Fastener Testing

By Alan Thomas, market manager at ZwickRoell Ltd

The Rockwell hardness test is the most commonly used hardness test method and is generally easier to perform than other types of hardness testing methods.

The test method is used on all metals, except in conditions where the test metal structure or surface condition would introduce too much variation, for example where the indentations would be too large for the application or where the sample size or shape prohibits its use.

The Rockwell method measures the permanent depth of indentation produced by a force on an indenter. First, a preliminary test force, commonly referred to as preload or minor load, is applied to a sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of surface finish. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.

After the preload, an additional load, call the major load, is added to reach the total required test load. This force is held for a predetermined amount of time to allow for elastic recovery. This major load is then released, returning to the preliminary load. After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the baseline and final depth measurements and this distance is converted to a hardness number. The preliminary test force is removed and the indenter is retracted from the test specimen.

It is also important that there should be no deflection of the test sample or testing machine during the loading application – from conditions such as dirt under the test specimen or on the machine elevating screw. It is important to keep the surface finish clean and decarburisation from heat treatment should be removed. It should also be noted that sheet metal can be too thin and too soft for testing on a particular Rockwell scale – without exceeding minimum thickness requirements and potentially indenting the test anvil – resulting in erroneous test results.