Study on the unstable crack propagation mechanism in turning machinable ceramics based on the energy conversion principle and the fracture mechanics theory

Through machinable ceramic turning tests, the crack propagation at the machined surface and subsurface is explored, and the mechanism of unstable propagation is studied. The arrangement of the grains affects the direction of crack propagation, and the instability deflection angle (IDA) has a positive correlation with the instability deflection length (IDL). Crack propagation occurs in four stages: initial crack formation stage, metastable propagation stage, unstable propagation stage, and upward propagation stage. Based on the energy conversion principle and the fracture mechanics theory, a binary model of unstable crack propagation is developed. The IDA and IDL are quantified together with their positive correlation and accuracy. The experimental validation results reveal that the theoretical values of the IDA and IDL approach the experimental values, indicating that the model is accurate and may be utilized as a theoretical basis for forecasting the unstable crack propagation behavior in brittle material cutting.