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

节理和断层是岩石中公认的两种类型的裂缝。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，一个流行的裂缝发育模型是基于莫尔-库仑破坏准则的。该模型认为岩石在拉伸状态下的破坏准则是连续的和抛物线的，描述了节理和断层的形成，并预测在压应力和拉应力的混合状态下形成混合型裂缝(即具有节理和断层中间特征的裂缝)。然而，由于在混合应力状态下进行的实验很少，所以关于节理到断层过渡的破坏模式和力学行为仍然存在许多问题。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了断裂类型连续体中的末端成员。在德克萨斯农工大学岩石变形实验室，利用在围压下拉伸的狗骨形状的卡拉拉大理岩样品进行的初步实验为混合裂缝的存在提供了强有力的证据。断裂样品在宏观断裂方向和表面形态上表现出从经典节理到库仑断层的连续过渡。然而，断裂强度似乎不符合抛物线Mohr-Coulomb破坏准则。通过实验岩石变形和拉压混合应力状态下脆性破坏的岩组研究，研究了卡拉拉大理岩和贝里亚砂岩中节理到断层的转变。这些实验被用来确定从节理到断层的故障包络的确切形式。声发射被监测，以确定导致宏观破坏的微裂缝事件的相对时间和数量，而顺序应变实验为裂缝发育的不同阶段的岩组分析提供样本。用光学和扫描电子显微镜对岩石组构进行了详细的分析，并用表面轮廓术对宏观断口的形貌进行了表征，确定了微观结构和变形机制。晶内断裂、晶界断裂、解理和孪晶沿宏观断裂的相对贡献正在被量化。该项目的具体目标包括测试用于混合骨折形成的阶梯裂纹模型，开发与实验观察相一致的裂缝发展的微观力学模型，以及推导出用于混合骨折的失效准则。

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.