13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Table 3. Determination of average crack propagation rate from interrupted tests on KB bearing welds Δσ = 279 MPa, ground bottom surface After 7,000 cycles (≈ 20% of lifetime) After 15,000 cycles (≈ 40% of lifetime) ΔK after opening of KB (MPa√m) 4.3 4.3 ΔK after fatigue crack propagation (MPa√m) 5.2 - 5.8 5.5 - 6.1 Average value of crack growth (10-9 m/cycle) 4.3 - 7.1 2.6 - 4.0 4.3. Effect of welding conditions on fatigue crack initiation and propagation In KB and Gap bearing specimens, the defect clearly affects crack initiation mechanisms, leading to a strong reduction in fatigue lifetime at high stresses. The fatigue lifetime at low stresses cannot be related to an obvious effect of defects on crack initiation, hardness or tensile strength, while it is significantly affected by the change in welding conditions. Thus, changing the welding conditions not only produced “visible” defects, but also influenced the sensitivity of the welds to fatigue crack initiation even away from those defects. Consequently, the effect of such changes on the fatigue strength cannot be inferred from using only tensile test results and metallographic inspection of the welds. For a maximal stress lower than the yield strength of the nugget (horizontal line in Fig. 5), residual stresses cannot be relieved during the fatigue test and have to be taken into account to further interpret data from e.g. Gap3 and Gap7 bearing specimens. The estimated crack propagation rate in KB bearing specimens seems to be close to that reported in literature (Fig. 9b), indicating that crack propagation could be less affected than crack initiation by the change in welding conditions. More experimental data points are needed to confirm this result. 5. Conclusions Tension-tension fatigue tests on 2198-T851 butt friction stir welds led to the following conclusions. • The effect of the joint line remnant is negligible in the investigated conditions. • The kissing bond opens during tension or during the first fatigue cycle if the maximum stress is higher than about 280 MPa. The crack growth rate (4-7.10-9 m/cycle) is consistent with literature data. The threshold stress intensity factor amplitude, ΔK, for crack propagation is close to 1 MPa√m. • A clearance of 10% of the sheet thickness is large enough to trigger local intergranular decohesions close to the lower surface of the nugget, for sufficiently high values of stresses. A clearance of 23% of the sheet thickness systematically leads to premature crack initiation by intergranular decohesions both in uniaxial tension and fatigue tests. The resulting low fatigue strength could be affected by residual stresses because no large scale plastic strain develops in that particular case. • The change in welding conditions seems to lower the fatigue strength even far away from the visible defects, while keeping hardness and tensile properties almost unaffected. The appearance of defects is not the only consequence of this change that influences fatigue properties. Acknowledgements This study was supported by the FNRAE (National Aerospace Foundation) under “MASAE” grant. References [1] P. L. Threadgill, A. J. Leonard, H. R. Shercliff, P. J. Withers, Friction stir welding of aluminium alloys. Int. Mater. Rev., 54 (2009) 49-93.
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