Fracture response of polyimide and polysulfone under hydrostatic pressure

Fracture response of polyimide and polysulfone in tension under superimposed hydrostatic pressures up to 100,000 psi is presented. The influence of hydrostatic pressure and the pressure medium on the specific surface energy γ is determined by utilizing the Griffith theory of brittle fracture. For polyimide as well as for polysulfone, γ is found to increase with increasing pressure. Furthermore, for polysulfone at any given pressure level, the value of γ is found to be lower in heptane medium than in kerosene. Heptane is known to be a stress-cracking agent for polysulfone. Fracture surface of the tested tensile specimens is examined by using a scanning electron microscope. The observed fracture features are correlated with the macroscopic deformation behavior and also with the effects of the pressure medium used. In polyimide, the region of crack initiation narrows down from a very broad region at atmospheric pressure to almost a point source along the outer periphery of the specimen at 100,000 psi. In addition, polyimide undergoes a transition in the nature of fracture response between 80,000 to 100,000 psi, and this is clearly indicated in the scanning electron micrographs. In polysulfone, the crack propagation appears to be faster when heptane is used as the pressure medium than when kerosene is used. The penetration of the medium into the specimens can be observed on the micrographs. Several scanning electron micrographs of the fracture surface suggest the possibility of a significant temperature rise in the specimen during fracture. This increase in the specimen temperature is roughly estimated from the stored elastic energy released upon fracture.