The effect of cyclic loading amplitude and notch crack inclination angle on the fracture toughness test on Brisbane tuff-multiple factorial analyses

Abstract

The Cracked Chevron Notch Brazilian Disc (CCNBD) method has been used to determine rock fracture toughness value of rocks in rock mechanics since many years. The CCNBD method has advantages over the other proposed fracture toughness tests in terms of the simplicity of sample preparation and less material requirement for testing. In this study, the specifications of Brisbane tuff sample geometry have been selected according to the suggestions of International Society of Rock Mechanics (ISRM). The main aim of this research is to evaluate the effect of amplitude variations and change of chevron notch angles on the fracture toughness of rocks under both static and cyclic loading. The cyclic loading was applied in three different levels of amplitudes, 10%, 20% and 30% of the static ultimate loading (SUL). In addition, three different chevron crack angles 30°, 45° and 60° have been chosen to investigate the effect of initial crack angle on the rock fracture toughness value. A series of multiple variation analyses by using Analysis of Variance (ANOVA) were carried out in this study to evaluate the effect of amplitude and inclination angle of chevron crack on the rock fracture toughness value of rocks. Statistical results demonstrated that rock fracture toughness values are very sensitive with the change of amplitudes less than 20% of SUL whereas rock fracture toughness is less sensitive with the change of amplitudes between 20% and 40% of SUL. Moreover, 45? notch crack inclination angle used to investigate the mixed Mode I-II fracturing behavior of rocks was found the most critical inclined pre-existing crack under various amplitude cyclic loading in other crack inclination angles. These outcomes are believed very important findings for many rock mechanics applications such as investigations of behavior of bedded rocks, anisotropic rocks and discontinuities in rock masses encountered with dynamic loads and fatigue.