Several testing methods have been proposed for evaluating the mode I fracture toughness of rocks. Since the fracture toughness of rocks estimated from the different testing methods is generally not compared, it is still not well-known the effect of type of the testing methods on the fracture toughness. Therefore, in this paper, three testing methods, chevron bend (CB) test, semi-circular bend (SCB) test and straight notched disk bending (SNDB) test, were performed using Kimachi sandstone in order to evaluate the mode I fracture toughness. The CB specimen is cylindrical shape with a V-shaped notch, called chevron notch, perpendicular the specimen axis. The SCB specimen is a semi-circular disk and has a straight edge notch throughout the specimen thickness. In the SNBD test, a circular disk specimen with a straight edge notch throughout the diameter is used. In a series of tests, load was applied by three-point bending to produce the tensile stress state at the notch tip indirectly for all the specimens, and controlled by the constant displacement rate. As a result, the average fracture toughness was estimated as 0.64 MN/m3/2 in the CB test, 0.66 MN/m3/2 in the SCB test, and 0.46 MN/m3/2 in the SNDB tests, respectively. The fracture toughness obtained from the CB and SCB tests were compatible each other. However, the SNDB tests estimated lower value of the fracture toughness than the others. Based on the results, the differences of the fracture toughness obtained from the three testing methods were discussed.

1. Introduction

Fracture toughness is one of the basic material properties in linear elastic fracture mechanics, and indicates the resistance to crack initiation. For brittle materials such as rocks, estimation of the fracture toughness is important for understanding of the fracture process. It has been applied as a parameter for classification of rock materials, an index for rock fragmentation, and a material property in interpretation of geological features and in stability analysis of rock structures, as well as in modeling of fracturing of rocks (Whittaker et al., 1992).

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