Body hardness and qualitative analysis

Hardness testing, in its most common form, involves pressing a low-deformability indenter (diamond cone or sphere, cobalt-bonded tungsten carbide or extra-hard steel) with a given force, normally against the surface of a workpiece, and measuring a length characteristic of the response of the workpiece material: indenter penetration, size of residual indentation. It is most often used to check the condition of a metal part after production, shaping, heat treatment, surface treatment, etc. Its local character also makes it an irreplaceable tool for characterizing heterogeneities in the mechanical properties of solid parts or thin films deposited on their surface; furthermore, the compressive nature of the stress state generated enables us to explore the plastic deformation capabilities of brittle materials such as tool materials or porous parts that break prematurely in the tensile elastic range. However, the physical significance of the quantities measured is often poorly understood by practitioners. In fact, if correctly conducted and interpreted, it can be used, like rheological tests such as tensile or torsional tests (see article [M 4 151] "Lois de comportement des métaux. Elasticity. Viscoelasticity") to measure, on a local scale, the strain-hardening curve, the Young's modulus of the material under test, and even its toughness if it is brittle and, if the test is carried out under heat, its viscoplasticity and creep properties. The penetration of the indentor produces plastic deformation in common metallic materials under a state of compressive stress; but the outside of the contact zone is the seat of tensile stresses which can cause damage in the form of cracks, or even material removal in the case of brittle materials such as tool materials. This is also where the material's sliding planes emerge.

The aim of this series of articles, divided into two parts, is to present these various aspects of hardness testing. The first part is devoted to the theoretical analysis of indentation testing using various types of indenters. In this article [M 4 154], after a brief history highlighting the difficulties encountered by scientists in precisely defining the physical quantity "hardness" (§ 1), we briefly recall the characteristics of the elastoplastic behavior of a cold and hot deformed metal alloy and qualitatively analyze the indentation test (§ 2). In a second article [M 4 155], we present theoretical analyses of the cold hardness test performed on a high Young's modulus material with the usual indentor shapes. In a third article [M 4 156], we analyze the effect of material elasticity. Finally, in a fourth article [M 4 157], we interpret the results of this test carried out on viscoplastic materials (hot metal), on a small scale by nanoindentation, on brittle or anisotropic materials....