on the indentation is the piling-up or sinking-in of the surrounding material. If the metal is work hardened it has a tendency to pile up and form a "crater". If the metal is annealed it will sink in around the indentation. Both of these effects add to the error of the hardness measurement. Relation to yield stress When hardness, , is defined as the mean contact pressure (load/ projected contact area), the yield stress, , of many materials is proportional to the hardness by a constant known as the constrain factor, C. where: The hardness differs from the uni-axial compressive yield stress of the material because different compressive failure modes apply. A uni-axial test only constrains the material in one dimension, which allows the material to fail as a result of shear. Indentation hardness on the other hand is constrained in three dimensions which prevent shear from dominating the failure. See also Leeb rebound hardness test Meyer's law References External links "Pinball Tester Reveals Hardness." Popular Mechanics, November 1945, p. 75. Bibliography . Hardness tests