Estimation of Principal Directions of Bi-Axial Residual Stress Using Instrumented Knoop Indentation Testing

We propose a novel method for estimating stress directionality p and principal direction theta(p) using four Knoop indentations. Indentation load-depth curves with a Knoop indenter are shifted with respect to indenter orientation to stress direction. Based on this phenomenon, we derived p and theta(p) theoretically in terms of the indentation load differences between a stressed and an unstressed sample. Plastic zone beneath the Knoop indenter was theoretically estimated based on expanding cavity model for confirming separate plastic zones of four Knoop indentations. The proposed models and algorithms were verified by indentation experiments under various applied stress states that are generated using a stress-generating jig with two independent orthogonal loading axes. Estimated principal directions showed good agreement with applied principal directions. For small target area when considering in-field application, we proposed an estimation method of one load difference with other three load differences, and it was verified as feasible method with experimental results.