Mechanical testing of metals - Hardness testing

In this section Mechanical testing of metals, readers can also refer to the following specialist articles:

• "Determination of behavior laws [M 120] ;

• "Formability testing [M 125] ;

• "Fracture testing [M 126].

While the notion of hardness is one of the most intuitive, in practice its measurement corresponds to that of the resistance to local penetration of the material under consideration. Hardness is a complex physical property that is difficult to interpret, and depends not only on the material's characteristics, but also on the nature and shape of the penetrator and the method of penetration. For example, cold-worked copper offers greater resistance to penetration than mild steel, but is scratched by it.

The usual hardness tests are simple, rapid and generally non-destructive, except in very localized areas. They therefore offer a very convenient means, widely used in workshops, of checking the evolution of the properties of a metal part, particularly during heat and mechanical treatments, or for checking the conformity of supplies. In addition, hardness can be used to assess, to a certain extent, mechanical strength, abrasion resistance, polish retention, difficulty of machining, etc. It can be used to assess the strength of metal parts. It can also be used to assess the resistance of brittle bodies (carbides, intermetallic compounds, etc.). Finally, the development of methods for measuring microhardness has made it possible to solve numerous problems: assessing the hardness of thin or superficial layers, exploring multi-phase alloys, evaluating local strain hardening, etc. In recent years, the development of nanoscale measurement techniques has opened up even greater possibilities in these fields.

A large number of hardness evaluation methods have been proposed. The most common and familiar consist in measuring resistance to penetration, but scratching, rebound or pendulum oscillation tests can in some cases offer interesting possibilities.

Acronyms and symbols are explained on p. 16.