NEW METHOD OF LIFETIME PREDICTION FOR BRITTLE-FRACTURE OF POLYETHYLENE

A new method for predicting the time to brittle failure of polyethylenes is proposed. The method includes modeling slow crack growth in polyethylenes and the experimental determination of material parameters for the model. The model is based on the concept of the crack layer, i.e., a system consisting of the strongly interacting crack and process zone and the kinetic equations which govern the crack layer growth. The process zone in poly ethylenes usually appears to be a thin strip of drawn material extending along the crack line. This permits a characterization of the crack layer by two parameters: the crack and process zone lengths. The two-parameter crack layer kinetic model allows description of slow crack growth as the discontinuous (stepwise) process which is commonly observed in the brittle fracture of polyethylenes. The model also predicts a relationship between time to failure and applied stress, identical to that established experimentally. The material parameters of the kinetic model can be determined by experiments on smooth specimens, i.e., are independent of slow crack growth and require relatively short-term observations. Thus, the combination of the material testing and the mathematical modeling of the crack layer evolution is proposed as a method for lifetime prediction in the brittle fracture of polyethylenes. (C) 1995 John Wiley & Sons, Inc.