D.R. Promer, Z.S. Najafabadi, J. Kim, D. Kujawski
Western Michigan University,
Keywords: instrumented indentation test, finite element simulation, plastic material properties
Summary:A new technique that can determine the elastic-plastic properties of metallic materials using the instrumented indentation test and finite element simulation is developed. This non-destructive technique can be applied to additively manufactured and/or surface treated metallic components in various scale. The measurement of material properties using the instrumented micro- or nano- indentation test is currently limited to the elastic modulus and surface hardness, and a number of experimental and numerical approaches have been suggested for prediction of monotonic properties of metallic materials including yield strength, strain hardening, ultimate strength, and fracture toughness. However, the past efforts to measure the stress-strain behavior using a single instrumented indentation test were not successful because there is no straightforward relation between force-displacement relation and the elastic-plastic relation. It is generally recognized that there are some reverse plasticity and continuous change in contact area during unloading. This is manifested in by nonlinear unloading curve. This observation is applied equally well to all indenter’s shapes including Berkovich, Vickers, conical, and spherical indenters. In this study, the elastic-plastic stress-strain relation is modeled using the Ramberg-Osgood equation. The peak of the force-displacement curve obtained from the instrumented micro-indentation test can be matched with the curve obtained from FE simulation with a different combination of material parameters in the Ramberg-Osgood equation (strength constant and hardening exponent). In other words, the same loading portion and peak of the force-displacement can be obtained from different materials. When the peak of the curve is matched, it is difficult to see the differences in loading curves (virtual experimental vs. simulated). However, the differences in unloading curves, especially the difference in the residual displacements is noticeable. The differences in the residual displacements are very small compared to the magnitude of maximum displacement, but it provides sufficient information to adjust plastic material properties. This study shows that unloading curves are highly affected by the elastic recovery process and the residual displacements give the critical information needed to adjust (increasing or decreasing) measured plastic properties. The developed technique in this study can be used for product design and fast prototyping with confidence in the understanding of material behavior.