The Paris Law for Continuous and Discontinuous Fatigue Crack Growth in Polycarbonate
Keiji OGI and Tetsuro Shiraishi
Abstract:This paper presents the Paris law for discontinuous crack growth (DCG) and continuous crack growth (CCG) in polycarbonate under cyclic loading. First, the length and height of the craze zone near the crack tip were expressed on the basis of fracture mechanics. Next, the craze fibril breakdown model for CCG was applied to DCG and correlated with the Paris law. In CCG, the microscopic crack propagation rate da/dN equals to macroscopic one da/dN. In contrast, da/dN is not equal to da/dN in DCG because the macroscopic crack does not propagate until the craze fibrils break after some number of cycles. Therefore, a failure criterion for unstable crack growth in the craze zone was introduced. As a result, it was concluded that the crack propagation exponent for DCG is the same as that for CCG, while the coefficient in the Paris law for DCG is larger than that for CCG. Finally, the experiment data for DCG and CCG in polycarbonate were fitted to the Paris law to verify the reasonability of the above conclusions. In addition, the number of cycles required for DCG predicted using the model was compared with the experiment result. Key Words:Continuous crack growth, Discontinuous crack growth, Fatigue, Polycarbonate, Craze