Accelerated strength testing of thermoplastic composites

Constant ramp strength tests on unidirectional thermoplastic composite spec imens oriented in the 90 degrees direction were conducted at constant tempe ratures ranging from 149 degrees C to 232 degrees C. Ramp rates spanning 5 orders of magnitude were tested so that failures occurred in the range from 0:5 s to 24 h (0.5 to 100 000 MPa/sec). Below 204 degrees C, time-temperat ure superposition held allowing strength at longer times to be estimated fr om strength tests at shorter times but higher temperatures. The data indica ted that a 50% drop in strength might be expected for this material when th e test time is increased by 9 orders of magnitude. The Shift factors derive d from compliance data applied well to the strength results. To explain the link between compliance and strength, a viscoelastic fractur e model was investigated. The model, which used compliance as input, was fo und to fit the strength data only if the critical fracture energy was allow ed to vary with stress rate. This variation in the critical parameter sever ely limits its use in developing a robust time-dependent strength model. Th e significance of this research is therefore seen as providing both the ind ication that a more versatile acceleration method for strength can be devel oped and the evidence that such a method is needed.