Abstract
Insights into the initiation of plasticity under contact loading of $A$ and $M$ planes of sapphire are provided by finite element analysis of the resolved shear stress acting on basal slip and basal twinning systems and by the evaluation of acoustic emission signals associated with the onset of plasticity. The analysis of acoustic emission activity utilizes wavelet-based signal representation in the joint time-frequency domain. The proposed model invoking loading-rate-dependent competition between basal slip and basal twinning predicts different yield-point mechanisms for $A$ and $M$ planes. The predicted mechanisms are consistent with the experimental plasticity initiation patterns. Room-temperature values of critical shear stress of 12.9--13.9 GPa for basal slip and 12.6--14.4 GPa for basal twinning derived from application of finite element analysis to the results of nanoindentation experiments are compared with the values expected from the results of theoretical modeling and extrapolation of high-temperature experimental results.
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