In [1-2] we have proposed an elastoplastic model of fragmentation at the high-velocity penetration of a periodic set of strings in a massive target. The model have allowed us to estimate the size, velocity and kinetic energy of the fragments knocked out of the target. However, this model assumed that the fragments formed instantaneously. For a more adequate description of the fragmentation in present paper we propose to take into account the fact that the formation of the fragments occurs in a finite time.