A controversial issue in mechanical property extractions from instrumented indentation applied load (P)-penetration depth (h(s)) curves concerns the possibility of finding more than one solid that essentially exhibits the same contact response. By recourse to finite element simulations of elastic-power law strain hardening solids, it was shown that for both the elasto-plastic and the fully plastic contact regimes it becomes possible to find an arbitrary number of solids whose P-h(s) curves are visually similar [L. Wang and S.I. Rokhlin, J. Mater. Res. 21, 995 (2006)]. Although this assertion supports prior findings [e.g., C.M. Cheng and Y.T. Cheng, J. Mater. Res. 14, 3493 (1999); K.K. Tho et al., Mater. Sci. Eng. A 390, 202 (2005); and J. Alkorta et al., J. Mater. Res. 20, 432 (2005)], it apparently contradicts the work by Casals and Alcala in which, through similar computational procedures and constitutive theory, it was shown that there can be "only" two solids with strictly the same P-h(s) curve [O. Casals and J. Alcala, Acta Mater. 53, 3545 (2005)]. The purpose of this comment is to reconcile the different views on the multiplicity of inferred properties that can be extracted from a single P-h(s) curve, as well as to address the issue of whether visually similar P-h(s) curves can be taken to be statistically identical both from analytical and experimental standpoints. Some considerations are also given regarding experiments performed with dual indenters.
