Chapter 9: Hardness Testing Methods

Hardness Testing Methods begins by distinguishing between the three major categories of hardness measurements: scratch hardness, often utilized in mineralogy via the Mohs scale; indentation hardness, which is the primary focus for engineering applications; and rebound or dynamic hardness, measured by energy absorption. The text details the Brinell hardness test, where a hard steel or carbide sphere creates an impression, and the Brinell Hardness Number (BHN) is derived from the load divided by the surface area of the indentation. A critical discussion on geometric similitude explains that to obtain consistent BHN values across different loads, the ratio of the load to the square of the ball diameter must remain constant. The concept of Meyer hardness is introduced as a more fundamental physical definition based on the projected area of the impression, leading to Meyer's law, which mathematically relates the applied load to the indentation diameter and correlates with the strain-hardening coefficient of the material. The chapter delves into the mechanics of indentation, utilizing slip-line field theory and elastic-plastic analysis to define a constraint factor, typically around three, which relates the mean indentation pressure to the material's flow stress. This relationship allows for the approximation of the true-stress-true-strain flow curve and yield strength from hardness data. The Vickers hardness test is described as utilizing a square-based diamond pyramid to ensure geometrically similar impressions at all loads, avoiding the size-dependence issues of spherical indenters, though it is subject to errors from pincushioning or barreling distortions. The Rockwell hardness test is highlighted for its industrial utility, measuring the depth of penetration rather than diameter, and employing various scales like Rockwell B and C to cover a wide range of materials. Furthermore, the text covers microhardness testing using Knoop and Vickers indenters for analyzing small areas or specific microconstituents, noting the importance of surface preparation. The chapter concludes by addressing the empirical limitations of converting between different hardness scales and examines the temperature dependence of hardness, illustrating how it decreases exponentially as temperature rises.