Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

Hardness testing is one of the most universally valuable mechanical tests available to engineers, as it is both simple and relatively inexpensive for the wealth of information and data it produces. Hardness testing, generally in the form of a surface penetration test, is both quicker and less destructive than tensile testing. Hardness provides a linear relationship with tensile strength over a wide range of strengths for many materials, such as steel. Hardness tests are empirical, rather than derived from theory, as the results conflate effects from many different materials properties (Young's modulus, yield strength, etc.).

Hardness is a characteristic of a material used to describe how much plastic deformation (yield) that a material will undergo when a known force is applied). One can characterize hardness in three manners: scratch, indentation, and rebound hardness. A common early example of a hardness (scratch) test is the Mohs scale (1820), derived for minerals, and in which talc has a value of 1 and diamond a value of 10. In indentation testing using the Rockwell approach, small indenters are used with different loads. The most common are the Rockwell Hardness B (HRB), which uses a 1/16 in. hardened steel ball indenter along with a 100 kg weight, and the Rockwell Hardness C (HRC), which uses a diamond cone indenter along with a 150 kg weight. HRB tests are conducted for materials with low range hardness, such as aluminum, brass, and soft steels, whereas HRC tests are used for materials with high range hardness, such as harder steels. Smaller weights (15 to 45 kg) are used for the Rockwell superficial hardness test such as the HR15W, which uses a 1/8 in. steel ball with a 15 kg weight. With its lower load and shallow impression, a superficial Rockwell hardness test is ideal for very thin or brittle materials. An example of a rebound test is the Schmidt hammer, used to measure the strength of concrete. In this test, a steel mass is shot at the surface with a known force and the rebound of the ball is measured. In all types of hardness testing, it is imperative to conduct extensive calibrations if reliable results are to be obtained.

For this experiment, we will be examining the Rockwell hardness test, which measures indentation hardness of untreated and heat-treated aluminum.