On the measurement of yield strength by spherical indentation

Abstract

Over the past 10 years, a number of investigators have proposed methods to measure the yield strength of metals using instrumented indentation experiments performed with a sphere [Ma et al., J. Appl. Phys. 94 288 (2003); Cao and Lu, Acta Mater. 52 4023 (2004); Yu and Blanchard, J. Mater. Res. 11 2358 (1996); Field and Swain, J. Mater. Res. 10 101 (1995)]. Most of these proposed methods have yet to be rigorously verified experimentally. The objective of this work is to contribute to experimental verification by testing four contemporary models against their ability to accurately determine the yield strength of the aluminium alloy 6061-T6 using the smallest sphere commercially available. The four models selected for this review are those in the references mentioned above. The tensile and indentation samples were taken from the same 3.175-mm thick sheet and the surface of the indentation sample was given the best possible mechanical polishing. The indentation experiments were performed using a 90° diamond cone with a mechanically polished radius of 385 nm. The procedures proposed by Ma et al. and Cao and Lu were inconsistent with experimental observations and could not be implemented. Yu and Blanchard's model overestimated the yield strength by approximately 55%. Field and Swain's procedure overestimated the tensile flow curve by roughly 40%, which precluded obtaining a meaningful estimate of the yield strength. Among the most likely explanations for these surprisingly poor results are the effects of roughness and contaminants on the surface, and the possibility of an indentation size effect.