The load-displacement softening response of quasi-brittle solids exhibits an unstable structural behavior, which is characterised by a negative slope in the post-peak regime. In severely brittle situations, the post-peak behaviour can show a virtual positive slope, the fracture propagation occurring unexpectedly with a catastrophic loss in the load-carrying capacity. In this case, if the displacement controls the loading process, the curve exhibits a discontinuity and the representative point drops to the lower branch with a negative slope. On the other hand, in order to obtain a stable crack growth, a decrease both in load and in displacement is required. In the last forty years, in-depth study of the so-called snap-back instability was conducted in relation to crack propagation phenomena in quasi-brittle materials. In the present work, the structural response of two brittle-matrix specimens is analysed: the first contains a distribution of collinear micro-cracks, whereas the second presents multiple parallel reinforcing fibres embedded in the matrix. In both cases, it is shown that the structural response presents a discrete number of snap-back instabilities with related peaks and valleys, the crack propagation occurring alternately within the matrix and through the heterogeneities. Thus, the strong analogy between weakened and strengthened zones consists in a multiple snap-back mechanical response, where descending branches of propagating cracks alternate with ascending (linear) branches of arrested cracks.

中文翻译：

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复合材料中的微损伤不稳定机制：开裂聚结与纤维延展性和滑移


准脆性固体的载荷-位移软化响应表现出不稳定的结构行为，其特征是在峰后状态中呈负斜率。在极度脆性情况下，峰后行为可能显示虚拟的正斜率，裂缝扩展意外发生，导致承载能力的灾难性损失。在这种情况下，如果位移控制加载过程，则曲线将呈现不连续性，并且代表点以负斜率下降到较低的分支。另一方面，为了获得稳定的裂纹扩展，需要同时降低载荷和位移。在过去的 40 年里，对所谓的回弹不稳定性与准脆性材料中的裂纹扩展现象进行了深入研究。在本工作中，分析了两个脆性基体试样的结构响应：第一个包含共线微裂纹的分布，而第二个则呈现嵌入基体中的多个平行增强纤维。在这两种情况下，都表明结构响应呈现离散数量的回弹不稳定性以及相关的峰和谷，裂纹扩展在基体内和通过非均质替发生。因此，弱化区和加强区之间的强烈类比在于多个回弹机械响应，其中扩展裂纹的下降分支与停止裂纹的上升（线性）分支交替出现。