Crack Front Geometry and Stress Intensity Factor of Semi-circular Bend Specimens with Straight Through and Chevron Notches

The semi-circular bending (SCB) test is one of the most useful testing methods for determining the mode-I fracture toughness of rocks. An SCB specimen with an artificial notch is loaded at three points including two lower points and a single upper point during the test. In general, there are two types of geometries for artificial notches: straight through and chevron notches. The straight through notch is commonly adopted for the SCB (STNSCB) test as the suggested method for estimating mode-I fracture toughness of rocks in ISRM, while the cracked chevron notch SCB (CCNSCB) test using a specimen with a chevron notch has been performed by some researches. In this paper, by means of the commercial finite-element software ABAQUS, cracking behavior from the tip of an artificial notch during STNSCB and CCNSCB tests is analyzed with Extended Finite-Element Method (XFEM) to clarify crack front geometry in the process of cracking. The relationship between the crack length and the stress intensity factor can be obtained by analyzing stress intensity factors of the specimen with FEM, based on crack front geometries calculated with XFEM during the cracking process. Using this relationship, the minimum stress intensity factor at a critical crack length is determined for estimating mode-I fracture toughness of the rock for the CCNSCB test. Furthermore, by performing the SCB tests using Kimachi sandstone specimens with three different artificial notch geometries, fracture toughness of the sandstone is determined from the SCB test. Consequently, it is concluded that the values of fracture toughness from the test with three notch geometries are almost the same