Hardness as an indicator of material strength: a critical review

Hardness is a powerful property to evaluate the deformation behavior of materials. It serves as confident quality control for several processes, especially in the heat treatment of metals. With the advent of depth-sensing indentation, this technique embraces the determination of other mechanical properties. As proof, recognized standards are available to guide the evaluation of Young's modulus using instrumented indentation. However, there are continuous efforts to describe the strength using hardness apparatus. This critical review aims to compile all ways of correlation between hardness and uniaxial strength. This relationship is usually addressed by a single value, called constraint factor, vastly recognized in metals as approximately 3. From a theoretical point of view, this value works well for materials with rigid-plastic behavior, where hardening effects can be discharged. Divergent variations presented herein show difficulties in incorporating the effect of plastic properties on the constraint factor determination. In the same way, the empirical determinations did not consider the differences in hardening exponents, putting in the same statistical analysis diverse microstructures. A specific section discusses the constraint factor for nonmetallic materials. There are critical doubts for determining strength from hardness values in this case. The existence of several approaches to estimate the constraint factor in brittle materials did not assure yet a unique value for the same material, which put in evidence the lack of a robust physical basis to understand the plastic deformation under indentation. Future trends are indicated along with these observations to become practical the recent developments that have allied hardness and strength. The most important aspect is to combine adequately the experimental and simulation approaches, which can be supported by an analysis of residual imprints of hardness and finite element model.