Abstract: An elastic viscoplastic constitutive mode is given in this paper. Based on it, the numerical computation of lnR/r singular in the mode II dynamic crack’s tip field is carried out. The relationships between stress field distribution and viscosity coefficients and mach numbers are given. The problem existed in ln(R/r) singular zone is given. The cause of the transitional zone is given. The form of stress field in the transitional zone and it’s solving method are guessed rationally.

Key words: elastic-viscoplastic materials, viscosity coefficient, Mach number, dynamic crowing, simulation analysis