A modelling for craze behavior covering a wide range of strain rate and its application to simulation for fracture prediction of crystalline polymer

Polymers show peculiar mechanical responses that are not observed in metals, such as remarkable strain rate dependency and ductile fracture caused by craze which is an assembly consisting of micro-voids and fibrils. In the design process for polymeric products, we attempt generally to predict the fracture position by commercial FEM solvers. However, we can not precisely reproduce the fracture behavior of polymers, because a material model that can express an accumulation of craze is not installed yet on the current commercial solvers. Therefore, so as to predict fracture on the basis of craze behavior, we proposed a constitutive equation with craze effect, the craze evolution equation that can express propagation and growth cessation of craze, the evolution of mean normal plastic strain and criterion for craze initiation with strain rate dependency in our previous work. In this study, our craze evolution equation is extended to an enhanced type covering wide range of strain rate and is proposed as a material model by combining with non-coaxial elastoviscoplastic constitutive equation and the evolution equations proposed in the previous work. Then, numerical uni-axial tensile tests in which applied strain rate is given at five levels between 0.01s and 100s are conducted on a commercial FEM solver LS-DYNA to which the present material model is added via user subroutine. Furthermore, we predict computationally the fracture positions under the condition of wide range of strain rates by using the criteria of craze concentration and fibril strength.