified Gough Pollard method is valid for estimating the

fatigue strength of welds subjected to proportional and non-proportional loading. Modified

Wöhler curve method and vonMises hypothesis are observed better in estimating the fatigue

strength in high multiaxial stress ratio regime.

In the increased performance by post weld improvement techniques section, it is observed that

residual stresses induced in a HFMI treated welded joint relaxes during high constant

amplitude and variable amplitude loading. A numerical methodology is established for the

estimation of residual stresses in HFMI treated welded joints and simple plates. Furthermore,

it is concluded that residual stress relaxation can be estimated using numerical methods along

with combined material model. Analytical models developed for shot peening can also be used

to estimate residual stress relaxation, however development of more sophisticated analytical

models is required.

25

7 Contribution to the field Finite element models are developed for parametric study for dimensioning the weld against

static strength in S600, S700, and S960 steels

Analytical methodologies in various standard are verified and compared for designing welds

in steel of yield strength greater than 700MPa

Correlation factors are suggested for using design methodologies in Eurocode3 for design

welds in steels of yield strength greater than 700MPa

New test data generated for welded joints subjected to high multiaxial stress ratios and failing

at the weld root

Existing nominal and local stress based approaches are verified and compared for joints

subject to proportional multiaxial stress states

New test data generated for tube to plate welded joints failing in the weld root and subjected to

non-proportional stress states

Applicability of existing local stress based methodologies for the assessment of weld root

fatigue strength is compared and verified

Limitations of vonMises stress hypothesis, principal stress hypothesis, modified wöhler curve

method, and effective equivalent stress hypothesis are assessed.

The fatigue strength class and slope for assessing the fatigue strength of tube-to-plate welded

joints with effective equivalent stress hypothesis is recommended

Behavior of compressive residual stresses in HFMI treated joints subjected to constant and

variable amplitude loading is studied

Numerical models are established for estimating the residual stress state in HFMI treated

joints in S355, S700, and S960 steel grades

Experimental data for the residual stress state induced in HFMI treated welded joints and non-

welded plates is generated

The effect of HFMI input process parameters on the induced residual stress state is

investigated

A simulation loop is established for estimating residual stress relaxation

26

8 Summary of the appended papers In Paper A study has been carried out on the effect of strength mismatch in weld metal and

penetration ratio on the ultimate strength capacity of fillet welds and their failure modes. The ultimate

strength capacity evaluated with nonlinear Finite Element Analysis and testing is compared with the

ultimate strength capacity estimated by standards: Eurocode3, BSK 07 and AWS D1.1 Structural

welding code-Steel. Test results are used to establish correct material properties, to be input into the

finite element model. A criterion for selection of consumables has been developed, when two different

grades of high strength steels are to be joined. The results show that fully penetrated joint with under-

matched filler material is more ductile and the ultimate strength capacity of base plate can be achieved.

It is observed that joints with under-matched filler material are more sensitive to penetration ratio. The

test data correlates reasonably well to the results predicted by finite element analysis software and

design codes. It is also concluded that joint preparation has an effect on the ultimate strength capacity

of the joint.

In Paper B the influence of yield strength of the filler material and weld metal penetration on the load

carrying capacity of butt welded joints in steels of yield strength 700MPa and 960MPa is investigated.

These joints are manufactured with three different filler materials (under-matching, matching, and

over-matching) and full and partial weld metal penetrations. The load carrying capacities are evaluated

with experiments and compared with the estimations by finite element analysis and design rules in

Eurocode3 and American Welding Society Code AWS D1.1. The results show that load carrying

estimations by finite element analysis, Eurocode3, and AWS D1.1 are in good agreement with the

experiments. It is observed that the global load carrying capacity and ductility of the joints are affected

by weld metal penetration and yield strengths of the base and filler materials. This influence is more

pronounced in joints in S960 steel welded with under-matched filler material. Furthermore, the base

plate material strength can be utilized in under-matched butt welded joints provided appropriate weld

metal penetration and width is assured. Moreover, it is also found that the design rules in Eurocode3

can be extended to designing of welds in S960 steels by the use of correlation factor of one.

In Paper C the fatigue strength of inclined butt welds subjected to a proportional multiaxial stress

state generated by uniaxial loading is studied in nominal and local stress concepts. The local

methodologies studied included principal stress hypothesis, von Mises stress hypothesis and modified

Wöhler curve method. Nominal methodologies included modified Gough–Pollard interaction

equation, the design equation in Eurocode3 and the interaction equation in DNV standard. Results are

evaluated along with data published in relevant literature. It is observed that both local and nominal

stress assessment methods are able to estimate multiaxial fatigue strength. No obvious difference in

fatigue strength is observed in the nominal stress concept, but the notch stress concept is able to

capture a decrease in fatigue strength in shear-dominated joints. It is concluded that modified Wöhler

curve method is a suitable tool for the evaluation of fatigue strength in joints dominated by both

normal and shear stresses.

In Paper D the fatigue strength of fillet welded tube-to-plate joints failing at the weld root and

subjected to multi-axial stress states is investigated. The fatigue tested data is collected from the

literature and it is assessed together with the experimental data generated in this study. The fatigue

strength estimation capabilities of local stress based methods such as Principal stress hypothesis, von

Mises stress hypothesis, Modified Wöhler Curve Method, and Effective Equivalent Stress Hypothesis

are compared. The applicability of modified Gough Pollard algorithm in local stress system is also

assessed. It is observed that most of the proposed local stress assessment methods can estimate the

fatigue strength of fillet welds subjected to multiaxial stress states with constant principal stress

27

direction. In case of the load histories which produce varying principal stress direction with respect to

time, better estimation capability is shown by Modified Wöhler Curve Method and Effective

Equivalent Stress Hypothesis. In most of the cases of varying principal stress direction load histories,

von Mises stress hypothesis and Principal stress hypothesis fail to estimate the fatigue strength. The

fatigue strength of specimens tested with combined loading is reduced in comparison to the fatigue

strength of specimens tested with only internal pressure and only bending loading. Out of phase

loading does not affect the fatigue strength significantly for the specimens in this study. However; a

decrease in fatigue strength is observed for the test data for out of phase loading collected from the

literature.

In Paper E the behavior of compressive residual stresses induced in welded joints in high strength

steels by high frequency mechanical impact treatment has been investigated. Longitudinal non-load

carrying attachments in HSS are tested with constant amplitude and variable amplitude fatigue

loading. Stress concentration factors have been calculated using finite element analysis. Residual

stresses have been measured at different cycles during fatigue testing using X-ray diffraction

technique. It is observed that the induced residual stresses are quite stable with some relaxation in

constant amplitude and variable amplitude loading. The overloads in variable amplitude loading seem

to be more detrimental. Relaxation of residual stresses is more obvious in variable amplitude tests.

In Paper F residual stresses of the weld area and their modifications for improving the fatigue strength

are discussed. Both surface and depth distribution of residual stresses in as welded and post weld

treated weld seams have been measured. Four samples have been measured after fatigue testing. Post

weld treatment methods which have been studied are high frequency mechanical impact and laser re-

melting. Relative new method low-transformation temperature filler material samples have been

measured. Clearest strongest effect gives high frequency mechanical impact method. Results indicate

that thin plate is more sensitive to high tensile stresses than thicker plate. This can be related also to

different amount of layers. In the fatigue testing compressive residual stress have relaxed different

amount.

In Paper G the residual stress state induced by ultrasonic impact treatment in S355, S700MC, and

S960 grades steel is investigated experimentally and numerically. Plate specimens were manufactured

and treated with different treatment intensities i.e. vibration amplitudes of the sonotrode. The

indentation depths were measured by the aid of a laser scanner and residual stresses using X-ray

diffraction technique. The effect of steel grade and treatment intensity on the induced compressive

residual stress state was firstly studied experimentally. In addition, displacement controlled

simulations were carried out to estimate the local residual stress condition considering the effect of

different material models. Both the numerically estimated and experimentally measured residual

stresses were qualitatively in good agreement. Residual stress state in S355 and S700MC can be

estimated well using combined strain rate dependent material model. No significant effect of the

treatment intensity is observed on the indentation depth and residual stress state for S355 grade steel.

The indentation depth decreases with the increase in the yield strength of the steel.

In Paper H the effect of cyclic loading on the stability of compressive residual stress fields induced by

high frequency mechanical impact treatment is studied. First, the effectiveness of the post-treatment

technique is shown by fatigue tests with a mild steel S355 longitudinal stiffener specimen. Extensive

X-ray residual stress measurements support the beneficial impact of the method. It also illustrates that

cyclic-loading leads to a local relaxation of this condition. Second, a numerical simulation chain

incorporating a structural weld simulation, numerical analysis of the HFMI-treatment, and a final

cyclic loading step is set-up. The results show that the residual stresses at the surface of the weld toe

28

are in agreement to the X-ray measurements for both the as-welded and HFMI-treated condition,

which basically proofs the applicability of the manufacturing simulation. The numerical computation

incorporating the first five load-cycles demonstrates that the simulated behaviour again exhibits

consistent results with the measurements. An additional utilization of analytical relaxation models in

literature reveals that the estimation of the residual stress state in the high-cycle fatigue region fits also

well to the measured values confirming the applicability of the presented combined numerical-

analytical procedure to assess the cyclic residual stress stability of high frequency mechanical impact

treated welded joints.

29

9 Future work In the future, static strength of welded joints in steel of yield strength greater than 960 can be

investigated. This will develop database for extending design rules in standards to design of welded

joints in steels of yield strength greater than 700 MPa. There is very limited experimental data about

multiaxial fatigue published in the literature. More experiments about proportional and non-

proportional stress state can be carried out. Very limited data for tube-to-plate welded joints subjected

to internal pressure and torsion is generated in this study. Similar experiments are recommended in the

future to increase statistics. Linear elastic fracture mechanics can be used to assess the multiaxial

fatigue strength of inclined butt welds and tube-to-plate welded joints failing at the weld root.

Residual stress relaxation measurements have been done on very limited number of test specimens.

Residual stress measurements on more test specimens tested with higher fatigue loads is

recommended.

30

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