Fatigue assessment of laserbeam welded PM steel components...

881Mat.-wiss. u. Werkstofftech. 2011, 42, No. 10 DOI 10.1002/mawe.201100861

Fatigue assessment of laserbeam welded PM steelcomponents by the notch stress approach

Schwingfestigkeitsbewertung von laserstrahlgeschweißten gesintertenBauteilen nach dem Kerbspannungskonzept

Dedicated to Prof. Holger Hanselka on the occasion of his 50th birthday.

R. Waterkotte2, J. Baumgartner1, C. M. Sonsino1

The local fatigue strength of a laserbeam weld of a complex engine component, which joins aPM with a formed sheet component, was assessed by the notch stress concept with the ficti-tious reference radius of rref = 0.05 mm. First, simplified specimens, following the main geomet-ric dimensions of the parts, were manufactured. On these specimens the fatigue strength wasidentified by tests and the notch stresses calculated by finite element analysis. Based on theseresults a design SN-curve was derived to assess the fatigue strength of the engine component.The numerical assessment of the welded joint was verified by proof tests with the component.The assessment could be improved by considering statistical and stress gradient dependent sizeeffects according to the concept of the highly stressed volume.

Keywords: Camshaft phasing unit / PM steel / laserbeam welding / fatigue design / notch stress concept /

In dieser Arbeit wurde eine Bemessungsmethode f�r eine komplex gefertigte Motorelementkom-ponente, die Teil eines Nockenwellenverstellersystems ist, erarbeitet. Diese Komponente, diew�hrend des Betriebs unter pulsierendem Innendruck steht, besteht aus einem kaltumgeformtenStahldeckel, der mit einem Stahlsinterbauteil �berlappend laserstrahlverschweißt wird. Zur Be-wertung der Einflussgr�ßen Werkstoff, Gestaltung und Fertigung auf die Lebensdauer wurde daskomplexe Bauteil systematisch durch bauteil�hnliche vereinfachte Detailproben experimentellim Einstufenversuch untersucht. An den bauteil�hnlichen Versuchsk�rpern wurde mittels FEMdie Kerbspannung f�r einen Referenzradius rref = 0,05 mm berechnet und eine Kerbspannungs-w�hlerlinie abgeleitet. Diese Referenzw�hlerlinie wurde anschließend zur Auslegung des kom-plexen Bauteils verwendet. Zur Verifizierung der Berechnung wurde die Schwingfestigkeit desBauteils experimentell ermittelt. Hierbei zeigten sich konservative Ergebnisse. Durch Ber�cksich-tigung des h�chst-beanspruchten Werkstoffvolumens konnte die Absch�tzung verbessert wer-den.

Schl�sselw�rter: Nockenwellenversteller / gesinterter Stahl / Laserstrahlschweißen / Schwingfestigkeits-bewertung / Kerbspannungskonzept /

1 Introduction

Engine parts manufactured by joining of sintered (PM) andwrought (ingot) steel components can reach a high degree incomplexity and function integration. Due to the advantage of thelow production costs of PM parts as well as of formed sheets,such components can compete successfully in the market, if afast, cost-effective and reliable joining process is used. Suitable

for this purpose is the laserbeam welding process, which can beused for this material combination when considering some spe-cial requirements of the welding process. Additionally the struc-tural durability of the component has also to be assessed by anappropriate fatigue design methodology.

As a result of the global effort to reduce CO2-emissions newrestrictions for sources of these emissions are steadily put intoplace. Directly inflicted by these restrictions development cyclese.g. in engine construction business have to be drastically short-ened. This can only be achieved through early insights and over-views of development status and resulting reductions of proto-type production. Additionally, the required lightweight construc-tion can be achieved by choosing innovative manufacturing tech-nologies and reliable assessment methods.

In order to fulfill these requirements, a camshaft phasing unitwas developed, Fig. 1. The component, which is loaded with pul-

1 Fraunhofer Institute for Structural Durability and System ReliabilityLBF, Darmstadt/Germany

2 Schaeffler Technologies GmbH & Co. KG, Herzogenaurach/Germany

Correspondence author: J�rg Baumgartner, Fraunhofer Institute forStructural Durability and System Reliability LBF, Darmstadt / GermanyE-mail: [email protected]

i 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.wiley-vch.de/home/muw

J. Baumgartner et al. Mat.-wiss. u. Werkstofftech. 2011, 42, No. 10

sating inner pressure, consists of a formed steel part, which islaserbeam welded with a PM part to a complex unit. The develop-ment of the fatigue assessment method was performed in closecollaboration between Schaeffler Technologies GmbH & Co KGand the Fraunhofer Institute for Structural Durability and Sys-tem Reliability LBF.

The development of the fatigue design concept was based onmethods principally used in [1] and [2]. In these works fatiguestrength tests and numerical analysis on small scale specimenswere used to derive a design SN-curve. These curves describe thefatigue strength of a weld in global (nominal or structuralstresses) or local stresses (notch stresses or stress intensity fac-tors). In above mentioned investigations the material combina-tion steel-steel was used. Therefore, these achieved results couldnot be used directly for a design of welded components consist-ing of a sintered PM part and of a wrought steel part, like thecamshaft phasing unit, without any verification.

For a numerical fatigue strength assessment of differentdesigns of the camshaft phasing unit, the transferability of theprinciple method derived from welded steel-steel to PM steel-steel parts had first to be investigated. This was to be achieved bynumerical and experimental investigations on various speci-mens, where relevant parameters like the material combinationused, the chosen weld parameters or the heat treatment was var-ied. Based on the results a reference SN curve was to be derived,with which a fatigue strength assessment of the camshaft phas-ing unit had to be calculated. Finally, these results had to be veri-fied by fatigue test on the unit.

2 Assessment approach

Various approaches for assessing the fatigue strength of weldedcomponents exist [3]. The most commonly used approaches havebeen compiled in guidelines, for example the IIW recommenda-tions [3], Fig. 2. These approaches can be classified by means ofthe assessed stresses as global and local concepts, namely thenominal stress, structural stress and notch stress concept as wellas fracture mechanics concept.

Due to restrictions in the applicability of these approaches forthe case study reported here, only the notch stress concept wasused in the investigations.

2.1 Notch stress concept

Notches are inducing a local stress increase and therefore have ahigh influence on the fatigue strength of a part. In case of weldedcomponents under cyclic loading cracks may start in most casesat the weld root or the weld toe notches. For the application of thenotch stress concepts these notches are replaced by a notch witha fixed reference radius rref. For welded components made ofthick steel sheets (sheet thickness t = 5 mm) Radaj [3, 4] intro-duced the reference radius of rf = 1 mm considering micro-sup-port effects of the material according to Neuber.

Looking at laserbeam welded components made of thin steelsheets, an application of the reference radius derived for thicksheets would in most cases lead to a significant cross sectionalreduction. Initially for spot welded components, later also forwelded components, a reference radius of rref = 0.05 mm wasintroduced [5, 6]. Using this reference radius for a fatigue assess-ment on laser beam welded sheets, good results could beachieved in various investigations [1, 2, 7], so this concept isalready used in industrial application [8].

Apart from these investigations this method is at the momentonly verified for seam welds joining steel-steel and aluminium-aluminium parts.

2.2 Transferability

The assessment of welded components using the notch stressapproach is based on material dependent design SN-curves. In[1] a design SN-curve was derived for the material combinationsteel-steel by a huge number of experimental and numericalinvestigations on laser beam welded specimens. For all speci-mens cracks initiated at the weld root during fatigue tests. No sig-nificant influence of the steel grade could be identified, whichranged to a maximum yield stress of ry = 500 MPa.

The question arises, whether an assessment of welded compo-nents made of PM steel and wrought steel can also be success-fully performed with the notch stress concept and a referenceradius of rref = 0.05 mm.

To answer this question of transferability the concept in Fig. 3was followed. Analogous to the prior investigations the fatiguestrength of component-like specimens was tested under constantamplitude loading. For taking into account the influence of themanufacturing process, different types of specimens were pro-duced with variations in the production parameters. In conjunc-

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Figure 1. Principle design of a camshaft phasing unit.

Bild 1. Prinzipieller Aufbau eines Nockenwellenverstellers

Figure 2. Approaches for the fatigue strength assessment of weldedcomponents.

Bild 2. Bewertungskonzepte f�r zyklisch beanspruchte geschweißteBauteile.

Mat.-wiss. u. Werkstofftech. 2011, 42, No. 10 Fatigue assessment of laserbeam welded PM steel components by the notch stress approach

tion with numerically calculated notch stresses a design SN-curve was derived. This reference curve served on one hand asverification of the notch stress approach and on the other handwas used for a fatigue strength assessment of the camshaft phas-ing unit. Finally, the quality of the assessment was evaluated byfatigue strength tests on the units.

3 Experimental investigations

Basis of the performed investigations was a laserbeam weldedcamshaft phasing unit, Fig. 4. The housing of the camshaft phas-ing unit, shortly called “stator unit” afterwards, consists of a PMpart and a sealing cover made of a wrought case hardening steel,Table 1. After the forming the sealing cover was ground at thebottom side and welded with a laser beam to the stator. The unitwas loaded with inner pressure with constant amplitudes.

During fatigue testing cracks initiated at the five locationspointed out in Fig. 4 at the weld root. The cracks grew from theroot through the heat affected zone to the outside of the unit. Thecracks were detected by leakage of the pressure medium(hydraulic fluid).

In an early development stage of the camshaft phasing unitthe final dimensions of the laser beam weld were unknown due

to the special material combination chosen and the newly intro-duced production process. For an experimental testing of thefatigue strength of the unit, a prototype with a weld depth ofw = 1.5 mm was available.

3.1 Component-like specimens

For a numerical assessment of the fatigue strength of the cam-shaft phasing unit a design SN-curve was derived from compo-nent-like specimens. The specimens had a similar geometricaldimension as the phasing unit but a rotational symmetry, Fig 5.To gain information about the influence of the PM steels on thefatigue strength of the unit, two variants of the component-likespecimen were manufactured. For the first variant the stator wasmade of the same PM steel as used for the phasing unit, for thesecond variant the stator was made of the same steel as the seal-ing cover was made from. The parameters of the production

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Figure 3. Schematic illustration of the application of the notch stress concept.

Bild 3. Schematische Darstellung zur Anwendung des Kerbspannungskonzepts.

Figure 4. Stator unit and micro section of the weld in the area ofcrack initiation.

Bild 4. Statoreinheit und Schliff der Schweißnaht im versagenskriti-schen Bereich.

Figure 5. Variants of the component-like specimen.

Bild 5. Varianten der bauteil�hnlichen Versuchsk�rper.

Table 1. Physical data of the used materials

Tabelle 1. Physikalische Kennwerte der verwendeten Werkstoffe

Material E-Modul in GPa Poisson ratio

PM steelSteel

130200

0.230.30


process (sinter / forming, preparation of the parts, welding proc-ess and weld depth) were chosen according to the ones for theprototype of the phasing unit.

3.2 Fatigue strength tests

The fatigue strength tests on the component-like specimens andthe phasing unit were performed with a pressure medium(hydraulic fluid) and constant amplitude loading. The failure cri-terion was the leakage of the pressure medium due to cracksthrough the weld. All specimens which ran up to a life ofNg = 5 N 106 cycles were defined as run-outs and retested on anelevated pressure level.

The SN-curves of both variants of the component like speci-mens have a similar slope of k L 7 (inverse Basquin exponent) inthe range up to N = 1 N 106 cycles. The knee point of the SN-curvewas identified to be around Nk = 2 N 106 cycles, Fig. 6. The courseof the SN-curve behind the knee point was assumed to be k* = 22according to [9], which is equal to a reduction of the fatiguestrength of 10% per decade.

Looking at the SN-curves of both variants, no significant influ-ence of the chosen material of the stator could be identified; theSN-curves are nearly identical. The component-like specimensshow the same failure mode as the camshaft phasing unit: Thecracks start at the weld root and grow through the heat affectedzone of the cover.

4 Calculation of notch stresses

The calculation of notch stresses at the component-like speci-mens was performed in Abaqusm. For the model 2D-solid ele-ments with a quadratic shape function for rotational symmetry(CAX8) were used, Fig. 7. The boundary condition was a clamp-ing of the specimen on the flange. The model was loaded withinternal pressure. The meshing of the notches was the same forall investigated models.

The calculation of notch stresses at the weld root, where mate-rials with different stiffness are joined, demands a detailed con-sideration of modeling the local weld area around the notch. Theinfluence of the distribution of material on the calculated maxi-mum notch stresses was investigated in detail on different FE-Models, Fig. 8. Dark areas stand for a stiff (wrought steel), brightones for a less stiff material (PM steel).

Model 3 stands for a precise mapping of the real existing distri-bution of the material at the weld of the component-like speci-men with a stator of PM steel. This is done under the assumptionthat the material in the weld zone has a similar E-modulus as thecover material. Model 2 and 4 describe a possibility of how theefforts of modeling a complex 3D part can be reduced.

For the different models the linear-elastic notch stressesaccording to v. Mises are calculated, Table 2. The calculated notchstresses of model 3 and 4 are nearly identical, whereas themodel 2 yields stresses that are 10% higher. This comparisonshows that for geometrically similar and equally loaded compo-

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Figure 6. SN-curve of the component-like specimen.

Bild 6. W�hlerlinien der bauteil�hnlichen Versuchsk�rper.

Figure 7. FE-Model of the component-like specimen.

Bild 7. FE-Modell des bauteil�hnlichen Versuchsk�rpers.

Figure 8. Micro section and modeling of the material at the weld andthe notch.

Bild 8. Schliff und Modellierungsm�glichkeiten der Schweißnaht undder Kerbe.

Table 2. Impact of the local material distribution on the notch stress

Tabelle 2. Auswirkung der lokalen Materialverteilung auf die Kerb-spannung

Model Difference of notch stress compared to thereference model 3 in %

1 92 133 04 1


nents, the usage of model 4 results especially for complex 3Dmodels in a significant reduction of modeling time and gives inaddition a very good approximation of the stresses of the modelwith realistic material mapping.

4.1 Modeling techniques

To achieve consistent results for the calculation of notch stresses– not only for welds of dissimilar material – further investiga-tions of efficient and easy applicable modeling techniques havebeen performed. These techniques were summarized in a guide-line showing important hints about building up models andassessing the notch stresses for a successful assessment of seamwelds to application engineers.

The principle outline for the calculation of notch stresses isperformed in two steps. In the first step a global model is createdin which seam welds are included, however without consideringweld details such as e.g. notches. The main idea is that the globaldeformation state and the stress flow through the part are mod-eled correctly. In the following, the deformations of the globalmodel are transferred to a submodel, in which the weld geometrytogether with the notches is included. In these notches the notchstresses can be calculated [10]. To receive consistent results, sev-eral parameters have to be considered:* Element type (linear or quadratic shape function),* Geometry of the weld,* Meshing of the global model, especially the element length in

the weld area,* Geometry and meshing of the submodel,* Positioning of the submodel in comparison to the global mo-

del.Fig. 9 displays an example of meshing of an overlap weld and

the position of the submodel.

4.2 Verification of the finite element models

To assure the compliance of stresses between the FE-model andthe real existing specimen, an experimental strain analysis hasbeen performed. Therefore, strain gauges were applied on posi-tions close to the weld as well as on positions with low strain gra-dients to verify the overall loading. If at both positions a good cor-relation between experimentally measured and calculated strainsis achieved, the notch stresses from the finite element model aretrustworthy.

Fig. 10 displays the strains measured and calculated on thecomponent-like specimen. A good correlation can be seen.

The collaboration of the calculation with the design depart-ment as well as with the production process is of big importanceto include variations of the geometry of the parts and the weld.Gathering this information and having verified finite elementmodels at hand, statistical analysis can be performed to provideinsight into the expected scatter or the variance of fatiguestrength.

4.3 Derivation of a design SN-curve

A design SN-curve for the reference radius of rref = 0.05 mm canbe derived from the calculated linear elastic notch stresses andfrom the experimentally identified fatigue strength of the com-ponent-like specimen. For the component-like specimen with aPM stator, the modeling approach “4” was chosen for the speci-men with a steel stator approach “1”.

There are no significant differences in fatigue strengthbetween the two variants of the specimens, Fig. 11. Therefore, adesign SN-curve was derived from the results of both variants.The design SN-curve has a low scatter of Ts = 1 : 1.21 [1, 2] andhas a knee point at around Nk = 2 N 106 cycles. The slope was iden-tified to be k = 6.7.

5 Fatigue strength assessment

Having a valid design SN-curve at hand, a fatigue strengthassessment of the camshaft phasing unit can be performed. Thecalculation of notch stresses at the stator unit was performed inAbaqus using elements with quadratic shape function (C3D20,C3D10). Due to cyclic symmetry of the unit the model could be

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Figure 9. Model of overlap keyhole weld with positioning of the sub-model.

Bild 9. Modell der �berlappnaht mit Positionierung des Submodells.

Figure 10. Comparison of strain measured and calculated on component-like specimen.

Bild 10. Abgleich FEM und Versuch – bauteil�hnliche Versuchsk�rper.


simplified by building up a 72 8 degree segment and applying theaccording boundary conditions. The material distribution wasmodeled in accordance with the component-like specimen witha PM steel stator, following approach “4”. To assess the influenceof different welding depth on the fatigue strength of the unit, sev-eral variants were calculated, Fig. 12.

For all variants the notch stresses were determined by meansof submodels. Using the calculated maximum notch stressamplitude and the design SN-curves from the component-likespecimens, a SN-curve for the camshaft phasing unit could bederived.

To verify the validity and performance of the assessmentapproach, the fatigue strength of the stator units was experimen-tally tested. Loading, failure criterion and minimum pressurewas chosen as in tests with the component-like specimens. Com-parison of the calculated and the experimentally determinedfatigue strengths shows differences, Fig. 13. The calculatedfatigue strength at Nk = 2 N 106 load cycles is 18% lower than theexperimentally determined one. The calculation shows conserva-tive results by a factor fN = 2.50 in life and fS = 1.18 in stress.

The differences between calculated and experimentally deter-mined fatigue strengths might be attributed to differences in thehighly stressed volume (HSV) of the seam weld [11], Fig. 14. Thecomponent-like specimens have, due to the rotational symmetry,an equally stressed seam weld. The camshaft phasing unit hasonly five small areas of the seam weld which are highly stressed,Fig. 4. Comparing the highly stressed volumes, a ratio ofV80%,comp.-like specimen : V80%,uni L 1 : 0.10 could be identified. There-

fore, experimentally identified higher fatigue strengths (in localnotch stresses) might be explainable.

The reason for choosing the volume V80% instead of the usualV90% [12, 13] was to capture more material in order to compensatepossible local inhomogenities.

Taking into account the influence of the highly stressed vol-ume over the relation V�0:045

90% from investigations in [13], animprovement in the fatigue assessment was achieved. The differ-

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Figure 11. Notch stress SN-curve, derived on basis of tests with com-ponent-like specimens.

Bild 11. Kerbspannungsw�hlerlinie, berechnet auf Basis der Versuchemit bauteil�hnlichen Versuchsk�rpern.

Figure 12. Finite element model of the stator unit with differing welddepths w.

Bild 12. FE-Modell des Bauteils mit unterschiedlichen Einscheißtiefenw.

Figure 13. Calculated and experimentally determined fatiguestrengths.

Bild 13. Berechnete und experimentell ermittelte Schwingfestigkeit.

Figure 14. Highly stressed material volume V80% on a weld root underpeel loading.

Bild 14. H�chst beanspruchtes Werkstoffvolumen V80% an einer Wur-zelkerbe unter Sch�lbeanspruchung.

Figure 15. Endurable fatigue strength related to the highly stress vol-ume V80% for the failure criterion rupture of seam welds under normalstresses.

Bild 15. Ertragbare Beanspruchungen im Vergleich zum h�chst bean-spruchten Volumen V80% f�r Schweißn�hte unter Normalbeanspru-chung f�r das Versagenskriterium Bruch.


ence between calculated and experimentally derived SN-curves isreduced to a factor of TN = 1:1.70 in life and Tr = 1:1.10 instresses. The chosen exponent of e = –0.045 was identifiedthrough investigations on MIG welded specimens with failuresin the weld toe. Therefore, the influence of the highly stressedvolume on laser beam welded specimens with failures startingfrom the weld root might be different; at PM-steels an exponentof e = 0.061 was determined [12].

Summarizing the results of several other fatigue strengthinvestigations on laser beam welded specimens [1, 14, 15], aswell as results from internal research projects in LBF, an expo-nential relationship could be identified tork;a;vM;2E6 ¼ 390 � V�0:12

80% MPa between endurable notch stressesand highly stressed volume, Fig 15.

6 Summary and outlook

In the presented detailed investigations for assessing the fatiguestrength of laserbeam welded components made of PM steel andsteel parts the local notch stress concept proved to be successful.For this the variant with the small reference radiusrref = 0.05 mm was used. It yielded slightly conservative resultswhich could be improved by taking into account stress gradientand size effects using the method of the highly stressed volume.

The application of this method caused a significant reductionof development cycles due to the possibility to give reliable infor-mation about the fatigue strength of design variants. Furtherimprovements of the method might be achieved by incorporat-ing more results, especially gained at welds with a very smallhighly stressed volume or seam weld ends. Ignoring the sizeeffects might lead to over-conservative assessments resulting inuneconomically heavy components.

Acknowledgement

The authors give thanks to Dr.-Ing. K. Lipp and Dipl.-Ing. K. St�r-zel for their valuable support during the project.

This paper was presented at the World PM2010 Conference,10th – 14th October 2010, in Florence and printed in the confer-ence proceedings. Additionally it was published at Materials Test-ing, 52, 2010 in German language.

7 References

[1] M. Eibl, Berechnung der Schwingfestigkeit laser-strahlgeschweißter Feinbleche mit lokalen Konzepten, Dis-sertation, Technische Universit�t Darmstadt, 2003.

[2] J. Schlemmer, M. Bacher-H�chst, C. M. Sonsino, Schwing-feste Auslegung von d�nnwandigen Laserstrahlsch-weißverbindungen f�r Einspritzsysteme, Deutscher Ver-band f�r Materialforschung und -pr�fung (DVM), Berlin,DVM–Bericht Nr. 802, 2003, pp. 25–36.

[3] D. Radaj, C. M. Sonsino, W. Fricke, Fatigue Assessment ofWelded Joints by Local Approaches, Woodhead Publishing,Cambridge, 2006, 2nd Edition.

[4] D. Radaj, M. Vormwald, Erm�dungsfestigkeit, Grundlagenf�r Leichtbau, Maschinen– und Stahlbau, Springer Verlag,2003, 2. Auflage.

[5] G. Zhang, M. Eibl, S. Singh, O. Hahn, J. R. Kurzok, Schwei-ßen und Schneiden 2002, 54, 132.

[6] G. Zhang, B. Richter, Fatigue & Fracture of Eng. Mat. & Struc-tures 2003, 26, 507.

[7] M. Eibl, C. M. Sonsino, H. Kaufmann, G. Zhang, Int. J.Fatigue 2003, 25, 719.

[8] F. deBruyne, A. Hoppe, Virtuelle Schwingfestigkeitsbewer-tung von F�geverbindungen bei T�ren und Klappen VDITagung 2006: Berechnung und Simulation im Fahrzeug-bau, VDI Band Nr. 1967, 2006.

[9] C. M. Sonsino, Int. J. Fatigue 2007, 29, 2246.[10] W. Fricke, Guideline for the Fatigue Assessment by Notch

Stress Analysis for Welded Structures, IIW-Doc. XIII-2240r1-08/XV-1289r1-08, International Institute of Welding,Graz, 2008.

[11] C. M. Sonsino, Fatigue Design Concept for PM Parts andRequired Material Data – An Overview, Metal Powder Indus-tries Federation (MPIF), Princeton, USA, 2003.

[12] C. M. Sonsino, Konstruktion 1993, 45, 25.[13] C. M. Sonsino, Int. J. Fatigue 1995, 17, 55.[14] J. Baumgartner, Schwingfestigkeitsbewertung laser-

strahlgeschweißter Stahlstrukturen geringer Wanddickenaus dem Automobilbau, DVS-Report No. 256: Festigkeitgeschweißter Bauteile – Anwendbarkeit lokaler Nachweis-konzepte bei Schwingbeanspruchung, DVS Media GmbH2009, D�sseldorf, pp. 63–74.

[15] T. Bruder, K. St�rzel, J. Baumgartner, Fatigue Assessmentof Seam Welds of Automotive Components by Local StressApproaches, Proceedings of the 2nd Symposium on StructuralDurability in Darmstadt, 2008, pp. 183–197.

Received in final form: July 29th 2011 T 861

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