Studies of fatigue fracture for rigid plastics have indicated that fracture takes place by two mode. The first fails by softening accompanied with generation of heat in high stress level, while the second does by crack propagation in low stress level.
The purpose of this paper was to investigate the fatigue crack propagation properties and discuss a fatigue crack growth equation in low stress level for rigid plastics. Flexural fatigue tests of the specimens with a small blind hole of 0.3mm in diameter were carried out under a constant stress amplitude at room temperature. The materials used in the test were polycarbonate (PC), polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA). The main results obtained are as follows:
(1) In the range that the crack length l on the surface of specimen was small, the rate of crack propagation dl/dN was nearly proportional to σnl. The value of n was 3 for PC, PVC and PMMA.
(2) In the case that the crack lengths l were equal, the forms of crack surface were similar each other even though the specimens were subjected to different stress levels.
(3) The relation between the crack length l and the relative number of cycles (N-Nc)/(Nf-Nc) was independent of stress levels, where Nf is the number of cycles to failure, and Nc is that at crack length l of 0.4mm.
