13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- All cross-weld specimens failed by strain localization, followed by ductile fracture. Sound welds fractured from close to the TMAZ-HAZ boundary with mixed dimpled / “fibrous” fracture surfaces (Fig. 4a). No obvious effect of the JLR was evidenced, as already reported by [1, 5, 13-14]. Table 1. Yield strength, tensile strength and ratio of weld tensile strength to that of base metal (along TD) Weld Yield strength (MPa) Tensile strength (MPa) Ratio of weld strength to base metal strength Sound 282-302 412-429 0.80-0.83 JLR bearing 285-296 413-416 0.80 KB bearing 265-287 398-410 0.77-0.80 Gap3 bearing 282-284 401-404 0.78 Gap7 bearing 277-281 398-399 0.77 In KB bearing specimens, fracture initiated as a striated, locally ductile region whose location and geometry closely matched those of the KB itself (Fig. 4b), as in [7]. The crack then left the KB to propagate across the nugget. From in situ tensile tests, opening of the KB (Fig. 4c) occurred for an engineering stress higher than about 280 MPa (more details are reported in [11]). Multiple crack initiation sites were found in that case. No effect of “sensitive GBs” was detected for the Gap3 bearing welds, whereas intergranular cracking triggered fracture of Gap7 welds (Fig. 4d). As already reported in [9], a clearance of 10% of the parent sheet thickness may be high enough to induce a decrease in strength. A clearance of 23% of the parent sheet thickness is high enough, in the investigated conditions, to trigger intergranular crack initiation and propagation from the nugget root. Figure 4. (a,b,d) fracture surfaces (a) in the TMAZ (sound and JLR bearing welds), (b) of a KB bearing weld (the striated zone corresponding to the KB defect itself) and (d) close to the root of a Gap7 weld. (c) Onset of cracking at the lower surface of a KB bearing specimen, slightly polished and then pulled in tension in the SEM (loading axis horizontal) 3.2. Fatigue properties Fatigue properties of base metal and welds are summarized in Fig. 5 and Table 2. Despite of the high experimental scatter, the cross-weld specimens can be ranged as follows, with respect to their fatigue strength (Fig. 5 and Table 2): Sound > JLR bearing > KB bearing (as-welded) > KB bearing (ground) > Gap3, Gap7 (1) ND RD (b) RD TD (c) ND RD (d) (a) ND RD
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