Influence of residual stress and work hardening on instrumented indentation

Many forming and surface treatments for metallic materials introduce residual stresses and work hardening simultaneously in mechanical components. Instrumented indentation is a technique which is sensitive to both phenomena and can thus be used to quantify them provided that their influences on the experimental response can be separated. In the present paper, this question was addressed through a series of Finite Elements simulations of a spherical indentation test in which the residual stress and work hardening levels were varied independently. It was found that, for each given value of the compressive residual stress, there is a corresponding work hardening level (cumulated plastic strain) for which the two P-h curves (Force vs. Penetration Depth curves) are almost completely superposed. Therefore, it will be impossible to obtain a unique set of residual stress and work hardening from the analysis of the sole P-h curves. However, when the dimples left by the indenter are analyzed, it can be found that, for the same P-h curve, the two phenomena lead to a difference in pile-up value which is about 2% of the maximum penetration depth. The numerical simulations presented in the paper are obtained on a specific material with a spherical indenter but similar results were obtained on other materials with a spherical or a conical indenter.