The mechanisms of dynamic fracture of various engineering materials--aluminium and its alloys, copper, tough high-duty steels, are investigated in the case of back spalling of flat targets by means of high-speed impact of plates under conditions of uniaxial deformation. From a microstructural analysis it is found that there is a clear relation between the type of failure (fracture, shear, rotational faulting or combinations thereof) and the velocity distribution of particles of the medium (so-called 'mesovolumes') in the wave of the load, measured in each impact loading event by laser interferometry. In particular, if the width of the distribution of mesovolumes with regard to velocities ΔV is commensurate with the mean velocity of particles, a discontinuous sheer spalling mechanism operates, whereas for a δ - shaped distribution, only shear is involved. In the intermediate case, as a rule, dynamic fracture is by the rotational mechanism.