A predictor for fatigue crack nucleation of single-network hydrogels considering water diffusion effect

Owing to their excellent biocompatibility and capability of large deformation, hydrogels have attracted extensive attention for promising applications. When subjected to cyclic loads, hydrogels are susceptible to fatigue. To ensure their durability, it is crucial to have a deeper understanding on the fatigue mechanisms of hydrogels. However, there is a lack of study in the literature for predicting the fatigue damage of hydrogels under the coupling of large deformation and water diffusion. This work aims to formulate a fatigue life predictor for characterizing the crack nucleation of single-network hydrogels and unveil the effects of water diffusion on fatigue. Borrowing the concept of fatigue crack nucleation for rubber-like materials, the fatigue life predictor is developed within the framework of configurational mechanics. With the proposed predictor, the contributions of stretching and mixing to the fatigue damage of hydrogels are identified. Case studies with different swelling conditions are conducted to further distinguish various effects, including chemical potential, loading rate, and cyclic stretching amplitude, on both stretching and mixing induced damage. It is concluded that the fatigue damage accumulation in hydrogels under cyclic loading is the competing result of stretching and water diffusion. As the proposed predictor is capable of predicting the spatial fatigue damage of hydrogels, the current research can provide guidance on designing loading profiles to improve the fatigue life of hydrogels. In addition, by incorporating self-healing mechanism and multiphysics coupling, the proposed modeling framework can be further expanded to investigate the fatigue of other hydrogels, like double-network and stimuli-sensitive hydrogels.