13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- evidenced local opening of the KB over 60-80 µm (respectively, ~50 µm) followed by fatigue crack propagation over 30-40 µm (respectively, 30-55 µm) (Fig. 7c). Thus, even after only 20% of the fatigue lifetime, crack initiation and propagation was observed from place to place all along the KB as in [7, 8]. This confirms that at high stresses, early crack initiation occurs due to opening of the KB, most probably during the first cycle. At lower stresses, crack initiation occurred away from the KB but the lifetime was still significantly lower than that of sound or JLR bearing welds (data points 6 to 9 in Fig. 5). In KB specimens with an as-welded lower surface, the crack initiation site (either at the KB, or from surface defects far from the KB) did not significantly depend on the load level. This suggests that the effect of the as-welded KB on the fatigue lifetime could be of the same order as that of surface defects after welding. Figure 7. Fracture surface of KB bearing specimens (a) with a ground bottom surface, Δ σ = 306 MPa, lifetime ~13,000 cycles with the crack propagating in mode I along the exposed KB, (b) with an as-welded bottom surface; initiation at, and propagation away from the KB, Δ σ = 279 MPa, lifetime ~70,000 cycles; (c) with a ground bottom surface, Δ σ = 279 MPa, interrupted after 15,000 cycles. (1), (2), and (3) respectively denote cracking along the KB, away from the KB and under uniaxial tension. Average crack propagation indicated with a white arrow; same specimen orientation in (a,b,c) 3.2.3. Fatigue properties of Gap3 and Gap7 bearing welds The lifetime of Gap bearing welds was significantly lower than that of the other cross-weld specimens (Fig. 5 and Table 2). In all Gap7 specimens and in Gap3 specimens tested at higher stresses, fracture initiated from small regions close to the bottom surface and showing intergranular decohesions (Fig. 8), similar to those found after uniaxial tension (Fig. 4d). This suggests that in these conditions, the Gap-induced defects strongly affect the lifetime at crack initiation. Even when the fatigue crack did not initiate from such regions, the lifetime was low, as already found for KB bearing welds. Figure 8. (a) Fatigue crack initiation from “sensitive” GBs of a Gap3 bearing specimen, Δ σ = 256 MPa, lifetime ~26,500 cycles. The average crack propagation is indicated with a white arrow and the specimen orientation is the same as in Fig. 7a. (b) Detailed view of intergranular fracture of “sensitive” GBs 10 µm (b) 1 2 20 µm 1 2 3 (c) 10 µm 1 (a) 2 ND RD 20 µm (a) (b) 5 µm
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