Usage of the concept of the effectively shaped indenter for the determination of yield stress from Berkovich nano-indentation experiments

Determining the yield stress Y in addition to the indentation modulus EIT and hardness HIT from Berkovich nano-indentation experiments would widely open up their possible uses. Because of their self-similarity there is not enough information obtainable from such experiments to characterize the complete stress-strain curve of the specimen. Nevertheless it could be shown by Schwarzer and Pharr (2004) that for some selected materials the concept of the effectively shaped indenter (CEI) seems to provide good results when used to determine the yield stress Y from Berkovich indentations. However, further research providing a more complete overview why and under which circumstances the CEI is able to give such adequate results for the yield stresses is still missing.In this work, a wide spread database of simulated and experimental Berkovich nano-indentation force-displacement curves together with reference values for the yield stresses was utilized. Applying the CEI to these force-displacement curves revealed how the specimen material properties affect the usability of the CEI (Fig. 3). To give a better understanding of these phenomenological findings, an explanation of the behavior of the CEI based on the physics of the involved contact situations is given. The results lead to the conclusion that the spatial maxima of the von Mises stress delivered by the CEI do not generally represent the yield stress but rather refer to the indentation hardness HIT. This is due to the fact that the stress-strain field, especially the hydrostatic part, within the yield zone is not properly reflected by the elastic CEI model.