A method to identify the yield stress of metals using micro-indentation tests with a spherical indenter

The present study proposed an inverse analysis to identify the yield stress of metals using microindentation tests with a spherical indenter. The present inverse analysis utilized an updating response surface, relating the mechanical properties of metals including Young's modulus, yield stress and hardening exponent to the indentation depths, which was determined by direct numerical simulations with an elastic-plastic finite element model for an indentation test. First, we selected an optimum parameter for characterizing the load-displacement curve obtained from an indentation test, confirming the convergence of the identified value of yield stress to the actual value. Moreover, we demonstrated that the present method using updating response surface is effective for identifying yield stress in comparison with conventional method utilizing the approximate function for the representative strain during an indentation loading, because the accuracy of response surface can be improved around the actual values of mechanical properties with a small number of numerical simulation data. Finally, we demonstrated that the identified yield stresses from actual indentation tests on the specimens made of stainless steels or aluminum alloys were very close to those obtained by tensile tests, even if the residual stress existed in the material.