Influence of Schistosity Orientation on Failure Mode and Indirect Tensile Strength of Mica Schist

The indirect tension test is an important laboratory test for rock characterization. The presence of rock fabric, such as schistosity, complicates the assessment of test results. One hundred and forty-five indirect tension tests were conducted on mica schist specimens to investigate the effect of schistosity orientation on failure mode and tensile strength. Tensile strength results did not provide a clear relationship between schistosity orientation and tensile strength, so the failure patterns were investigated. A new naming scheme for failure modes was developed, incorporating fracture patterns observed in the specimen faces and edges. The Single Mode failure group specimens had only one failure pattern that appeared on both specimen faces, either axial failure (seventy-three specimens), schistosity failure (six specimens), or out-of-plane failure (seven specimens). The Mixed Mode failure group had thirty-two specimens that exhibited one failure pattern on one face and another on the other. The Hybrid Mode failure group had twenty-seven specimens with multiple failure patterns on both specimen faces. It was noted that Mixed Mode and Hybrid Mode specimens with components of axial failure had higher indirect tensile strengths than specimens without elements of axial failures. Statistical analyses of the tensile strength data using Levene's Test for equal variances and two-sample t-tests showed no statistical difference between the Mixed Mode and Hybrid Mode failure groups. However, there was a statistical difference between the tensile strengths of the Single Mode axial failure specimens and the combined Mixed Mode and Hybrid Mode failure groups. These results clearly emphasize that indirect tensile strength should be assessed using schistosity orientation and failure mode.