Hardness is the resistance of a material to penetration. This material
property is measured by pressing an indentor into the material with a
known force. The indenter plastically deforms the material forming a
permanent indentation. The size and depth of this impression is an
inverse indication of the hardness.
Relatively large indentations form in soft materials, whereas smaller indentations are made in harder materials. Thus, hardness is related to the ability to resist plastic deformation. Similarly, relatively large indentations form in low strength materials, whereas smaller indentations form in higher strength materials. From this description, it is apparent that hardness is related to strength.
The most common type of hardness test is the Rockwell test. The Rockwell test encompasses a set of tests for different hardness ranges. The Rockwell B (HRB) scale is used for aluminum and structural steels (softer materials) while the Rockwell C (HRC) scale is used for high carbon alloy steels (harder metals). These tests use different types of indenters and applied loads. These tests are performed in the lab using the Wilson Rockwell 523 hardness tester. Cylindrical tensile test specimens are commonly tested on the Rockwell testers. Since the specimen surface should ideally be flat on both sides to get an accurate reading, corrections are added when testing the rounded surface of a cylindrical specimen.
Brinell hardness tests are performed on the Wilson Model "J" Brinell Hardness tester. The Brinell scale is continuous over a wide range, suitable to almost every metal of interest.