+ & ? The Wear - out Approach for Predicting the c Remaining Lifetime

Failure models based on the Palmgren-Miner concept that material darnage is cumulative have been derived and used mainly for fatigue life predictions for metals and composite materials. Wexeview the principles underlying such models and suggest ways in which they may be best applied to polymeric materials in temperature environments. We first outline expectations when polymer degradation data can be rigorously time-temperature superposed over a given temperature range. For a step change in temperature after darnage has occurred at an initial temperature in this range, we show that the remaining lifetime at the second temperature should be linearly related to the aging time prior to the step. This predicted linearity implies that it should be possible to estimate the remaining and therefore the service lifetime of polymers by completing the aging at an accelerated temperature. We refer to this generic temperature-step method as the “Wear-out” approach. We next outline the expectations for Wear-out experiments when timetemperature superposition is invalid. Experimental Wear-out results are then analyzed for one material where time-temperature superposition is valid and for another where evidence suggests it is invalid. In analyzing the data, we introduce a procedure that we refer to as time-degradation superposition. This procedure not only utilizes all of the experimental data instead of a single point from each data set, but also allows us to determine the importance of any “interaction effects”.