When a rigid projectile penetrates a thick target, then the target material is displaced laterally, except close to the impacted surface when the surface around the impact either heaves, or is broken and scattered backwards. If the projectile energy is sufficiently great, then it will perforate the target. During the final phase the target material in front of the projectile is either displaced laterally-and-forwardly, without becoming detached, or it is broken and ejected as secondary fragments, which leave with about the same velocity as the projectile. In both cases the target material is subjected to great deformation. Some of the deformation is elastic and the rest is plastic. A model for the ratio between the plastic and elastic deformation energies is proposed in the report. This model involves the mass distribution of the secondary fragments. This model is compared with previous models for the energy required for rigid projectile perforation of target plates. The model is also applicable for non-rigid projectiles. In particular the production of secondary fragments by shaped charge jets is discussed and compared to experimental results.