Equivalent Thickness Conversion and Perforation Morphology of Typical Metallic Target Plates under Fragment Impact

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Abstract
Equivalent-thickness conversion among metallic targets is important for vulnerability assessment and protection modelling, but conventional coefficients are often treated as material constants and seldom include fragment morphology. Penetration tests were conducted on 616 armor steel, Q235 steel, and LY12 aluminum alloy using steel spheres, tungsten spheres, and tungsten block fragments. A V50-based database was established, and failure mechanisms were examined from perforation morphology, exit-side SEM, and metallography. A semi-empirical ballistic-limit model was developed from key variables, with a residual correction term to describe the coupled deviation between fragment geometry and target response. The corrected model achieved a mean absolute percentage error of 7.91% and a coefficient of determination of 0.852. The conversion matrices show strong dependence on fragment type. For 616 steel converted to LY12 aluminum alloy, the coefficient increases from 2.50 for spherical fragments to 3.48 for tungsten blocks, giving a morphology-induced deviation of 39.03%. Thus, conversion should be defined by fragment type and target material rather than by a single material constant.

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