Recalibration and Clarification of the Formula Applied to the ISRM-Suggested CCNBD Specimens for Testing Rock Fracture Toughness

The cracked chevron-notched Brazilian disc (CCNBD) was proposed by the International Society for Rock Mechanics (ISRM) to test the mode I (opening mode) fracture toughness of rock. The test method has been vigorously discussed and debated, despite being the subject of intensive research for decades. The minimum (critical) dimensionless stress intensity factors affiliated with the formula for calculating the fracture toughness using CCNBD specimens with different geometric parameters remain elusive and complex. The matter cannot be resolved by simply replacing the diameter in the original formula with the radius, as claimed by several authors. In this paper, the formula is fundamentally improved, as wide-ranging minimum dimensionless stress intensity factors pertaining to diversified CCNBD geometries are recalibrated by three-dimensional finite element analysis, and an expression with tabulated coefficients is obtained through curve-fitting the data obtained from the numerical calibration. The present results are shown to be more accurate than those in the literature. Furthermore, the importance of the reasonability of the results is highlighted; a comprehensive comparison of different values shows that the upper bounds of minimum stress intensity factors are violated by the above claim. The confusion resulting from the claim is, thus, clarified conclusively.