Constant ramp strength tests on unidirectional thermoplastic composite specimens oriented in the 90° direction were conducted at constant temperatures ranging from 149°C to 232°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°C, time-temperature superposition held allowing strength at longer times to be estimated from strength tests at shorter times but higher temperatures. The data indicated that a 50% drop in strength might be expected for this material when the test time is increased by 9 orders of magnitude. The shift factors derived from compliance data applied well to the strength results.
To explain the link between compliance and strength, a viscoelastic fracture model was investigated. The model, which used compliance as input, was found to fit the strength data only if the critical fracture energy was allowed to vary with stress rate. This variation in the critical parameter severely limits its use in developing a robust time-dependent strength model. The significance of this research is therefore seen as providing both the indication that a more versatile acceleration method for strength can be developed and the evidence that such a method is needed.