ORIGIN OF THE SIZE EFFECT IN INDENTATION OF METALS

A recently-proposed description of "the indentation size effect" in terms of a size-dependent error in effective load is shown to be accurate and to be consistent with a finite "plastic hinge" zone at the perimeter of the indentation. The implication for the theory of indentation is that strain-hardening materials do not deform in a size-independent geometrically-similar manner. For Vickers hardness testing of metals, the notional plastic hinge zone is a few micrometers wide and has little effect on the hardness values when the diagonal length of the indentation is > approximately 300-mu-m, but causes a considerable increase in apparent hardness when the indentation is smaller. The magnitude of the effect is governed by the strain-hardening propensity of the indented metal.