Role of Mesostructure Effects in Dynamic Plasticity and Strength of Ductile Steels

Dynamic deformation and fracture of solids are known to be the multiscale processes. Mesolevel-1 relates to deformation modes inside the structure elements while mesolevel-2 is self-consistent motions of conglomerates of the elements. Shock experiments and molecular dynamics simulation show a presence of local strain rate fluctuations (dispersion) which sensitively depend on the non-equilibrium of deformation process. Velocity dispersion at the mesolevel-1 (D2 m1) characterizes a relaxation of local stresses while the dispersion at the mesolevel-2 (D2 m2) is responsible for fragmentation of material. When Dm1 = Dm2 fragmentation is suppressed and material reveals maximum strength. At the unsteady stages of straining the particle velocity acquires defect of velocity which characterizes an intensity of energy exchange between scale levels. In the present paper the energy exchange is considered from the position of spallation. For five kinds of steel it is shown that spall-strength is maximum when the velocity dispersion at the mesolevel equals defect of the average velocity. Criterion for spallation takes into account the energy exchange between scale levels.