Establishing the evaluation technique of the biaxial residual stress determined with the surface strain measurement under the spherical indentation

In this research, a technique has been developed for quantitatively evaluating the amount and distribution of tensile and compressive biaxial residual stresses by using combined strain measurements under the spherical indentation loading together with the finite element method (FEM). When the spherical indentation is applied to the top surface of a steel plate, the elastic strain at an optimized position near the indentation is measured by strain gauges, where the biaxial residual stresses and applied indentation stresses are largely superimposed. In order to analyze the biaxial residual stress, FEM analysis has been conducted to establish the relationship between the elastic strain adjacent to the indentation and the indentation load for steel plates subjected to various uniform tensile and compressive biaxial bending stresses. A strain energy term (U*) has been newly introduced by integrating along the trajectory between the indentation load and elastic strain in a range from 0(zero) to the maximum load. The technique explained herein has been applied to quantify the biaxial residual stress from the maximum and minimum principal stresses at two directionally perpendicular points U*_ax and U*_by, which is expected contribute for enhancing reliability verification of steel parts.