86 S. Kramer et al. Fig. 12.1 (a) Cross-section of a 0.763-mm deep partial penetration laser weld in 1.532-mm thick 304L SS sheet with the laser weld having a keyhole shape and an unwelded ligament (the vertical line below the laser weld); (b) Cross-section of an EDM-notched 304L SS sheet with 0.236-mm notch width and a 0.789-mm notch depth that is similar to the unwelded ligament in the laser welded specimens insufficient to characterize the damage evolution of these laser welds. Experimental evidence of the damage mechanisms is required to guide improvements in these computational models. In this study, we performed interrupted tensile tests of laserwelded 304L SS to interrogate the local deformation and damage mechanisms in the laser weld. Additionally, we performed interrupted tensile tests of EDM-notched 304L SS specimens (see Fig. 12.1b), which have a similar pre-notch depth as the unwelded ligament of the laser-welded specimens, though not the same sharp notch tip, to compare the effect of the local geometry without the complications of the pre-existing porosity, heat affected zone, or change in grain structure in the laser weld as compared to the parent material. In this paper, we detail the experimental methodology of the interrupted testing and post-test measurements of damage, present the experimental results, discuss potential competing damage mechanisms, and identify future areas of research based on these initial findings. 12.2 Methodology The base material in this study was 304L SS sheet material, approximately 1.55-mm thick. Pairs of 76.2 101.6-mm plates were joined along the long edge with a single laser butt weld. These plates were cut into tensile coupons such that the laser weld was perpendicular to the tensile loading direction. In this study, the rolling direction of the parent 304L SS sheet material was along the short edge of the original plates, meaning the rolling direction of the material was in the direction of the global tensile load of the laser welded specimens. The notched tensile specimens were cut from EDM-notched plates (152.4 101.6-mm) of the 304L SS with the rolling direction along the tensile direction; the EDM notch was cut in the center of the plate with a nominally 0.2-mm wide EDM-wire, parallel to the short edge of the plate, resulting in a nominally 0.24-mm wide notch with a depth approximately half the thickness of the parent plate. All specimens had a nominal gage width of 6.35-mm. We performed base material tensile tests on a custom servo-hydraulic load frame with dual, horizontal, in-line actuators, allowing the center of the specimen to be stationary, which was useful for insituimaging of the local geometry of the notched and welded specimens during the interrupted tensile testing. The specimens were oriented with the width of the specimens along the vertical axis. The global displacement for the base material, notched, and laser welded 304L SS specimens was measured using a 25.4-mm gage length extensometer, attached on the thin side of the specimens; the extensometer body hung vertically below the specimen. Figure 12.2 shows the experimental setup, including the Navitar 6000 long-working distance microscope lens and CCD camera that imaged the through-thickness side of the specimens. The base material testing global actuator displacement rate was 12.7 m/s. The tensile test global actuator displacement rate for the notched and laser welded specimens was 1.27 or 2.54 m/s. We tested three base material specimens, one notched specimen to failure, five notched specimens to different displacement/load levels, seven laser-welded specimens to failure, and 14 laser welded specimens to different displacement/load levels. The notched specimens came from a single notched plate, but the notch depth varied slightly along the plate. The laser-welded specimens came from three different welded plates with slight
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