Determination of equi-biaxial residual stress and plastic properties in structural steel using instrumented indentation

This study describes a method that allows estimation of the equi-biaxial residual stress (σR), yield strength (σy), the strain hardening exponent (n), and the ratio (α) between strain at starting-point of strain hardening (εst) and yield strain (εy) of structural steel from the load-depth curve of one sharp indentation test. In this method, the relationships between the structural steel properties, residual stress, and the characteristics of the indentation loading-unloading curves were established in the form of dimensionless functions via extensive finite element (FE) analyses. Based on the FE analysis results of the indentation process with a large number of different material properties combinations, the effects of residual stress and plastic parameters on the indentation response were investigated, and a reverse algorithm for estimating four unknown quantities (σR, σy, n, and α) of steel from an indentation test was proposed. A stress-applying apparatus that allows the introduction of a compressive or tensile stress to the sample by applying a compressive or tensile force and maintaining the stress after releasing the force was also developed in this study. Indentation and tensile tests of structural steels (SS400 and SM490) were carried out and the proposed algorithm was validated through both numerical analyses and experimental results.