Assessment of Residual Stress and Determination of Stress Directionality by Instrumented Indentation Technique

Abstract

The instrumented indentation technique has taken the limelight as a promising alternative to conventional residual stress measurement methods for welds with rapid microstructural gradients because of its easy and nondestructive testing procedure. The technique is based on the key concept that the deviatoric-stress part of residual stress affects the indentation load-depth curve. By analyzing the difference between the residual stress-induced curve and residual stress-free curve, the quantitative residual stress of the target region can be evaluated. To determine the stress-free curve of the target region, we take into consideration microstructural changes that accommodate strength differences. In addition, we determine the ratio of the non-equibiaxial residual stress by using an asymmetric Knoop indenter, which has an elongated four-sided pyramidal geometry. We find that the load-depth curve is changed on penetration direction of the long diagonal for Knoop indenter, and derive a quantitative relation between the stress ratio and the load difference through both theoretical analysis and experiments. Finally, indentation tests and conventional tests were performed on the welded zone to verify the applicability of the technique. The estimated residual stress values obtained from instrumented indentation technique agreed well with those from conventional tests.