On the necessity of using critical distance model in mixed mode brittle fracture prediction of V-notched Brazilian disk specimens under negative mode І conditions

Fracture load is experimentally measured for round-tip V-notched Brazilian disk (RV-BD) specimen made of PMMA and loaded under mixed mode І/ІІ with negative mode І components. The experimental results are obtained for 90° notch angle and various notch tip radii. The experimental observations show that in some cases, fracture does not initiate from the notch border and it occurs from the edge of the central square hole of the specimen, which is not usually expected. It is shown by the finite element (FE) analysis that although the notch is under mixed mode І/ІІ loading with negative mode І conditions and the notch bisector line experiences compressive stresses, the notch round border is divided into two compressive and tensile sides, suggesting that two critical points may exist on the notch border. Moreover, the FE analysis shows that in some cases, the value of tangential stress on the notch border is larger than that on the edge of the central square hole; but according to the maximum tangential stress (MTS) criterion, which is based on the Theory of Critical Distances (TCD), because the value of tangential stress at the critical distance from the notch border is lower than that on the edge of the hole, fracture is not expected to occur from the notch border. In such cases, the experimental observations are consistent well with predictions of the MTS criterion for the fracture initiation points. The results demonstrate the necessity of using the critical distance in brittle failure prediction of engineering components weakened by blunt V-notches.