The Rockwell hardness test is a way of measuring the surface strength of different materials. The basic idea is to determine how resistant a material is to deformation and convert that into a number. When performing the Rockwell hardness test, scientists will indent the materials in question with different amounts of weight. The test is performed using a special device, which measures the depth of the indentations to create a number. Different amounts of weight and scales are used for different kinds of materials.
There are several steps that scientists use when performing the Rockwell hardness test. First, the material is placed in the device, which allows weight to be applied through an indenter. The indenter usually has a wedge shape, but sometimes it can be rounded, depending on the material and the parameters of the test.
Once the material is in place, a relatively small amount of weight is generally applied. This will normally deform the material to a small extent, and scientists will take a measurement. Then, a heavier weight is added, which will deform the material further. In many cases, the second weight will be left in place for a set period of time. At this point, testers will remove the second weight while leaving the first weight in place and then take another measurement.
In a typical circumstance, the removal of the second weight won't allow the material to return to the same depth it had after the small weight was initially applied. Generally, the heavy weight will cause a certain amount of permanent deformation. This permanent change is what scientists are normally trying to measure for when doing the Rockwell hardness test, so the measurement they're interested in is the difference between the depth when the small weight is initially applied and the depth after the heavy weight is removed.
The measurement itself is typically a number based on units consisting of 0.002 millimeters each, although this can vary some depending on the materials workers are testing. A very small measurement unit is useful for the Rockwell hardness test because many of the materials tested are quite strong, and they aren't going to deform very much. For example, Rockwell hardness is often used to measure the effectiveness of different metallic tempering techniques, and tempered metals are generally very resistant to deformation.
The Rockwell hardness test isn't the only way to measure the hardness of materials. There are a few other competing systems out there, but the Rockwell test is generally more popular. The biggest advantages for the Rockwell test compared to other methods are convenience and testing speed.