Craze growth in polymethylmethacrylate: a fracture mechanics approach

Abstract Test specimens of polymethylm ethacrylate (PMMA) have been subjected to stress in the presence of methanol and the resulting craze growth behaviour has been investigated by means of the concepts of fracture mechanics. Sharp-notched tension specimens have been tested over a wide range of constant applied loads to produce craze initiation and propagation. In this way it has been found that, for tension tests under constant load, both the initiation and growth characteristics are controlled by the initial stress intensity factor, K0, and not by the applied stress p, as previously thought. Two distinct growth patterns have been observed, one type leading to eventual craze arrest and the other to final fracture. In the former, the craze growth rate decreased, whereas in the latter case the growth initially decreased but then eventually continued at a constant rate. Both types of growth are functions of the initial stress intensity factor. A model for craze formation and growth has been proposed based on a crack opening displacement approach in conjunction with a simple flow analysis to describe the movement of the liquid environment within the craze. The model provides equations of motion for a craze and these have successfully been used to explain the observed behaviour. The size of the void spacing within a craze has been calculated to be 2.48 x 10-4 mm, this being in close agreement with the result obtained by Zhurkov, Kuksenko & Slutsker (1969) using direct methods. A stress equilibrium equation has enabled a craze yield stress pc to be derived as 9.06 N mm-2 for tests at 20 °C.