大鼠皮肤的循环力学性能研究

Cyclic Mechanical Behavior of Rat Skin

通过循环疲劳试验,研究了平均应力和应力幅值对大鼠皮肤力学性能的影响.结果表明:大鼠皮肤在拉-拉循环载荷作用下表现出明显的应变累积特性(即棘轮效应),棘轮应变曲线可划分为瞬态、稳态和加速失效3个阶段;棘轮应变及其应变率随平均应力或应力幅值的增加而增大,导致大鼠皮肤的损伤累积加速进而降低其疲劳寿命;循环应力-应变曲线中的滞后环面积、切线模量能够表征大鼠皮肤的循环特性,滞后环面积和切线模量呈现负相关的关联;对应循环疲劳失效的整个过程,滞后环面积经历减小、稳定、迅速增加3个过程;应力-应变滞后环面积随应力幅值的增加而增大,但平均应力对应力-应变滞后环面积的影响没有线性关系,在试验范围内当平均应力为12,MPa时滞后环的面积最小.结合Basquin模型和SWT模型,可以预测在不同平均应力与应力幅值作用下大鼠皮肤的低周循环疲劳寿命.通过石蜡切片技术和HE染色方法,研究拉伸和循环加载对皮肤微观结构的影响.结果表明,拉伸破坏和循环疲劳破坏都归因于纤维束的滑移和断裂,但两者发生断裂破坏的表现形式不同,前者表现为长裂纹扩展导致的拉伸断裂,后者则为短裂纹扩展导致的疲劳断裂.

The effects of mean stress and stress amplitude on the mechanical behavior of rat skin are analyzed by per- forming cyclic fatigue tests. It can be concluded that when the rat skin is subjected to cyclic loading in tension, the accumulation of plastic strain, which is called ratcheting, will take place. The ratcheting strain evolution can he di- vided into three stages： the transient ratcheting, the steady ratcheting and the accelerated failure stage. The ratchet- ing strain and its rate, increasing with the increase of mean stress or stress amplitude, will accelerate damage accu- mulation and eventually reduce the fatigue life of rat skin. In addition, the area of the stress-strain hysteresis loop and the tangent modulus can be used to elucidate the cyclic properties of rat skin, and the area of hysteresis loop is nega- tively correlated with the tangent modulus. Furthermore, the hysteresis loop＇s area experiences three processes, namely decrease, stabilization and prompt increase corresponding to the evolutionary process of cyclic fatigue fail- ure. The stress-strain hysteresis loop＇s area increases with the increase of stress amplitude, but there is no linear rela- tionship between mean stress and stress-strain hysteresis loop＇s area. Within the experimental scopes, the minimum value of the hysteresis loop＇s area is obtained at the mean stress of 12 MPa. Combined with the Basquin model and the SWT model, it is found that modified model can present a fairly good prediction on low cyclic fatigue life of rat skin under different mean stresses and stress amplitudes. Finally, the microstructures of rat skin under monotonic tension and cyclic tensile loading are observed based on paraffin section technique and HE staining. The results show that both monotonic tensile fracture and cyclic fatigue fracture of the rat skin are attributed to the slip or rupture of fiber bundles, but the destruction form in tension is different from that in fatigue： the former is caused by long crack propagation, while the latter arises from short crack propagation.