Constitutive Model for Stress Softening Behaviour of Skin Tissue under Cyclic Loading

Skin tissue is a complex heterogeneous material abundant with fibers. Under the cyclic loadings, it is observed that the stress response of the skin tissue decreases with the loading cycles and finally stabilizes to a steady value. This phenomenon is known as the stress softening behavior. A new constitutive model is developed to relate the macroscopic response with the microstructural alteration to investigate the stress softening behavior of skin tissue. Two mechanisms are assumed to play roles in the deformation of the skin tissue, i. e., the microstructural alteration due to fiber uncrimping and the evolution of microscopic damage during the loading process. When the external force is small, the former mechanism is dominant, while when the external force is large, the two mechanisms both influence the evolution of the microstructures. A good agreement has been found between the theoretical results and the ex-vivo experimental data from pig skin under cyclic loading condition, indicating that the proposed constitutive model can capture the stress softening behavior observed during the cyclic loading condition.