International Journal of Fatigue 33 (2011) 466–476

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International Journal of Fatigue

journal homepage: www.elsevier .com/locate / i j fa t igue

Evaluating the fatigue initiation location in friction stir welded AA2024-T3 joints

H.J.K. Lemmen ⇑, R.C. Alderliesten, R. BenedictusDelft University of Technology, Faculty of Aerospace Engineering, P.O. box 5058, 2600 GB Delft, The Netherlands

a r t i c l e i n f o

Article history:Received 13 April 2010Received in revised form 28 September 2010Accepted 5 October 2010Available online 12 October 2010

Keywords:Friction stir weldingFatigue initiationFractographyMicrostructureResidual stress

0142-1123/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.ijfatigue.2010.10.002

⇑ Corresponding author.E-mail address: H.J.K.Lemmen@tudelft.nl (H.J.K. Le

a b s t r a c t

This paper presents observations on fatigue initiation in friction stir welded AA2024-T3 joints. Based onthese observations, the influence of the weld on the location of initiation is discussed. These locations ofinitiation are correlated to stress results from numerical analysis. Furthermore, fractography was per-formed to investigate the role of the microstructure in the weld on the location of initiation. As a result,it has been observed in which cases the residual stress determines the location of initiation and in whichcases the microstructure.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Friction stir (FS) welded joints are complex considering themicrostructure and the mechanical properties. Several zones existwhich received different heat treatments and/or mechanical defor-mations. Moreover, the welds exhibit significant residual stresseswhich are highly anisotropic and predominantly oriented parallelto the weld [1]. To investigate the influence of different locationsin the weld on the fatigue initiation (FI) behaviour, centre notch fa-tigue specimens were tested as is reported in a previous paper [2].In that paper it was reported how the residual stress parallel to theweld affected the FI behaviour significantly when loaded parallel tothe weld (L–T specimens). Because the residual stresses wereknown, it was possible to obtain the fatigue stress at the notchand with that predict the FI behaviour. However, that predictionmethod did not include the influence of the microstructure onthe FI behaviour. Neither was analysed whether the through thick-ness variations of the stress could be related to the locations of FI.

It is expected that the location of FI along the notch in the thick-ness direction is mainly determined by the location of largest meanstress for the L–T specimens. However, due to the anisotropicresidual stresses, it is expected that the microstructure determinesmainly the location of FI for the T–L specimens (i.e. the weld per-pendicular to the applied load). This paper presents the micro-scopic analysis of the fracture surfaces with the purpose ofobtaining the locations of FI along the notch. These locations ofFI were correlated with the microstructure of the weld and the

ll rights reserved.

mmen).

fatigue stress along the notch obtained by FE analysis for the L–Tspecimens. As a result these correlations should prove whetherprevious expectations are realistic or not.

In addition, the investigation on the orientational dependencyof FI on various parameters was further investigated by perform-ing FI tests on specimens with a weld oriented at 45� with respectto the load. Together with the results of the L–T and T–L speci-mens this provided the FI behaviour as a function of the angle be-tween the weld and the applied load. It is expected that the FIbehaviour from the 45� specimens is in between the FI behaviourof the L–T and T–L tests.

Another research question which was not covered in previouspaper, is about the influence of the asymmetric welding processon FI. FS welding is an asymmetric process due to the rotation ofthe welding tool resulting in different microstructures at bothsides of the weld centre. Because FI is predominantly affected bythe microstructure in the T–L specimens, the FI behaviour shouldalso be asymmetric. To investigate this, FI tests were performedon T–L specimens with the notch at either side, but equal distancefrom the weld centre.

2. Experimental

Centre hole FI specimens were used to investigate FI for differ-ent orientations of the weld (Fig. 1). The centre notches weredrilled at different locations in the weld to investigate the influ-ence of the different zones, i.e. nugget, Thermo Mechanically Af-fected Zone (TMAZ) and Heat Affected Zone (HAZ) (Fig. 2b). Toobtain the FI behaviour from a specific location in the weld, a holewas drilled with a diameter of only 1 mm. This small diameter

Fig. 1. Geometry of three test specimens.

Fig. 2. (a) FSW tool and definition of axis; (b) different weld zones.

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captured the FI behaviour of a single weld zone but resulted in anincreased fatigue limit due to the size effect [3,4]. Because all holesin this research have the same diameter, including the tests onbase material, the results can be compared.

1 The work presented in reference [11] was never published, however, some resultsare also presented in [3].

2.1. Specimen preparation

The welds for this research were produced at EADS in Munichwith an ESAB FS welding machine. The FS weld tool (Fig. 2a) con-tained a shoulder and threaded triflute conical pin with diametersof 13 mm and 5 mm respectively. All welded and un-welded mate-rial for this research was obtained from the same sheet AA2024-T3.Welding was performed parallel to the rolling direction with a toolangle of a = 2� (Fig. 2a), a welding speed of 350 mm/min, a rota-tional speed of 550 rpm and a downward welding force of 19 kN(see for more details: [2]). The rotation of the weld tool createsan asymmetric microstructure in which one side is defined as theadvancing side (AS) and the other as the retreating side (RS)(Fig. 2a).

Vicker hardness (load = 200 g) measurements were performedat the centre line of the welded sheet to characterize the weld(Fig. 3). The hardness profile reflects the material behaviour inthe weld, and the geometry of the weld zones [5–10]. The positionsof the holes in the FI specimens were chosen based upon this hard-ness profile (Table 1). Three locations were tested in three direc-tions to investigate the influence of the orientation of the weldon the FI behaviour. Furthermore, three locations were tested atboth sides of the weld centre to investigate whether the FI behav-iour is symmetrical. For the L–T test this resulted in asymmetricspecimen geometries because the weld is at the centre of the spec-imen and the hole had to be drilled at one side of the weld. Eachlocation will be indicated in this paper by its y-value which isthe distance from the FS weld centre to the hole centre, accordingto the sign convention in Fig. 2a.

The detrimental effect of the weld surface on the FI behaviour isremoved by machining and grinding the specimens up to P1000.

This treatment provided a well-defined specimens surface. Theremaining thickness of the specimens was between 2.2 and2.4 mm. The holes were drilled and finished by reaming. All spec-imens were produced such that the crown side of the weld wasalways at the front side of the specimen (Fig. 4). The base materialspecimens were produced with the same geometry, except for thesurface treatment because no rough weld surface was present.

2.2. Experimental procedure

The fatigue tests were performed using a MTS 100 kN servohydraulic machine with an applied constant amplitude loading, astress ratio of R = 0.1, and a frequency of 10 Hz. Each location inthe weld was tested at a variety of stress amplitudes, ranging from60 MPa to 100 MPa, to obtain the S–N initiation curve (S–Ni) foreach location. The tests were stopped after 106 cycles if initiationdid not occur. At intervals of 2500 baseline cycles, marker loads[11]1 were applied consisting of 5000 cycles with a stress ratio ofR = 0.7, but with the same maximum stress. The maximum stress le-vel was kept the same to avoid the influence of overloads on the fa-tigue life. As a result of the marker loads, small bands in the fracturesurface could be observed easily with optical microscopy. The bandswere used to trace the location and moment of initiation. After FI oc-curred, the crack lengths were measured using optical microscopesat intervals of 2500 cycles right before the marker loads wereapplied.

Different definitions exist for the term fatigue initiation. In thisstudy the definition of Schijve [3] is followed in which fatigue ini-tiation is defined as the phase of the fatigue life including; cyclicslip, crack nucleation and micro crack growth. A crack length of0.3 mm was chosen to be the FI crack length, because this lengthwas short enough to exclude significant macroscopic fatiguefracture features and long enough to be determined sufficiently

Fig. 3. Hardness profile of an FS weld in AA2024-T3.

Table 1Location of the holes in the weld, with corresponding weld zone.

Zone HAZ TMAZ/Nugget/TMAZ HAZ

Location (mm) �10.5 �7.5 �3 0 3 5.5 7.5 10.5 12L–T x x x x x x45� x x xT–L x x x x x x x x x

Fig. 4. Definition and terminology of samples cut from the fatigue test specimen for optical microscopy investigation.

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accurate in all specimens. As a consequence, this approach doesnot relate to the metallurgic definition of FI life, but is rather anengineering approach; this crack length can be measured and thedifference with the metallurgic FI life should be equal for all spec-imens tested at the same load.

2.3. Fractography

The fracture surfaces were harvested from the FI specimensafter terminating the test. By doing so, four samples per specimenwere obtained: two upper parts from the left and right side of thehole and two lower parts from the left and right side of the hole(Fig. 4). The upper samples were prepared for fractographic inves-tigation using an Olympus stereo microscope. The location of initi-ation and the distance between the marker load bands werecounted and measured. The marker load bands were correlated

to the crack length measurements performed during the tests toenable the calculation of the initiation life.

The lower samples containing the weld were embedded inEpofix and prepared for metallurgic research. These samples wereground and polished to 3 lm and electrolytically etched with aBarker etch (200 ml distilled water and 5 g fluoboric acid 35%). ThisBarkers etch provides an oxide layer with an orientation equal tothe crystals in the grains underneath. As a result, the grain struc-ture can be revealed using polarised light and a 1/4 lambda filter.The orientation of the crystals determines the observed color of agrain. Grains with the same crystal orientation exhibit the samecolor.

2.4. Residual stress

The residual stresses in the FS welds which are subject of thispaper, have been measured using X-ray diffraction [1]. Different

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welds were subjected to these measurements resulting in multipleprofiles for the FS welds in AA2024-T3. Most measurements wereperformed on welded sheets with a size of �20 � 30 cm in as-welded condition. One measurement was performed on a FI L–Tspecimen prior to testing to obtain the residual stress profile afterthe production steps of this specimen (Fig. 5). FS welding intro-duces significant residual stresses in longitudinal direction, i.e. par-allel to the weld, with negligible stress in transverse direction. As aresult, the FI behaviour from the same location is different for dif-ferent orientations of the weld [2,1]. Depending on whether thestress is tensile or compressive, the L–T specimens exhibit a short-er or longer FI life compared to the T–L specimens. The fatigue lifeof the 45� specimen should be in between the results from the L–Tand T–L specimens. Because the residual stresses have no influenceon the T–L specimens, these specimens show only the influence ofthe microstructure.

2.5. Finite element analysis

For prediction purposes, a three-dimensional (3D) elastic–plas-tic FE analysis was performed on the L–T specimens [2,12]. The FEmodels had the same geometry as the FI specimens, with a plane ofsymmetry in the centre of the specimens. The weld is simulated inthis analysis by the residual stress field and the yield strength pro-file of the weld as they were measured in [1]. Two load cases wereapplied in the FE analysis; the maximum and minimum load of thefatigue cycle. As a result, the fatigue stresses, i.e. mean stress andstress amplitude, at the notch for various test locations in the weldwere obtained. The residual stress affects only the mean stress andnot the stress amplitude. Therefore, the FI behaviour is only a func-tion of the mean stress at the notch. Because a 3D representation ofthe FS weld was used, a through-thickness mean stress distributionis obtained for each notch location. For each notch location, thisenabled the identification of the location along the notch withthe highest mean stress. This location should correspond to thelocation of FI observed on the fracture surfaces. Moreover, themean stress differs at both sides of the hole, and thus initiationis expected first at the side with highest mean stress.

3. Results

3.1. Fractography results

The visibility of the marker load bands is dependent on the sur-face roughness, the distance between the marker load bands andthe width of the marker load bands (Figs. 6–9). Therefore, it was

Fig. 5. Residual stress profile measured i

not always possible to trace all the marker load bands up to thepoint of initiation. The fatigue life of each marker load band is indi-cated in the figures.

Two different surface topologies are distinguished on the frac-ture surfaces, a smooth surface in the nugget and TMAZ (Fig. 6)and a rough surface in the HAZ (Fig. 7). The fracture surfaces ofthe T–L tests at y = 3 mm and the L–T tests at y = 0 mm show bothsurface topologies (Figs. 8 and 9). The boundary between the twosurface topologies is sharp at the advancing side whereas at theretreating side it is wider and smoother (Fig. 9). An interesting fea-ture of the smooth surface is the good appearance of the markerload bands. Furthermore, wave like shapes are observed in thenugget region of the fracture surface (Figs. 6 and 9). The geometryof these waves is equal to the banded structure in the nugget (alsoreferred to as onion rings).

By the aid of the marker load bands, the number and locationsof nucleation were measured. These locations of initiation are visu-alized in Figs. 10 and 11. These figures represent a cross section ofthe FI specimens with the locations of the holes and the FS weldwhich is indicated by the boundary between the nugget and theTMAZ. For each specimen two points are plotted; one for each sideof the hole. In the case of multiple locations of initiation at oneside, only the earliest point of initiation is plotted. Initiation at bothsides of the hole did not occur simultaneous; therefore the pointwhich initiated first is indicated by triangles whereas the dia-monds represent the side of the secondary initiation points.

It was observed that all locations of initiation for the T–L tests aty = 3 mm are situated in the rough surface outside the nugget. Forthe T–L tests at y = 0 mm most cracks initiate at the root side cor-ner of the weld. The point of first initiation is for the L–T tests aty = 3, 5.5, 7.5 and 10.5 mm always at the weld centre side (left side)of the hole.

3.2. Microscopic investigation

Figs. 12–15 present the microstructure together with the crackpath from different specimens. The nugget is recognised in the cir-cular banded structure, which are a direct result of the materialflow around the weld tool. The bands are distinguished, becausethe grains have equal color within a band and thus exhibit lowangular boundaries [10]. In contrast to the images made from theroot side, the bands are barely visible at the crown side (Fig. 15)[13]. These figures show that the transition from the nugget tothe HAZ is sharp at the advancing side and more smooth at theretreating side.

n an FS welded L–T FI specimen [1].

Fig. 6. Fracture surfaces of T–L test (upper), location: y = 0.0 mm, amplitude: 90 MPa.

Fig. 7. Fracture surfaces of T–L test (upper), location: y = 5.5 mm, amplitude: 65 MPa.

Fig. 8. Fracture surfaces of T–L test (upper), location: y = 3.0 mm, amplitude: 65 MPa.

Fig. 9. Fracture surfaces of L–T test (upper), location: y = 0.0 mm, amplitude: 65 MPa.

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The crack paths, which are the upper boundaries of the samplesin Figs. 12–15, show significant interaction with the microstruc-ture of the weld. In Fig. 12 the crack path has a preference to follow

the banded microstructure. At some locations secondary fracturesare observed. The same preference for the banded microstructureis observed for the T–L test, where the fracture crosses perpendic-

Fig. 10. Schematic cross section of the FS weld with the locations of initiation in the T–L test specimens for the different hole locations

Fig. 11. Schematic cross section of the FS weld with the locations of initiation in the L–T test specimens for the different hole locations.

Fig. 12. Side view of etched sample (lower) with fracture (L–T test).

Fig. 13. Side view of etched sample (lower) with fracture (T–L test).

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Fig. 14. Side view of etched sample (lower) with fracture (45� test).

Fig. 15. Side view of etched sample (lower) with fracture (45� test).

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ular to the weld (Fig. 13). Remarkably, when the fracture ap-proaches the weld, it turns first in another direction than the orien-tation of the banded structure. Once it reached the nugget, thefracture follows the circular pattern of the microstructure.

Figs. 14 and 15 present the fractures in a 45� specimen, with thehole located in and outside the nugget, respectively. The fracturefrom the hole in the nugget, is initially not affected by the bandedstructure. Once it reaches the edge of the nugget, the fracture ischanged to an orientation parallel to the weld. The specimen inwhich the fracture entered the nugget from the outside, showeda remarkable behaviour; the crack path was turned more than65� and continued to grow in this direction outside the weld(Fig. 15). The crack path was interrupted once the fracture at theother side of the hole reached the edge of the specimen. In contrastto the previous specimens where the fracture followed the bandedmicrostructure, crack growth in this specimen is opposite to themicrostructure direction as indicated in the figure.

3.3. Fatigue initiation test results

Two sets of results are presented in this section; the S–Ni curvesfrom the T–L tests performed on both sides of the weld center

(Fig. 16), and the S–Ni curves from locations tested in all three weldorientations (Fig. 17). For comparison and visualization purposes, afour parameter Weibull S–Ni curve is fitted through the datapoints. Note that due to curve fitting each data point is consideredof equal weight, which may be considered inadequate for the run-outs.

No significant differences are observed between either side ofthe weld centre for the locations y = ±7.5 and ±10.5 mm (Fig. 16).For the location y = ±3 mm the fatigue limit is higher at the retreat-ing side than at the advancing side. This might be attributed to themicrostructure in the TMAZ, because this zone received a largerheat input at the advancing side than at the retreating side. Initia-tion occurred in all y = ±3 mm specimens at the TMAZ side of thefracture as is illustrated by Fig. 8.

Three locations, y = 0 mm, 5.5 mm and 10.5 mm, were tested forall three orientations of the weld (Fig. 17). The anisotropic charac-ter of the residual stresses should be reflected in the FI behaviour.Consequently the S–Ni curve of the 45� specimens was expected tobe situated in between the FI behaviour of the L–T and T–L test.This is not the case for the FI behaviour at 0 mm, which is anindication that more parameters than the residual stress affectthe FI behaviour. When the residual stresses are compressive

Fig. 16. Comparison of the fatigue initiation data from locations symmetric about the weld centre.

Fig. 17. FI test results at location y = 0 mm, 5.5 mm and 10.5 mm for different loading directions.

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(y = 10.5 mm), the S–Ni curve of the L–T exhibits longer fatigue lifethan the curve for the T–L test and visa versa for tensile stresses(y = 0 and 5.5 mm).

3.4. Results from the FE analysis

The FE analysis has been successfully verified with a meshdependency analysis and validated with the results from basematerial specimens [12]. As a result of the FE analysis, the stressesat the notch for each location in the weld that was tested were ob-tained. Because a 3D FE analysis was performed, the mean stress atthe notch is not constant through the thickness (Fig. 18). The meanstress shows coherence with the residual stress profile of the weld[12]. At y = 7.5 mm the mean stress is highest, which coincideswith the peak in the residual stress profile (Fig. 5). Furthermore,the mean stress is different for both sides of holes which are posi-tioned at locations where the residual stress profile exhibits aslope. The locations y = 10.5 and 12 mm are in a region with com-pressive stresses, therefore the mean stress is below the basematerial curve.

The FE analysis showed a significant impact of plasticity on themaximum stresses at the notch. The theoretical linear-elasticstress levels at the notch are fairly high because the stress concen-tration factor of Kt = 2.94. However, plasticity acts as a damping ef-fect on the stress concentration factor. The consequence of thisdamping effect is an inverse relation between the applied nominalstress amplitude and the mean stress at the notch, i.e. when the

applied stress amplitude is increased; the mean stress at the notchis reduced. This non-linear behaviour implies that FE analysis is theonly method to obtain the influence of residual stress in a weld onthe mean stress at the notch.

Unlike the mean stress, which is dependent on the maximumand minimum stress, and thus limited by local material properties,the stress amplitude is not affected and is only dependent on thestress concentration factor Kt. Therefore, the order of the meanstress profiles in Fig. 18 is not changed for different stress ampli-tudes. Only the distance between the curves is changed becauseof the non-linear response of the mean stress, discussed in previ-ous paragraph.

4. Discussion

The application of marker loads was considered a success, be-cause it enabled a thorough investigation of the fracture surfacesby optical microscopy without using time consuming ScanningElectron Microscopy (SEM). In most cases, it was easy to recognisethe marker load bands on the fracture surfaces. For low stressamplitudes, the distance between the marker load bands was toosmall to distinguish small crack lengths. Here, low stress ampli-tudes imply even smaller stress amplitudes of the marker loadscausing marker bands too narrow to be visible. The marker loadscould have been adjusted to the stress level, but that was consid-ered an influence on the FI behaviour.

Fig. 18. FE analysis: through thickness longitudinal mean stress for different locations in the weld, for an applied stress amplitude of 70 MPa.

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The effect of the marker loads on the FI life is unknown and it isrecommended to verify this influence. For this research the effect isassumed to be negligible, because the marker loads are applied toall the specimens in the same way and thus any effect, if present,should be similar. Besides, for the macroscopic crack growth phase,the influence of the marker loads is visible but small, because thegrowth rate in the marker load bands is significantly lower thanthe growth rate during the base fatigue loading (see Figs. 6–9). Thisindication is only valid for the fatigue crack growth behaviour andnot for the FI behaviour, because different damage mechanisms areactive in the two fatigue phases.

The different surface roughnesses observed on fracture surfacesin the FS weld are a result of the different grain sizes in the nugget,TMAZ and HAZ. The small grains in the nugget reveal a smooth sur-face, while the large grains from the parent material result in arough surface. The boundary between the microstructure in thenugget and the TMAZ is evident on the fracture surfaces of locationy = 0 mm and 3 mm (Fig. 9). The transition is sharp at the advanc-ing side, whereas the retreating side exhibits a more smooth tran-sition. This observation is in line with the microstructure observedin Figs. 12–14. The difference between the two nugget boundariesis a direct effect from the asymmetric weld process [9,10].

The wave-like geometry of the fracture surfaces in the nugget iscreated by the banded microstructure. The banded microstructureis formed by different material flows around the weld tool duringthe weld process [14]. These material flows exhibit different ther-mal and mechanical histories. As a result, the different bands havedissimilar mechanical behaviour [14,13]. The fracture surface re-flects these differences by small plane change in each band, result-ing in a wave like topology.

4.1. Locations of initiation

It is generally known that grain boundaries form a threshold formicro crack growth in the crack initiation phase [3]. This thresholdbehaviour supports the initiation locations at y = 3.0 mm in Fig. 10,where all fatigue cracks initiated outside the nugget in the regionwith larger grains. It is not possible to attribute this behaviour tothe grain size only, because several other properties affect the FIbehaviour too, such as the difference in precipitation hardening,the lack or presence of a precipitation free zone around the grainboundaries, the presence of low angle grain boundaries in the nug-get, etc [10]. Moreover, these aspects have different influences onthe various phases of fatigue initiation, i.e. cyclic slip, crack

nucleation and micro crack growth. This means that a positive ef-fect of one property in one phase can be leveled out by a negativeeffect of another property in the next phase. This can be the reasonwhy initiation in the different microstructures did not lead to a sig-nificant difference in the FI life, for instance between locationy = 0 mm and 3 mm.

Fig. 10 shows a large amount of corner cracks at locationy = 0 mm at the root side of the FS weld. One explanation whichcan be put forward here, is the presence of a root flaw as investi-gated by Dickerson [15], but no proof of the presence of a root flawwas observed. The root surface has been ground, which removesdetrimental surface features like the root flaw. Moreover, Dicker-son explains that it is difficult to obtain a significant root flaw, be-cause FS welding is extremely robust. In order to do so, weldingparameters had to be used that were far outside the normal pro-cess envelope, which was not the case for this research. Therefore,the root flow is not considered to be the reason for this behaviour.Another more likely explanation is the local microstructure, be-cause during welding the nugget material at the root exhibitedhigher deformations than the material at the crown. The materialflow at the root exhibits large vertical movements and is forcedunderneath the pin [14]. As a result, the material at the root exhib-its lower resistance against FI.

For the results of the L–T specimens it was assumed that thelocation of initiation along the hole is determined by the locationwhere the highest mean stress is present. In Fig. 19 the mean stressobtained by FE analysis from Fig. 11 and the location of initiation inthe L–T specimens from Fig. 18 are plotted together. The meanstress is plotted for a single applied stress amplitude of 70 MPa,whereas the locations of initiation are plotted for all applied stressamplitudes used in this study. The order of the mean stress curvesis the same for all applied stress amplitudes, therefore, the curvesfor 70 MPa represent the variation within the mean stress curvesfor other stress amplitudes too.

A region is indicated for each hole in Fig. 19B where the highestmean stress is located according to the FE analysis (Fig. 19A). Thiscomparison shows indeed that for most locations, i.e. y = 1 mm,3 mm and 7.5 mm, initiation occurs at the location with highestmean stress. For other holes, the locations of initiation do not oronly partly coincide with the indicated range.

For all the locations where the mean stress is significantly dif-ferent at both sides, the first initiation point is situated on the sidewith highest mean stress. For the locations y = 7.5 mm and10.5 mm the mean stress is highest on the left side of the hole (side

Fig. 19. (A) FE analysis FI L–T specimens; through-thickness mean stress at left (RS) and right (AS) side of the holes (definition Fig. 4); applied stress amplitude is 70 MPa;(B) locations of FI data from Fig. 11.

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closest to the weld centre). This corresponds to the locations offirst initiation (indicated by triangles) which are indeed at thesame side. Furthermore, at locations y = 3 mm and 12 mm the dif-ference of the mean stress is smaller, but the locations of FI are stillon the side with the highest mean stress. For the location y = 0 mmand 5.5 mm, the difference between the mean stress at both sidesof the hole is insignificant and the observed locations of initiationare distributed over both sides of the hole.

4.2. Microscopy

Figs. 12–15 show that an interaction exists between the micro-structure of the nugget/TMAZ and the crack path. The bandedmicrostructure in the nugget attracts the fracture as if it providesa path with low fatigue resistance. The low angular grain bound-aries might play a role in this, or the bands which exhibit largeamount of intermetallics [13,6]. The fatigue crack is not only at-tracted by the banded microstructure in the nugget, but also bythe boundary between the nugget and the HAZ as is shown inFig. 14. Also the fatigue crack in Fig. 15 follows a path which isnot determined by the banded microstructure. For this specimenit is believed that the residual stress plays an important role. Ingeneral, fatigue cracks are affected by the microstructure and theresidual stresses in the FS weld. But they depend on the locationin the weld, which effect is largest, and how the crack path is chan-ged by it.

4.3. FI behaviour

The asymmetry of the FI behaviour in the weld was investigatedby testing three locations at both sides of the weld centre (Fig. 16).Only for the location y = ±3 mm the FI lives were significantly lar-ger at the retreating side (y = �3 mm) than at the advancing side(y = +3 mm). The locations of initiation are for these tests all situ-ated in the TMAZ (Fig. 10). Therefore, the asymmetric behaviourcan be attributed to the TMAZ. The other locations did not showa significant asymmetric behaviour. The asymmetry of the weldprocess is only present in the nugget and TMAZ region, becausethere the rotation of the tool results in different microstructures

and mechanical behaviour. The HAZ exhibits symmetric behaviour,because this zone is only affected by the heat trespassing from thetool to the surrounding material. The temperature is higher at theadvancing side, but only the heat treatment is slightly affected bythat, not the microstructure. Therefore, it is concluded that the FIbehaviour is asymmetric in the nugget and TMAZ, but not in theHAZ.

Two of the three locations tested for all three weld orientationsshow FI behaviour which is expected from the anisotropic residualstress, i.e. y = 5.5 and 10.5 mm (Fig. 17). The FI behaviour for the45� specimens at these notch locations is in-between the resultsfrom the L–T and T–L specimens at the same location in the weld.However, for y = 0 mm the FI behaviour from the 45� specimen isnot in-between the FI behaviour from the L–T and T–L specimens.This could be attributed to the microstructure in the nugget whichis highly anisotropic. This explanation is speculative, because noproof exists yet. The scatter in FI life might also affect the results,because the amount of tests on 45� specimens was limited. How-ever, the other tests showed a good consistency between the testsresults from equal location and configuration.

The conclusion which can be drawn from previous discussion isthat the FI behaviour in the nugget is affected by more parametersthan the residual stress only. Whether the influence of the micro-structure on the crack path is a result of the grain size or orienta-tion, the precipitation distribution, or the properties of the grainboundaries, is yet unknown.

5. Conclusions

The location of initiation is not only determined by the residualstress, as is concluded before [2], but also by the microstructure.Especially at the boundary between the nugget and TMAZ, thelocation of initiation has a preference for the TMAZ. Moreover,the TMAZ at the advancing side exhibits lower FI properties thanthe TMAZ at the retreating side. This asymmetric behaviour is fullyattributed to the asymmetric welding process.

The microstructure in the weld has direct influence on theappearance of the fracture surface. The fracture surface has a‘smooth’ appearance in the fine grained nugget and a ‘rough’

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appearance in the HAZ. The rough appearance of the surface wassimilar to what was observed for the base material.

The microstructure in the nugget affects the crack path. For cer-tain angles of the weld, the fracture follows the banded microstruc-ture in the nugget resulting in significant change of the fatiguecrack propagation direction.

First initiation occurs always on the side of the hole whichexhibits the highest mean stress. Only when the difference in meanstress is small, initiation occurs randomly at both sides. The loca-tion of initiation along the hole edge is determined by the locationof highest mean stress for most locations.

Acknowledgments

The authors wish to thank Dr. J. Silvanus from EADS Innovationworks Germany, Metallic technologies and Surface engineering, forthe ability to use the friction stir weld machine to produce thespecimen. Mr. E.R. Peekstok from the Delft University of Technol-ogy, department: Micro structural Control in Metals, for his helpby the preparation of the microscopy samples and the use of themicroscopes.

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