用圆孔平台巴西圆盘确定岩石拉伸强度的非局部应力方法

Nonlocal Stress Method for Determination of Rock Tensile Strength Holed-Flattened Brazilian Disc Specimen

对用圆孔平台巴西圆盘确定岩石拉伸强度的方法进行了研究。用有限元法分析了试件加载直径上的双向应力分布。由于加载直径靠近孔边处存在较大的拉伸应力梯度,如果以加载直径与中心圆孔相交点的最大拉伸应力来确定岩石的拉伸强度σt,拉伸强度的试验值会随着中心圆孔与圆盘半径之比r/R的增加而减小。采用非局部应力方法,考虑双向应力状态作用下发生破坏的Griffith强度准则的等效应力σG,在加载直径上选取材料特征尺寸,让σG"在特征尺寸上积分后取均值后得到岩石的拉伸强度σt。利用该方法得到的圆孔平台巴西圆盘测得的σt值不随r/R的变化而变化,其平均值大约为平台巴西圆盘测试值的1.23倍左右。

A method for the determination of the rock tensile strength with the holed-flattened Brazilian disc specimen is proposed. The biaxial stress distribution in the loading diameter of the specimen is analyzed with finite element method. As large tensile stress gradient exists in the loading diameter near the centre hole, if the tensile strength σt is determined from the maximum tensile stress at the intersecting point of the circular hole and the loading diameter, the obtained value decreases with the increment of the radius ratio r/R, where r is the radius of the centre hole, and R is the radius of the disc. Based on the Griffith strength criteria considering the equivalent stress σG for the biaxial stress condition, the tensile strength is determined with a nonlocal stress method, which demonstrates that the material fails when the stress averaged over a characteristic distance along the prospective fracture path reaches the tensile strength. The experimental results of the tensile strength σt obtained with this nonlocal stress method do not vary with the radius ratio r/R, and the average value is about 1.23 times that tested with the flattened Brazilian disc specimen.