PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 346–361 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 5. a. Top view of canister test panel. Welding was done in the left-right direction. Laser peening was done with aluminum tape ablation layer with 4 inch (10 cm) wide coverage over the weld and heat affected zone. Stress measurements were performed using the Contour method in 2-dimensions across the full width of the panel. b. Side-on view shows stiffening ribs used to help prevent straining as would be the case in the geometric constraint of a canister during laser peening. Figure 5a shows a plan view of a panel that was roll formed and welded per spent fuel canister fabrication procedures. The weld runs left to right. Figure 5b shows strongbacks that were lightly stitch-welded to the back of the plate to help provide similar constraints as found in the canisters, thus to hold the cylindrical shape from straining during the laser peening. Stitch welding was used to minimize stress generated during attachment of the stiffeners.

Figure 6. Contour measurement of residual stress in welded panels of 316L Stainless Steel. The welding jointed two separate horizontal pieces in a process as done for canisters. The vertical piece was tacked on as a stiffener to limit straining during laser peening. The upper panel (SN1) was welded and then laser peened and the lower panel (SN2) welded and left un-peened. Note the deep compressive stress in the exposed surface (lower surface) of the upper panel. One panel was left in the as-welded condition and used for measurements of residual stress for the un-peened state. It was anticipated that the un-peened panel would have tensile stress in the welded area as measurement confirmed. A second panel was laser peened over the weld area. Peening was done at 4 GW/cm2 irradiance, 18 ns pulse duration and with 3 layers of coverage. Spot size was 4.7 mm square (6.6 mm diagonal). Each layer of peening provides fully 100% coverage with the square laser spots robotically positioned within 0.1 mm precision of one next to another and using approximately 3% spot length overlaps. The peening area was 4 inches (10 cm) wide covering the weld and heat affected zones across the width of the panel. Residual stress measurements were performed across the full width of the panel by Hill Engineering using the two-dimensional Contour method. In this technique a panel is systematically cut by an electro-discharge machine (EDM) wire and the resulting cut-surface deformations micro-measured across