Experimental and Petrofabric Study of Hybrid Fractures and the Transition From Joints to Faults

节理和断层是岩石中常见的两种断裂类型。在许多大学和研究生水平的构造地质学和构造学教科书中提出的裂缝发展的流行模型是基于莫尔-库仑破坏准则。该模型表明，岩石的破坏标准在拉伸状态下是连续的和抛物线的，描述了节理和断层的形成，并预测混合裂缝（即，具有介于节理和断层之间的特征的裂缝）在混合的压应力和张应力状态下形成。然而，许多问题仍然存在关于跨关节故障过渡的故障模式和力学行为，因为很少有实验已经进行了混合应力状态下。因此，关于岩石断裂的一个基本问题是，节理和断层是否代表断裂类型连续体中的端元。初步实验使用狗骨形样品的卡拉拉大理石在围压下扩展在岩石变形实验室在得克萨斯州A M大学提供了强有力的证据混合裂缝的存在。破裂的样品似乎显示出连续过渡的宏观断裂方向和表面形态从经典的关节库仑故障。断裂强度，但是，似乎不符合抛物线莫尔-库仑破坏criterion.The联合断层过渡卡拉拉大理石和Berea砂岩正在调查通过实验岩石变形和岩石组构研究脆性破坏在混合拉伸和压缩应力状态。实验正被用来确定从节理到断层的破坏包络线的确切形式。声发射监测，以确定导致宏观故障的微破裂事件的相对时间和数量，和连续的应变实验提供样品的断裂发展的不同阶段的岩组分析。详细的岩石组构分析，使用光学和扫描电子显微镜，表面轮廓术被用来表征宏观断裂表面的形态，并确定微观结构和变形机制。沿着沿着宏观断裂的晶内断裂、晶界断裂、解理和孪晶的相对贡献被量化。该项目的具体目标包括测试混合裂缝形成的阶梯裂纹模型，开发与实验观察一致的裂缝发展的微观力学模型，并推导出混合裂缝的破坏准则。

Joints and faults are two types of fractures generally recognized in rock. A popular model for fracture development presented in many college and graduate level textbooks on structural geology and tectonics is based on the Mohr-Coulomb failure criterion. The model suggests that the failure criterion for rock is continuous and parabolic in the tensile regime, describes the formation of both joints and faults, and predicts that hybrid fractures (i.e., fractures with characteristics intermediate to joints and faults) form under mixed compressive and tensile stress states. However, many questions remain concerning the mode of failure and mechanical behavior across the joint to fault transition because few experiments have been conducted under mixed stress states. As such, a fundamental question regarding fracture of rock is whether joints and faults represent end-members in a continuum of fracture types. Preliminary experiments using dog-bone shaped samples of Carrara marble extended under confining pressure in the rock deformation laboratory at Texas A&M University provide strong evidence for the existence of hybrid fractures. Fractured samples appear to display a continuous transition in macroscopic fracture orientation and surface morphology from classic joints to Coulomb faults. Fracture strength, however, does not appear consistent with a parabolic Mohr-Coulomb failure criterion.The joint to fault transition in Carrara marble and Berea sandstone is being investigated through experimental rock deformation and petrofabric study of brittle failure under mixed tensile and compressive stress states. The experiments are being used to determine the exact form of the failure envelope from jointing to faulting. Acoustic emissions are monitored to determine relative timing and number of microfracture events leading up to macroscopic failure, and sequential strain experiments provide samples for petrofabric analysis of different stages of fracture development. Detailed petrofabric analysis using optical and scanning electron microscopy, and surface profilometry are used to characterize the morphology of the macroscopic fracture surface and determine the microscopic structure and mechanisms of deformation. The relative contribution of intragranular fracture, grain boundary fracture, cleavage, and twinning along the macroscopic fracture is being quantified. Specific goals of the project include testing the stepped-crack model for formation of hybrid fractures, developing a micromechanical model of fracture development consistent with experiment observations, and deriving a failure criterion for hybrid fracture.