A Four-Parameter Fatigue Life Prediction Model for Flexible Polymers: Mechanism and Threshold-Driven Behavior

Strain fatigue critically impacts polymer applications such as bionic skin and wearable devices. However, existing prediction models derived from metallic materials fail to account for the viscoelastic nature of polymers. In this paper, we propose a four-parameter strain-fatigue life prediction model incorporating a fatigue threshold (epsilon th) to address the limitations of traditional linear damage accumulation frameworks in capturing the viscoelastic behavior of flexible polymers. Experimental validation across five thermoplastic polymers demonstrates the superior accuracy of the four-parameter model in high-cycle fatigue prediction compared to the Manson-Coffin model. For instance, the proposed model achieved R2 values of 0.98, 0.93, 0.89, and 0.93 in PA6, PC, PE-LLD, and PE-LLD, respectively, which were significantly better than the Manson-Coffin model (R2 = 0.95, 0.88, 0.78, and 0.84). The fatigue threshold (epsilon th) has been proven to effectively quantify the critical strain limit of irreversible damage accumulation, which is 1.96%, 5.55%, 8.14%, and 9.53% in PA6, PC, PE-LLD, and PE-LLD. This work reveals that plastic deformation below epsilon th does not lead to fatigue accumulation, challenging traditional damage accumulation paradigms, and provides a robust framework for predicting the strain fatigue life of flexible polymers, with significant implications for material design and durability assessment.