Relationship Between Hardness and Strength

A relationship between hardness and strength is discussed. 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 affects surface quality. Slip-line field theory usually is used to develop the hardness relationship. The metal is assumed to be isotropic and that slip occurs at a shear stress assumed to be one-half the yield stress. The downfall of this analysis is that metals work-harden during the indentation and thus, the hardness correlates much better with the ultimate tensile strength rather than the yield strength. Ductile irons also are limited in tensile strain, or elongation, but not as severely as gray irons because the graphite is nodular rather than flake