Subsurface strain distribution around Vickers hardness indentations in annealed polycrystalline copper

Abstract

Subsurface strain distribution around relatively large Vickers diamond indentations of a diagonal length of 2.003 mm in annealed copper has been determined by making a survey of Vickers hardness, using a low indenter load of 50 gf, of a sectioned plane which was normal to the specimen surface and an indentation edge and which was close to the median plane containing the indentation tip. The maximum strain hardening, corresponding to an estimated equivalent strain of between 0.25 and 0.36, occurs at the tip of indentation. It is shown that the measured strain distribution is distinctly different from the strain distribution that would occur if the displaced material were to flow out uniformly along radial directions. A discussion of the equivalent strain introduced within the sample by a Vickers indentation and the ratio of Vickers hardness to the uniaxial flow stress is also given. It is argued that the often-quoted value of 0.08 as the equivalent strain for a Vickers indentation may not be as unique as previously thought and may indeed be quite misleading. It is also shown that even for a metal, such as the annealed copper used in this work, the ratio of Vickers hardness to the prior flow stress may vary in the range from 2.8 to 11.5, depending upon the prior strain in the specimen.