循环加载作用下皮肤应力软化的连续介质模型

皮肤组织作为富含纤维的非均匀材料，具有复杂的力学特性．皮肤组织在循环加载作用下，随着循环次数的增加，加载过程的应力响应逐渐降低，并最终达到不随循环次数增加而改变的稳定状态，这种现象被称为应力软化行为．本文对加载过程中纤维的延展机制对宏观力学响应的影响进行研究，认为在外界载荷较小时该机制主导了宏观层次上的应力软化行为，随着外界载荷的增大，拉伸过程中微观结构损伤的演化开始产生影响，而且此时内部微观结构的演化由两种机制共同影响，据此建立了连续介质模型，将宏观尺度上应力软化行为和微观结构的演化相关联．将所获得的应力响应理论结果与猪离体头部皮肤在循环加载作用下的实验结果进行对比分析，证明了该模型能够合理地描述皮肤组织在循环加载作用下的应力软化行为．

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.