19.3 Results and Discussion To validate the technique, aluminum alloy targets were impacted at ~1.2 km/s and a comparison of the resultant data obtained from high-speed image analysis, 3D DIC, PDV was made. Figure 19.2 illustrates the damage that occurred due to copper impacts at more than 1 km/s. Note that the through hole generated (0.60–0.7500) was slightly larger than the impactor diameter of 0.500, and takes place in approximately 10–15 microseconds. Thus, the evolved deformation represents a highly localized damage region. 19.3.1 High-Speed Imaging Figure 19.3 shows high-speed shadowgraph images that were captured during the impact as well as images converted into outlines using edge-detection algorithms. The images show the respective times after damage begins to evolve on the back surface. The images reveal that a crater is formed in the target plate due to plastic flow (to accommodate the volume of the striking projectile), creating a bulge on the rear face of the plate. The high shear stresses imposed on the target material just ahead of the projectile generate a plug. Failure in the metal target occurs within ~ 15 microseconds, and a plug is ejected from the rear of the plate. The images also show the deformed shape of the projectile long after impact, which becomes semicircular and has a residual velocity of more than 800–900 m/s. 19.3.2 Edge-On (2D) Image Analysis Using an edge-on shadowgraph imaging technique, the images were processed using MATLAB to compute the back face deflection and velocity fields (see Fig. 19.4a,b). Note the bulge symmetry, which is expected for a “reasonably” isotropic material. Also, notice that the velocity of the bulge is approximately constant at all times during the damage process. From the data, the maximum values of deflection and velocity during the deformation process were determined and tracked to evaluate the displacement- and velocity-time histories at the shot line. Here, it was observed that the displacement history is approximately linear to almost 8 mm over 13 microseconds prior to failure and spalling. Similarly, a steady rise in the back face velocity is seen until a near constant velocity of 850–900 m/s is reached. Beyond 13 microseconds, the back face ruptures and a spall is ejected. Fig. 19.2 Post-mortem back face images of (a) bare and (b) “speckled” targets which have been impacted by 0.500 copper spheres to demonstrate the affected area 19 Instrumented Penetration of Metal Alloys During High-Velocity Impacts 141
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