页岩Ⅰ型断裂韧性测试及跨尺度裂缝表征研究

Study on Shale Fracture Toughness and Micro-characterization of Mode Ⅰ Crack Using DCB Specimen and SEM Method

断裂韧性作为表征岩石断裂特性的一项重要参数,在岩石裂缝的起裂和扩展中起着至关重要的作用。在近几十年来,前人开展了大量的断裂韧性测试工作。此项研究基于双悬臂梁断裂韧性测试方法,对页岩的本征尖锐裂缝的断裂韧性进行测试,并通过环境扫描电子显微镜进行裂缝的跨尺度表征。结论表明5 mm厚度的样品足以保证测试的准确性和可重复性。同时,线弹性断裂力学中对过程区尺度的预测在实验中得到了很好的验证。裂缝的迂曲度的分形特征表明裂缝在不同尺度下有着不同的裂缝长度。在微观尺度下,裂缝以微裂纹连接的方式进行扩展。整个裂缝扩展过程中,缝尖附近的微裂纹始终处在相互竞争的状态之中,竞争成功的微裂纹最终成变为主裂纹,竞争失败的微裂纹在主裂纹继续扩展之后会因为应力释放而闭合。此项研究对岩石裂缝的测量和表征有着指导性意义。

Fracture toughness is an important property of rock,especially in the cases regarding fracture initiation and propagation.For the past forty years,enormous work has been done on rock fracture toughness testing,some were included in the suggested methods of International Society for Rock Mechanics(ISRM).In this study,shale rock fracture toughness testing was performed using Double-Cantilever-Beam(DCB)method.Fractures with highly repeatable toughness were induced with intrinsically sharp crack tip vanishing within the specimen.Microscopic features of shale crack were characterized using Scanning Electron Microscope(SEM).Conclusions show that thickness of 5 mm is an ideal value for testing repeatability of shale fracture toughness.The study also shows that Linear Elastic Fracture Mechanics(LEFM)literally gives good prediction on the size of process zone even in heterogeneous geo-materials with abundant defects.In micro scale,modeⅠcracks of shale has different crack length when measured at different scale.The crack also shows high tortuosity and continuously identifiable process zone in ways of crack discontinuities and branching.The crack tip is never an explicit one as micro cracks around the crack tip are in constant competition become the main crack when.The study gives a better understanding of shale fracturing process and its microscopic properties.