# Interface Characterization within a Nuclear Fuel Plate
## 4 Results and Discussion
### Fuel-Cladding Interface in Variable Bond Strength Specimens  
Thus far, all LST testing of fuel plate specimens containing dissimilar interfaces have produced debonds. This was also the case with LANL-fabricated specimens tested in this work; however, reliable testing results were only obtained after machining the fuel plates to a final thickness matching the historical nominal thickness for finished fuel plates. The as-received LANL-fabricated specimens were not initially machined to typical finished fuel plate final thickness. As-received, the specimens were 2.64 mm thick, whereas finished fuel plates are typically \(\sim\)1.4 mm thick. Initial shock testing was performed on the thicker as-received specimens and it is believed that debonding was achieved. The issue was that the determination of debonding was hard to distinguish in the LUT C-scan images. Figure 6: Diagrams showing the plate geometry and nomenclature used for laser shock testing. (**A**) is the cross-sectional figure, and (**B**) shows the experimental geometry. The usual debond signatures were either not present or not readily identifiable in the thicker plates. Thus the testing produced indeterminate results. A decision was made to machine the plates down to thicknesses of 1.65 mm prior to further LST testing. The LST testing was then repeated in the thinner fuel plates. The resulting ultrasonic C-scans showed the same distinct signals and signatures as observed historically during testing with the typical final fuel plate thickness [3; 4; 5; 6; 7]. The changes in the ultrasonic signals could have been brought about by the geometry changes and/or the work hardening by the surface machining. This remains an area for further investigation. Since the stresses at the interfaces are complex and are influenced by geometry and propagation speed of the shock wave (which are influenced by residual stresses and work hardening of the Al), there may be certain plate geometries that are not conducive to laser shock testing. All of the results reported in this section were obtained on the machined plates having a nominal thicknesses of 1.65 mm. Figure 7 shows the resulting maximum surface velocities obtained for the LANL variable contamination specimens. The reference specimen is 4E5 with the Zr coating that was cleaned using typical procedures prior to HIPing. The contaminated samples 4D5 and 4A5 show some interface strength degradation, which might be expected. Sample 3C5 with a thick hydrocarbon layer shows a slight increase in interface strength with respect to reference specimen 4E5. These results also suggest that surface preparation has minimal effect on bond strength. Although the increase in the interface strength is not statistically significant, it is interesting to note that the bulge testing performed at LANL on specimens taken from the same fuel plates also shows the thick hydrocarbon sample having the highest interface fracture energy [10], see Figure 8. At present, the bulge test is the only known technique to compare results and help validate the laser shock technique. The debond areas for the variable strength specimens are shown in Figure 9. These images are only used for a debond or bond determination. A debond has been arbitrarily defined to be two or more black pixels in the image. The size of the debond is not a concern at present. Interesting qualitative observations are made from the images For example, in most laser shock samples tested to date, the maximum debond area is on the order of the incident shock laser beam diameter, and may be smaller depending on the severity of the debonding force. As shown in Figure 9, the debond area in these specimens was typically less than the incident shock laser beam size that had a diameter of approximately 23 mm. Figure 7: This graph shows the measured maximum surface velocity measured in the various Los Alamos National Laboratory (LANL) interface samples with varying levels of contamination.