All well-known experimental methods for studying phenomena of spalling provide only indirect information about destruction process and stresses. More complete data on target stresses during spalling can be obtained from dynamic measurements under shock loading. However, in this case the high tensile stresses are estimated via deformation parameters measured on a target free surface distanced by the thickness of spall layer. Numerical simulations of spalling allow performance properties and, in particular, spallation strengths of materials to be evaluated. In this work, numerical simulation results of contact interaction of a deformable steel impactor and a steel composite target are presented. An effect of gap width in the composite target on its stress strain state under dynamic loading is investigated. Impact velocity is set at 260 m/s, and maximum gap width between plates of the target is set at 0.12 mm. The gap width of several micrometers is considered as a defect caused by a previous shock loading. Thus, numerical simulation of artificial spalling allows to evaluate fracture and destruction parameters for targets with spall cracks from previous shock loading. Results of the simulation have good correspondence with results of real experiments.