反平面剪切(III型)加载下脆性岩石的断口分析

FRACTURE MORPHOLOGY ANALYSIS OF BRITTLE ROCK UNDER ANTI-PLANE SHEAR(MODE III) LOADING

采用反平面冲剪和反平面剪切盒实验研究反平面剪切(III型)加载下脆性岩石断裂特征。反平面冲剪试样的断裂轨迹是一空间螺旋面,断口微观特征表现为沿晶断裂,晶体大部分有3个陡峭的台阶面,晶面出现较多的河流状和鱼骨状张拉条纹,试件破坏以张拉断裂(I型)为主。反平面压剪试样的断裂轨迹基本上沿原裂纹面,断口微观特征表现为穿晶断裂,晶面上多而密的平行线条纹和较多的岩屑表现出强烈的剪切破坏特征,试件破坏以剪切断裂(III型)为主。增加侧压可抑制裂纹尖端的拉应力,导致岩石产生沿原裂纹面扩展的反平面剪切(III型)断裂。反平面压剪实验是实现岩石III型断裂和测定岩石III型断裂韧度KIIIC的一种有效实验方法。

An anti-plane punch-through shear test and an anti-plane compression-shear box test are applied to the study of fracture mechanism of brittle rock under the anti-plane shear（mode III）loading. For the anti-plane punch-through shear specimen, the fracture trajectories is a helicoid and the microscopic surface is formed by intergranular fracture. The most crystals have three steep step surfaces with many fiver-typed patterns and fish- born patterns, which represent the tensile fracture（mode I）. For the anti-plane compression-shear specimen, the crack appears and propagates almost along its original plane. The microscopic characteristic is of transgranular fracture. The crystal surface displays many dense parallel patterns; and rock fragments indicate the shear fracture（mode Ⅲ）. Finite element method is used to investigate the fracture mechanism under anti-plane shear loading. Numerical results show that both tensile and shear stresses exist at the crack tip. The addition of compressive stress on the original crack plane can depress the tensile stress at the crack tip and facilitate the crack to initiate and propagate along its original plane; i.e. anti-plane shear mode III fracture. The ratio of maximum shear stress to maximum tensile stress τmax/σ1 is 5 - 10. Since too large or too low inclination angle α can result in small compressive stresses or local crush; the suggested inclination angle α is 55°- 70°. The anti-plane compression-shear box test is a potential method for realizing fracture mode III and determining its toughness KⅢc of brittle rock.