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Residual Stress Analysis and Fatigue Assessment of Welded Steel Structures
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.ORCID iD: 0000-0003-4180-4710
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life.

The work that is reported in this doctoral thesis is part results of the Nordic R&D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life.

Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured.

Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses.

Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure.

A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading.

A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature.

Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , iv, 36 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2008:11
Keyword [en]
fatigue failure, welding, weld defects, welded joints, stress concentration, residual stress, linear elastic fracture mechanics, welding simulation, finite element analysis, fatigue crack growth.
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4687OAI: oai:DiVA.org:kth-4687DiVA: diva2:13428
Public defence
2008-04-25, E1, E-huset, KTH, Lindstedtsvägen 3, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100706Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2011-04-12Bibliographically approved
List of papers
1. Fatigue Assessment of Cruciform Joints Welded with Different Methods
Open this publication in new window or tab >>Fatigue Assessment of Cruciform Joints Welded with Different Methods
2006 (English)In: Steel Research International, ISSN 1611-3683, Vol. 77, no 12, 882-888 p.Article in journal (Refereed) Published
Abstract [en]

The advantages of the relatively new laser welding method are low welding distortions and high productivity, and it opens new opportunities for welding of steel structures in the automotive and heavy machinery industries. There is a need for understanding the fatigue strength and to develop fatigue design curves for laser welded joints. In this paper fatigue testing and weld quality of Hybrid Nd: YAG laser/MAG and MAG welded non-load carrying cruciform joints is examined. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analysed. The measured parameters were used to calculate the stress concentration factors by implementing FE-models, in order to anticipate the influence of size of weld geometrical parameters on the stress concentrations. Residual stress measurement was carried out close to the toe region using the X-ray diffraction method and weld defects (cold laps) in the cracked specimens was measured.

Keyword
fatigue life; stress concentration; welds defects; residual stress; and laser welding
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8170 (URN)000243162500006 ()2-s2.0-33846689914 (Scopus ID)
Note

QC 20100702. Original title "Assessment of Cruciform Joints Welded with Different Methods"

Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2014-12-16Bibliographically approved
2. Fatigue Assessment and LEFM Analysis of Cruciform Joints Fabricated with Different Welding Processes
Open this publication in new window or tab >>Fatigue Assessment and LEFM Analysis of Cruciform Joints Fabricated with Different Welding Processes
2008 (English)In: Welding in the World, ISSN 0043-2288, Vol. 52, no 7-8, 93-105 p.Article in journal (Refereed) Published
Abstract [en]

In this study fatigue testing and defect assessment were carried out on specimens welded with robotic and manualwelding using fl ux cored (FCAW) and metal cored (MCAW) fi ller materials in order to study the effect of the weldingmethod on the fatigue strength and weld quality. Thirteen different batches were investigated of which two wasshot peened before fatigue testing. The local weld geometry was measured for all the specimens before testing.The specimens welded with fl ux cored weld wire showed the best fatigue strength, small defects and low residualstresses. Large scatter in the fatigue data is observed, especially when manual welding is employed. The few largestdefects were removed by the shot peening process, although small defects survived. This led to a smaller scatterin fatigue live for the shot peened specimens. Linear elastic fracture mechanics, LEFM, was employed for analysisof the fatigue test results. The fatigue life predictions using a 2D LEFM FE-model for simulating a continuous coldlap defect along the weld toe showed a qualitative agreement with the fatigue test results. The 2D analysis showedthat a continuous cold lap defect should be no more than 0.5 mm deep in order to comply with the requirement offatigue lives for normal weld quality according to the IIW design rules. For larger defects (> 0.8 mm) an increased toeradius will have a small effect on the fatigue strength. A 3D LEFM analysis of crack growth from a spatter-inducedcold lap defect was also carried out. This showed similar trends in crack growth compared to the 2D analysis ofa continuous cold lap, although the spatter-induced cold lap defect (semi-elliptical) had a longer fatigue life (x2.7),and hence is less dangerous from a fatigue point of view.

Keyword
Arc welding; Defects; Fatigue tests; Fracture mechanics; Mechanical tests; Reference lists; Residual stresses; Stress distribution
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8171 (URN)2-s2.0-47149110006 (Scopus ID)
Note
QC 20100702 Uppdaterad från accepted till published i DiVA 20100702Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-02Bibliographically approved
3. Residual Stress Analysis and Fatigue of Multi-pass Welded Tubular Structures
Open this publication in new window or tab >>Residual Stress Analysis and Fatigue of Multi-pass Welded Tubular Structures
2008 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 15, no 7, 863-874 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study is to investigate the residual stresses near the weld root and the weld toe for multi-pass welded tube-to-plates. Two different tubular joint configurations were studied; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. A 2D axi-symmetric finite element model was developed to calculate the temperature distribution, HAZ, penetration depth and the residual stress distribution for the sequentially coupled thermo-mechanical analysis. The calculated residual stresses was compared with experimental results and showed qualitatively good agreement. Torsion fatigue tests were performed in order to study crack propagation from the weld root, lower and upper weld toe in mode III. Some of the tube structures were loaded with a static internal pressure in order to separate the root crack and initiate the crack growth in mode III. Another batch was PWHT and fatigue tested, in order to study the influence of residual stresses.

Keyword
welding simulation; residual stresses; tubular joints; fatigue; fracture mechanics; CRACK-GROWTH; MODE; DEFORMATIONS; SIMULATION; DEFECTS; JOINT
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8172 (URN)10.1016/j.engfailanal.2007.11.016 (DOI)000256631600007 ()2-s2.0-41849142003 (Scopus ID)
Note
QC 20100702Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2017-12-14Bibliographically approved
4. Residual Stress Prediction and Relaxation in Welded Tubular Joint
Open this publication in new window or tab >>Residual Stress Prediction and Relaxation in Welded Tubular Joint
2007 (English)In: Welding in the World, ISSN 0043-2288, Vol. 51, no 1/2, 23-30 p.Article in journal (Refereed) Published
Abstract [en]

In this paper a three-dimensional welding simulation was carried out in the FE software ANSYS in order to predicttransient temperatures and the residual stresses in a three-pass welded tubular joint structure. The thermal analysisand the moving heat source were verified with temperature measurements, and the computed residual stresseswere verified with hole drilling measurements. The calculated and measured temperatures were in good agreement,and the computed residual stresses were in qualitatively good agreement with the experimental results. Residual stressrelaxation analyses were carried out with similar load as in earlier fatigue testing on the same tubular joint structure.The FE residual stress relaxation analysis showed a small amount of relaxation early in the fatigue life.

Keyword
Computation; Fatigue life; Finite element analysis; Measurement; Prediction; Reference lists; Residual stresses; Simulating; Stress distribution; Stress relaxation; Tubular constructions
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8173 (URN)2-s2.0-34247234147 (Scopus ID)
Note
QC 20100702Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-11-09Bibliographically approved
5. Residual stress effects on fatigue life of welded structures using LEFM
Open this publication in new window or tab >>Residual stress effects on fatigue life of welded structures using LEFM
2009 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 16, no 1, 449-467 p.Article in journal (Refereed) Published
Abstract [en]

In this paper a welding simulation procedure is developed using the FE software ANSYS in order to predict residual stresses. The procedure was verified with temperature and residual stress measurements found in the literature on multi-pass butt welded plates and T-fillet welds. The predictions show qualitative good agreement with experiments. The welding simulation procedure was then employed on a welded ship engine frame box at MAN B&W. A subroutine for LEFM analysis was developed in 2D in order to predict the crack path of propagating fatigue cracks. The objective was to investigate fatigue test results from special designed test bars from the frame box where all test failed from the non-penetrated weld root. A subroutine was developed in order to incorporate the predicted residual stresses and their relaxation during crack propagation by isoparametric stress mapping between meshes without and with cracks, respectively. The LEFM fatigue life predictions shows good agreement with the fatigue test result when the residual stresses are taken into account in the crack growth analysis.

Keyword
Welding simulation, FEM, Residual stresses, Fatigue crack growth, Linear elastic fracture mechanics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-8174 (URN)10.1016/j.engfailanal.2008.06.017 (DOI)000261260600040 ()2-s2.0-53849144973 (Scopus ID)
Note
QC 20100702 Uppdaterad från submitted till published (20100702)Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2017-12-14Bibliographically approved
6. Simplified FE welding simulation of fillet welds: 3D effects on the formation residual stresses
Open this publication in new window or tab >>Simplified FE welding simulation of fillet welds: 3D effects on the formation residual stresses
2009 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 16, no 7, 2281-2289 p.Article in journal (Refereed) Published
Abstract [en]

In this study two- and three-dimensional finite element welding simulations have been carried out. The welded component studied is a T-type fillet weld which is frequently used in the heavy vehicle machine industry with plate thicknesses of eight and 20 mm, respectively. The software's used for the welding simulations is MSC.Marc and ANSYS. The objective is to study the formation of the residual stresses due to 3D effect of the welding process. Moreover, welding simulations using solid models and contact models in the un-fused weld roots were carried out in order to investigate the possible effect with respect to the residual stresses. Residual stress measurements were carried out using X-ray diffraction technique on the manufactured T-welded structure. The 2D residual stress predictions shows good agreement with measurements, hence the 2Dwelding simulation procedure is suitable for residual stress predictions for incorporation in further fatigue crack growth analysis from weld defects emanating from the weld toe and the un-fused root.

Keyword
Welding simulation; FEM; Residual stresses; Fillet welds; FATIGUE
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8175 (URN)10.1016/j.engfailanal.2009.03.018 (DOI)000268566500030 ()2-s2.0-67649591918 (Scopus ID)
Note
QC 20100705. Uppdaterad från submitted till published (20100705). Tidigare titel: FEM welding simulation of fillet welds: 3D effects on residual stresses.Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2017-12-14Bibliographically approved

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