Biologically-derived and chemically-treated collagenous tissues such as glutaraldehyde-treated bovine pericardium (GLBP) are widely used in many medical applications. The long-term cyclic loading-induced tissue fatigue damage has been identified as one of the primary factors limiting the durability of such medical devices and an in-depth understanding of the fatigue behaviors of biological tissues is critical to increase device durability. However, a limited number of fatigue damage experiments were performed on biological tissues due to complexity and time-consuming nature of such fatigue experiments. Consequently, accurate constitutive models for fatigue damage are also lacking. In this study, we performed a rigorous fatigue experiment on GLBP tissues. The stress, strain and permanent set at a maximum of 8 different fatigue cycles, up to 15 million cycles, were obtained, which demonstrated a nonlinear stress softening and a nonlinear permanent set accumulation. Based on the experimental data, we developed a novel residual stiffness-based fatigue model. The fatigue model considers the fatigue-induced reduction of initial stiffness and stiffening effect, in contrast to our previous damage-based model that only considered the fatigue-induced reduction of the initial stiffness. Moreover, a new constitutive relation was proposed to describe how the fatigue life (the cycle number at failure) depends on the equivalent strain, analogous to the stress versus fatigue life (S-N) curve for traditional engineering material. The new fatigue model can characterize the stress softening and nonlinear permanent set effects when referring to the pre-fatigued configuration. It can also describe the nonlinear stress stiffening effect when referring to the post-fatigued configuration. The model predictions are in good agreement with the experiment. The experimental results and the novel model could be applied to fatigue analyses of medical devices to improve the durability.

生物衍生和化学处理的胶原组织，例如戊二醛处理的牛心包（GLBP）被广泛用于许多医疗应用中。长期循环加载引起的组织疲劳损伤已被确认为是限制此类医疗器械耐用性的主要因素之一，深入了解生物组织的疲劳行为对于提高器械的耐用性至关重要。然而，由于这种疲劳实验的复杂性和耗时的性质，因此对生物组织进行了有限数量的疲劳破坏实验。因此，也缺乏用于疲劳损伤的准确本构模型。在这项研究中，我们对GLBP组织进行了严格的疲劳实验。应力，应变和永久变形最多可设置8个不同的疲劳周期，获得了多达1500万个循环，这表明了非线性应力软化和非线性永久变形的累积。基于实验数据，我们开发了一种基于残余刚度的新型疲劳模型。与我们之前基于损伤的模型（仅考虑疲劳导致的初始刚度降低）相比，疲劳模型考虑了疲劳导致的初始刚度和刚度降低。此外，提出了一种新的本构关系来描述疲劳寿命（失效时的循环次数）如何取决于等效应变，类似于传统工程材料的应力与疲劳寿命（SN）曲线。当参考预疲劳配置时，新的疲劳模型可以表征应力软化和非线性永久变形效应。当提及后疲劳配置时，它也可以描述非线性应力加强效果。模型预测与实验吻合良好。实验结果和新颖的模型可用于医疗器械的疲劳分析，以提高耐用性。