Development of a residual stress evaluation technique based on surface strain measurements in indentation process of spherical indenter

In this research, a technique has been developed for quantitatively evaluating the magnitude of tensile and compressive residual stresses that is based on the surface strain measurements in the indentation process of the spherical indenter with the finite element method. 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 residual stress and applied indentation stress are superimposed. In order to analyze the 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 that are subjected to various tensile and compressive stress in the uniform residual stress and bending residual stress. A strain energy U* has been newly introduced. U* expresses the integrating along the trajectory about between the indentation load and elastic strain in a range from zero to the optimum load. U* is decreasing when the residual stress is at tensile state and it behavior has been verified by indentation test and finite element method. The application of this technique could contribute to the improved reliability of steel parts.