Comments on "Further investigation on the definition of the representative strain in conical indentation" by Y. Cao and N. Huber [J. Mater. Res. 21, 1810 (2006)]: A systematic study on applying the representative strains to extract plastic properties through one conical indentation test

Cao and Huber [J. Mater. Res., 21, 1810 (2006)] proposed parameter-dependent representative strains, which have the potential to measure plastic properties from one conical indentation test. However, the potential performance of such a technique was not systematically analyzed. In this commentary, through a comprehensive numerical investigation, it is found that the two basic variables in Cao and Huber's formulation are not completely independent, and it is difficult to obtain the two independent (sufficiently separated) representative stress-strain points needed for determining the plastic properties. Consequently, systematic errors (which could well exceed 100%) are generated for a wide range of materials, and the results are quite sensitive to small perturbations. As a complementary and critical examination of the original article, this commentary suggests that it is unreliable to use the representative strains proposed by Cao and Huber [J. Mater. Res., 21, 1810 (2006)] to measure the material plastic properties from one indentation.