90 S. Kramer et al. Fig. 12.6 Polished cross-sections from select notched and laser-welded specimens (oriented 90-degrees and inverted from corresponding Fig. 12.5 images), with inset electron backscatter diffraction SEM images near the crack tip to show void nucleation: (a) notched 98% peak-load (before estimated peak load) specimen, 52% displacement to failure; (b) notched 90% peak-load (after peak load) specimen, 85% displacement to failure; (c) Plate 4 98% peak-load (before estimated peak load) specimen, 63% displacement to failure; and (d) Plate 5 90% peak-load (after peak load) specimen, 82% displacement to failure from this view. In contrast, the crack growth into laser welds is apparent in both laser-welded specimens. All four of these specimens show evidence of void nucleation ahead of the free surface in the electron backscatter diffraction (EBSD) SEM view of the root of the notch/crack tip. Figure 12.7 includes SEM images of the root of the notch or laser weld from the perspective of the advancing deformation/crack; these images were taken prior to sectioning and view the central portion of the root of the notch or laser weld. Figure 12.7a, b shows the roots of the notches of the 98% peak load (before estimated peak load) and 90% peak load (after peak load) specimens, which are the same as those in Fig. 12.6a, b. Figure 12.7a clearly shows horizontal cracks at the root of the notch; higher magnification images (not provided here) show micro-scale cracking as well. Figure 12.7b shows the crack more prominently at the root of the notch. Figure 12.7c–f shows the damage and crack propagation of laser-welded specimens with increasing global deformation [note: the image magnification changes with each successive level of deformation]. Figure 12.7c has clear evidence of ductile rupture and horizontal cracks for the least global deformation level, just beyond the slope change of the load-displacement curve; also the preexisting pores from the welding process are deforming and are cracking open, seen as the vertically elongated oval structure in the root of the weld. Figure 12.7d–f shows evidence of extensive plastic deformation, ductile rupture, and pore deformation and coalescence with the main crack.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==