剪切盒加载下岩石剪切(Ⅱ型)断裂韧度K_(ⅡC)的测定

Determination of shear (Mode Ⅱ) fracture toughness K_(ⅡC) of rock using shear-box test

采用剪切盒实验获得了岩石的Ⅱ型断裂并测定了岩石的Ⅱ型断裂韧度KⅡC ,通过有限单元法中的位移法推导出剪切盒加载下双切口试样的Ⅱ型应力强度因子KⅡ 的一般计算公式 ,并探讨了KⅡC的尺寸效应和Ⅱ型断裂机理 .实验及数值计算结果表明 :在剪切盒加载下 ,裂纹尖端最大拉应力始终低于岩石的拉伸强度 ,最大剪应力大于其对应压应力下的剪切强度 ,且Ⅱ型最大应力强度因子KⅡmax为Ⅰ型最大应力强度因子KⅠmax的 2～ 4倍 ,从而导致产生Ⅱ型断裂 ;测得的KⅡC值随无量纲切口长度 (2a W)的增加而降低 ,当 2a W≥ 0 .7,且B≥W ,α为 6 5°～ 75°时 ,KⅡC趋近于一个常数 ,该常数为Ⅰ型断裂韧度KⅠC的 2～ 3倍 ,可认为是较合理的岩石Ⅱ型断裂韧度值 ;剪切盒实验是一种测定岩石KⅡC 的行之有效的方法 .

Shear box test was used to conduct Mode Ⅱ fracture of rock and to determine Mode Ⅱ fracture toughness K ⅡC . Under shear box loading the formula for calculating the shear stress intensity factor K Ⅱ of double notched specimen was obtained by finite element method through displacement approach. The effects of the dimensionless notch length (2 a/W ) on K ⅡC and Mode Ⅱ fracture mechanism were studied. Test and numerical results show that under shear box loading the major principal stress σ 1 at the notch tip is smaller than the tensile strength σ t and the maximum shear stress τ max is larger than the shear strength σ c in the presence of the compressive stress. Furthermore, the maximum Mode Ⅱ stress intensity factor K Ⅱmax is 2 4 times as large as the maximum Mode Ⅰ stress intensity factor K Ⅰmax . Therefore Mode Ⅱ fracture occurs. The measured value of K ⅡC decreases with the dimensionless notch length (2 a/W ) and becomes a constant when 2 a/W ≥0.7 and B≥W in the range of α =65°～75°. K ⅡC is 2～3 times K ⅠC and can be regarded as a reliable and reasonable value of Mode Ⅱ fracture toughness of rock. Shear box test is a potential method for determining K ⅡC of rock.