A stress-based model for fatigue life prediction of high density polyethylene under complicated loading conditions

Fatigue life prediction is crucial to the long-term durability evaluation of high density polyethylene (HDPE) which may experience stress-controlled fatigue or strain-controlled fatigue depending on the external loading conditions. Although abundant studies have been carried out on stress-controlled fatigue, investigations on strain-controlled fatigue are insufficient, and the relationship between these two types of fatigues is unclear. In addition, approaches for strain-controlled fatigue life prediction of HDPE are scarce. The present paper aims at revealing the relationship between strain-controlled fatigue and stress-controlled fatigue and proposing a method for fatigue life prediction. Firstly, the stress-controlled fatigue tests were performed to study the influence of stress ratio on fatigue life, and the strain-controlled fatigue tests were successfully conducted thanks to the application of anti-buckling devices. Furthermore, our results showed that the stress level (i.e. stress amplitude and stress ratio) determined the fatigue life for both stress-controlled test and strain-controlled test. More importantly, a stress-based numerical model was established to predict the fatigue life under complicated loading conditions. The numerical model was built based on the constitutive relation for the computation of the stress response, the Walker method for the evaluation of the stress ratio influence, and the damage accumulation for the calculation of the fatigue life. The proposed numerical method was validated by comparing the predicted fatigue lives with the experimental data of the strain-controlled tests.