Small correction required when applying the Hertzian contact model to instrumented indentation data

Instrumented indentation testing (IIT) is a relatively new form of mechanic al testing which significantly expands on the capabilities of traditional h ardness testing. In an IIT experiment, an indenter of known mechanical prop erties is pressed into contact and then withdrawn from a test material. The fundamental measurements during an IIT experiment are the applied load and the resulting penetration of the indenter into the test surface. The Hertz ian contact model, or a derivative thereof, is often employed to relate the se measurements to interesting mechanical properties of the test material. This article argues for a small correction to the Hertzian contact model wh en applied to instrumented indentation data. The magnitude of the correctio n primarily depends on Poisson's ratio of the test material and the contact radius normalized by the radius of the indenter tip. Neglecting this corre ction can cause significant errors in the calculation of elastic modulus an d hardness from instrumented indentation data.