Micromechanics analysis of crazing phenomenon in polymers

The use of polymer glasses to make structural components has increased significantly over the last two decade but is still limited by the tendency of these materials to fail in a macroscopically brittle manner, i.e., with no large scale plastic deformation before cracking. The occurrence of fracture is a major concern and this is reflected in the large number of tests which are used to simulate the various conditions which promote failure. The brittleness of polymers can be traced to the formation under tensile stress of small crack-like defects called crazes. A craze is not a crack, but is localized yielded region, consisting of interpenetrating polymer fibrils and voids. The fibrils are made of primary and secondary or cross-tie fibrils connecting the opposite faces of the craze so that loads can be transmitted across the faces through the fibrils. Crazing is a precursor to brittle fracture. So far a lot of experiments have been done to study the microstructures of crazed zones and to predict the propagation of crazes, but most of the work is limited in two-dimension. In this paper three-dimensional stress analysis for crazed polymers under tensile stress has been carried out with the aid of micromechanics theory to investigate the mechanical properties of crazed polymers. The craze zone is modelled as a transversely isotropic spheroidal inhomogeneity whose elastic moduli along the stress applied direction are much larger than the other two directions. The energy absorbed by crazes and the effective elastic moduli of crazed polymers are evaluated since those values are most concerned in engineering structures made by polymer materials. © 1995.