MECHANICAL-BEHAVIOR OF POLY(METHYL METHACRYLATE) .2. THE TEMPERATURE AND FREQUENCY-EFFECTS ON THE FATIGUE CRACK-PROPAGATION BEHAVIOR

The effect of temperature and cyclic frequency on fatigue crack propagation (FCP) rates were investigated for poly methylmethacrylate (PMMA). Test temperature ranged from -30 to 100° C which includes the β transition and approaches Tg. FCP experiments were conducted at frequencies of 1, 10, 50 and 100 Hz. In general, the FCP rates increased with increasing temperature and decreasing cyclic frequency. The crack growth rate is near a maximum at 80° C and 10 Hz. When the experimental conditions approach the glass transition region, e.g. studies at 100° C, the failure mechanism changes and the material becomes more fatigue resistant while simultaneously softening. The frequency dependence of da/dN provided by the Michel-Manson (M-M) model is shown to be valid up to about 80° C, in absence of extensive plastic deformation. FCP results exhibit an interaction between the thermal and mechanical driving forces which is not consistent with the M-M model. Plots of FCP rate were made against the reciprocal of temperature. The apparent activation energies calculated from these plots varied from 13 to 44 kJ mol-1, and show that the activation energy increases as Δ/K increases and as cyclic frequency decreases. © 1990 Chapman and Hall Ltd.