13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- as for JLR but with retracting the pin by 80 µm to modify mixing conditions within the weld. “Gap bearing” welds were realised under load control with a constant clearance between the sheets, of either 0.3 mm (“Gap3”: 10% of the parent sheet thickness) or 0.7 mm (“Gap7”: 23% of the parent sheet thickness). Only KB bearing welds cracked under severe bending conditions (width 10 mm, bending angle 90°, bending radius 7 mm, and bottom part of the weld outside). Figure 1. General view (a) of the grain structure of the studied sheet and (b) of a FSW joint (TMAZ: thermo-mechanically affected zone; HAZ: heat-affected zone) The JLR appears as a dark line across the nugget (Fig. 2a). The KB has the same appearance as the JLR (Fig. 2b). Detailed observations revealed that this defect is intergranular [11]. In the Gap bearing samples, small regions with deeply etched grain boundaries (“sensitive GBs” hereafter) were observed from place to place in the nugget, at the bottom surface (Fig. 2c). The hardness profile (Fig. 3a) is identical for all welds. A minimum in hardness was detected at TMAZs, as already reported e.g. in a 2098-T8 friction stir weld [12]. Figure 2. Cross-section observations of weld defects: (a) JLR; (b) KB (bottom) and JLR (upper, nearly horizontal part); (c) etched grain boundaries (bottom) and JLR in a Gap3 bearing weld Figure 3. (a) Hardness profiles across a sound weld. (b) Tensile curves of the various kinds of welds. Drawings of full-thickness (c) cross-weld and (d) base metal fatigue specimens (dimensions in mm) ND TD 100 µm KB (b) 50 µm (c) 50 µm (a) JLR RD TD ND (a) Retreating side Advancing side TMAZ TMAZ Nugget HAZ HAZ (b) ND TD Distance to weld centreline (mm) Weld Retreating side Advancing side Upper part Mid-thickness Lower part -30 -20 -10 0 10 20 30 100 120 140 160 180 HV0.1 (a) Average elongation (%) 0 5 10 15 20 0 100 200 300 400 Engineering stress (MPa) (b) 7 7 r = 62.5 21 90 (c) (d) weld
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