中心直裂纹半圆盘试样的石灰岩断裂韧度尺寸效应试验研究

Experimental study of size effect of fracture toughness of limestone using the notched semi-circular bend samples

为了考察试样尺寸和预制裂缝长度对岩石断裂韧度测试值的影响,采用4种圆盘半径分别为25.0、37.5、50.0、75.0 mm,预制裂缝长度与圆盘试样半径比值(无量纲裂缝长度)在0.3～0.7范围内的中心直裂纹半圆盘石灰岩试样(NSCB)进行了三点弯曲断裂试验。结果表明:NSCB试样的破坏过程表现出明显的"脆性"破坏特征。与预制裂缝长度相比,试样尺寸对峰值载荷和载荷-位移曲线的形态影响更显著;随着试样尺寸的增加,试样的断裂韧度测试值不断增加,增加幅度随着预制裂缝长度的增加而加剧;当试样尺寸一定时,随着无量纲裂缝长度的增加,断裂韧度测试值有一定程度降低;为减小尺寸效应的影响,建议采用圆盘半径大于37.5 mm,无量纲预制裂缝长度范围在0.4～0.6内的NSCB试样测试岩石的断裂韧度。所得结果为推广NSCB试样国际建议方法及准确测试岩石的断裂韧度提供了一定的参考。

This study is aimed to investigate the effects of the specimen size and prefabricated crack length on the rock fracture toughness. Then three-point bending tests are conducted on semi-circular limestone specimens(NSCB) with four disc radii of 25.0,37.5, 50.0 and 75.0 mm, respectively. The ratio of prefabricated crack length to the radius of disc specimens(dimensionless crack length) ranged from 0.3 to 0.7. The results show that the failure process of NSCB specimens exhibits the characteristic of an obvious brittle failure. Compared with the prefabricated crack length, the specimen size has more significant influence on the peak load and the shape of load-deformation curve. As the specimen size increases, the testing values of fracture toughness increase continuously.Moreover, the increase rate increases with the length of the prefabricated crack length. For a selected-size specimen, the testing value of fracture toughness reduces with the increase of prefabricated crack length. In order to reduce the effect of size effect in the fracture toughness test, it is suggested that the radius of NSCB specimen is greater than 37.5 mm and the dimensionless prefabricated crack length is in the range of 0.4~0.6. This study provides certain references for the promotion of the international method and the accurate test of rock fracture toughness using NSCB specimens.