THE process of indentation of a metal surface by a hard spherical indenter has been examined in considerable detail by Tabor1 and others. If the indenter makes contact with the surface over a circular area of radius a under the action of a load W, the mean contact pressure pm=W/πa2. Tabor showed that plastic yielding begins beneath the surface of the metal when pm reaches the value 1.1 Y (where Y=yield stress of the metal in simple tension). As the load is increased and a marked plastic indentation develops, the mean pressure approaches a value which is independent of the load given by: where c has a value of approximately 3. This is the state of affairs in a Brinell hardness test.