Crack interactions leading to shear localization were quantified using microstructural analysis for brittle faults and high-temperature ductile faults formed during experiments on quartz sandstone. In both faulting regimes, the nucleation of macroscopic faults results from the interactions of microfractures at two length scales in ensemble. Brittle faults nucleate when the longest mesoscale shear fractures and transgranular tensile cracks critically interact. In contrast, ductile faults nucleate when the longest mesoscale shear fractures and multi-grain scale intergranular shear cracks critically interact. For both faulting regimes, we conclude the interaction and coalescence of the longest mesoscale shear fractures is the fundamental process responsible for fault nucleation. Hence, mesoscale shear fractures, which accommodate the majority of axial strain prior to shear localization in both faulting regimes, also serve as the nucleus of macroscopic faults. Locally, the growth of the mesoscale shear fractures is promoted by the interaction and coalescence of the multi-grain scale cracks in both faulting regimes. We hypothesize that attainment of a critical microstructure for shear localization (i.e., local clustering of the longest microfractures) requires a characteristic amount of plastic axial strain, which depends on deformation conditions. In brittle faulting, distributed microfracturing is confined within limited regions of the rock volume, which expedites crack clustering and fault nucleation at low characteristic strains. In ductile faulting, distributed microfracturing occurs more uniformly throughout the rock volume, delaying shear localization to high characteristic strains. Accurate prediction of shear localization requires models that describe crack interactions of the largest flaws that account for crack clustering.

中文翻译：

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脆性场和延性场变形多孔粒状岩石中裂纹相互作用对剪切局部化的作用

通过对石英砂岩实验中形成的脆性断层和高温韧性断层的微观结构分析，对导致剪切局部化的裂纹相互作用进行了量化。在这两种断层状态下，宏观断层的成核是由集合中两个长度尺度上微裂缝相互作用的结果。当最长的介观剪切断裂和穿晶拉伸裂纹严重相互作用时，脆性断层就会成核。相反，当最长的中尺度剪切断裂和多晶尺度晶间剪切裂纹关键相互作用时，延性断层就会成核。对于这两种断层状态，我们得出结论，最长中尺度剪切裂缝的相互作用和合并是断层成核的基本过程。因此，中尺度剪切裂缝在两种断层区域的剪切定位之前容纳了大部分轴向应变，也充当了宏观断层的核心。就局部而言，两种断层区域中多粒尺度裂缝的相互作用和合并促进了中尺度剪切裂缝的生长。我们假设，获得剪切局部化的关键微观结构（即，最长微裂纹的局部聚集）需要特征量的塑性轴向应变，这取决于变形条件。在脆性断层作用中，分布式微破裂被限制在岩石体积的有限区域内，这加速了低特征应变下裂纹的聚集和断层成核。在延性断层中，分布的微裂缝在整个岩石体积中更均匀地发生，从而延迟了剪切局部化到高特征应变。剪切局部化的准确预测需要模型来描述最大缺陷的裂纹相互作用，从而解释裂纹聚集。