Hardness tests often are used to quantify strength and are considered to be nondestructive in most applications because the indentations are small and do not adversely affect surface quality. In the case of steel, there is a common relationship between the Brinell hardness number (BHN) and the ultimate tensile strength (UTS) given in pounds force per square inch (psi), or MPa:

UTS(psi)=500BHN

It often is assumed that this relationship is only valid for tempered martensitic structures. However, similar relationships can be shown for brass, aluminum and cast irons. A relationship between yield strength and hardness usually is not shown, and it may be instructive, and perhaps surprising, to show that these relationships have a theoretical basis.