Damage behavior in multi‐stage fatigue testing of glass‐fiber‐reinforced polyamide 66

Abstract With plastics being increasingly applied to moving parts today, predictions of continuous fatigue life under identical conditions (ambient temperature, frequency and load stress) are not sufficient. Rather, there is an urgent need to develop a method of predicting fatigue life under multi‐stage loading involving intermittently changing conditions. In this study, by applying a temperature‐ and frequency‐superposition method to fatigue life prediction of plastics, it was found that an exponential type of cumulative damage law could be applied to a two‐stage fatigue process. Specifically, when fatigue conditions (interval time, ambient temperature, frequency and load stress) are changed between several stages, the resulting damage behavior follows an exponential law representing the ratio of the number of cycles in each stage to the continuous fatigue life under those conditions. It is shown here that the exponent m has a single value and consists of the product of the independent functions of the factors of the fatigue conditions. It is also shown that the use of this exponential law reproduces very well the damage behavior in a two‐stage fatigue test in which each fatigue condition is changed and in three‐ and four‐stage fatigue tests in which the interval time is varied.