A damage model of characterizing the internal damage evolution for DN hydrogel using Multi-Particle Tracking (MPT) method

Under large deformation, Double-network (DN) hydrogels show necking phenomenon with the damage propagating inside the polymer network. Although the internal damage mechanism of DN hydrogel has been extensively studied, there is still a lack of the quantitative description for the internal damage evolution of DN hydrogel. In this work, an optical full-field deformation measurement, named as multi-particle tracking (MPT) method, is developed to measure the inhomogeneous deformation field of DN hydrogel. According to observation of the damage accumulation and propagation of DN hydrogel using MPT method, we propose a damage model to describe the entire damage process of DN hydrogel. In the proposed damage model, damage propagation is characterized by phase transition, and stress softening due to damage accumulation is described by a damage constitutive model. The proposed damage model can predict the overall stress-stretch relationship of DN hydrogel in the monotonic loading and cyclic loading tests. Furthermore, we predict size of the fragments of the first network, and explain the mechanism behind the high toughness of DN hydrogel. This study characterizes the internal damage evolution and proves a quantitative description for the inhomogeneous deformation of DN hydrogel.