APPLICATION OF INSTRUMENTED INDENTATION TECHNIQUE TO ESTIMATE STRENGTH AND RESIDUAL STRESS

摘要

The instrumented indentation technique has recently attracted significant research interest because of its various advantages such as nondestructive specimen preparation, easy process, and high spatial resolution. In particular, this technique is a promising alternative to measurement methods of tensile properties and residual stress. We can evaluate tensile properties and residual stress by analyzing the indentation load-depth curve. Tensile properties of materials, primarily yield strength and tensile strength, are obtained by defining representative stress and strain, through the numerous investigations of instrumented indentation curves, and the results were discussed by comparing with results of uniaxial tensile tests. Residual stress measurement technique is based on the key concepts that the deviatoric stress part of residual stress affects the change in indentation load-depth curve, and then by analyzing difference between residual stress-induced indentation curve and residual stress-free curve, quantitative residual stress of target region can be evaluated. In determining the stress-free curve of the target region, we take into consideration microstructural changes accommodating the strength difference. Micromechanical contact analyses for the residual-stress-induced load shifts yield stress values comparable to the applied in-plane stresses. This study supports the proposition that the surface stress in an arbitrary biaxial state can be evaluated through a theoretical model combined with the ratio of two principal stress components. Instrumented indentation tests and conventional tests were performed to verify the applicability of the suggested technique, and the estimated residual stress values obtained from the indentation technique showed good agreement with those from conventional tests.