A model is developed to estimate the force on a conical-nosed penetrator for normal entry into geological targets that have linear hydrostats and fail according to a Mohr-Coulomb criterion with a tension cutoff. The model is applicable to targets with shear strength which either increases with pressure (dry rocks) or is constant (sea ice, concrete, saturated rocks). For high enough penetrator velocity the target response is elastic-plastic, but at lower velocities stresses exceed the target tensile strength and the response includes an additional cracked region. Parametric results are obtained via a similarity transformation and solution to a nonlinear wave propagation problem. Predicted and measured penetrator decelerations are compared for a field test into a dry rock target and reasonable agreement is shown.

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