A simple model is suggested for deep penetration resistance of rigid, axially-symmetric, nose-pointed projectiles penetrating ductile metal targets. Model is based on power balance between the power supplied by the projectile during steady-state penetration and elastoplastic power needed for expanding penetration hole. The latter is approximated with the aid of the spherical specific cavitation energy (spherical cavitation pressure). Frictionless model includes an empirical parameter which is found to be practically independent of target material characteristics but reflects the shape of head profile. For ogival nose projectiles (with the spherical nose as a special case) the empirical parameter values are limited to a narrow range, slightly lower than one. Friction resistance is included by the Prandtl friction model leading to a general expression for the deep penetration hardness. Experimental data of spherical and ogival nose projectiles penetrating aluminium targets is compared with analytical predictions for penetration depth and an optimal ogival nose projectile is discussed.
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