19.2 Experimental The impact experiments were conducted using spherical copper (C101; Grade 200) projectiles of 0.500 diameter at 1.2 km/s. Copper was chosen due to its widely characterized and well-known properties and deformation behavior. The projectiles were fired from a 0.50-caliber smooth-bore powder gun using a two-piece discarding sabot with obturator. Projectile striking velocity (VS ) was measured using two make-screens set at a predetermined distance. The impact occurred at normal incidence on metal target plates (600 600) of aluminum alloy of thickness 0.2500. Relevant physical, mechanical, and shock properties for the projectile and target material are listed in Table 19.1. In situdiagnostic techniques employed to characterize the time-dependent failure behavior included high-speed imaging, DIC, and PDV. A make screen placed directly in front of the impact surface of the target provided a common trigger signal for the high-speed cameras, illumination, and PDV. The make screen was connected directly into the camera(s), and contact of the metallic (i.e. conductive) penetrator on the make screen triggered the camera. The cameras then generated a 5 V TLL pulse out of the auxiliary ports to the light strobes and PDV system. For high-speed imaging and DIC, high-speed cameras (Specialized Imaging Kirana) were used to capture the damage sequence at 2 million frames per second (fps) with an interframe time of 0.5 microseconds. A series of tests were performed using only one camera oriented edge-on with the target to capture the back face damage evolution by obtaining high-speed shadowgraphs (see Fig. 19.1a). Here, 2D image analysis was employed to track the back face deflection profile and velocity histories using edge detection and curve fitting algorithms. Another series of tests employed two cameras in a stereographic configuration to acquire images of “speckled” targets for 3D DIC analysis. For 3D DIC, the two cameras were positioned 1275 mm from the target and spaced 725 mm from one another in order to view the target at a relative (perspective) angle of 30 (see Fig. 19.1b). A first surface mirror was placed behind the target at 45 to achieve the desired view of the rear face of the target. The images were then processed using VIC-3D™(Correlated Solutions) to obtain a 3D measurement and visualization of the time-resolved deformation process. Illumination for the high-speed imaging and DIC was provided by two light strobes (Photogenic 2500DR-UV Powerlight). In each of the tests where edge-on high speed imaging was employed, PDV was used concurrently to provide timeresolved rear surface velocity measurements. The PDV system used in the current study was described by Ostrand et al.[2]. Each PDV probe consisted of a GRIN lens attached to a fiber pigtail (AC Photonics 1CL15A070LSD01); collimated light over a 300 mm working distance to a spot size of ~0.8 mm. Probes were aligned parallel to the shot line using a polycarbonate fixture. This facilitated measurement of velocities at several positions on the rear surface of the target sample, as shown in Fig 19.1a. Velocity records were measured directly on the shot line and at several distances away from the shot line. The PDV data was recorded using an effective bandwidth of 16 GHz, a sampling rate of up to 80 Gigasamples/s, and a maximum of 2 Gigapoints (Keysight DSOV174A Infiniium V-Series oscilloscope). The data was then reduced using proprietary PDV post-processing software developed by W.K. Bowman and D.S. Lowry to compute displacement- and velocity-time pairs for each channel. Table 19.1 Properties of the projectile and target materials (Matweb.com) Material Density (g/cm3) Elastic modulus, E (GPa) Yield strength, σy (MPa) Hardness, H (GPa) Poisson’s ratio, ν Shock impedance, Z Zplate/ Zprojectile Copper (C101) 8.89–8.94 115 69–365 0.75–0.9 0.31 32.0 – Aluminum (6061-T6) 2.70 69 276 1.07 0.33 13.6 0.43 Fig. 19.1 Schematic of experimental test setup: (a) Edge-on (2D) image analysis and PDV, (b) 3DDIC 140 P. Jannotti et al.
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