Hardness Tester

Hardness is one of the important material properties of solids. This is not only true for parts that have to withstand high loads. In order to be able to test the hardness of raw materials or parts without destroying the test specimen, special methods and devices have been developed. Some of the hardness testers are only suitable for a certain material or group of materials, the other models can be used for different materials. In most cases, hardness tests are performed according to special standards so that the result of the tests can be compared with the corresponding specifications for the hardness. These standards specify the exact procedure for the test and the specifications for the hardness tester to be used.

In order for the measurements to be reliable and accurate, certain requirements must be met. These include ensuring that the hardness tester selected is suitable for the material properties, shape and dimensions of the parts to be tested. The instrument should have the specifications given in the material-specific testing standards and be easy to operate. In addition, the measurements with the hardness tester should be as non-destructive as possible, quick and easy to perform.

Test methods for hardness determination
One of the first distinguishing criteria of the hardness tester is the measuring method according to which the tester works. If it is clear that the hardness tests are to be carried out, it is often already clear which standard the test should or must be based on. If it goes about the in-house tests or materials or workpieces for which there are no specifications or several possibilities, it must be weighed up which method is most suitable for the respective testing task.

VDI/VDE Guideline 2616 describes the most important common hardness testing methods. Sheet 1 compares the test methods for metallic materials and Sheet 2 compares the test methods for plastics and elastomers. Some of the digital hardness testers can also convert the determined measurement results into other hardness scales. However, since the hardness is usually only determined according to one method and the stored conversion factor is usually only valid for certain materials, the other values can only be used to a limited extent.

Design and functions of the hardness tester
The harder a material is, the greater the stress it may be subjected to during the hardness measurement. For this reason, the hardness testers that measure according to the same principle, e.g. via the penetration depth or via the strength of the rebound, can be further differentiated according to the energy used and the design of the indenter or impact body. Some models with external probe for tests according to the Leeb method offer the possibility to connect different probes. With such a hardness tester, for example, the hardness tests according to Leeb C, Leeb D and Leeb G can thus be performed.

Some hardness testing methods can also be performed using analogue hardness testers. The analogue instruments are inexpensive, easy to use, and do not rely on batteries or other power sources. They are not as sensitive to temperature and humidity as electronic instruments and are excellent for quick tests. In addition, with analogue models equipped with a drag pointer, the maximum value of the test remains convenient to read. A digital hardness tester generally displays the maximum value of the test. Many digital models can also show the maximum, minimum and average value of the measurement series.

Digital hardness testers that have data memory simplify the subsequent documentation and evaluation of the measurement results. If many tests are to be performed and documented, the hardness tester with memory and data interface should be selected. The data can then be very easily and quickly transferred to a computer for further processing. Models that can display the result in several hardness scales offer further possibilities for evaluation. However, it is important to consider whether the additional scales also apply to the material being tested.

Mobile hardness testers can also be used for measurements on large or already installed parts. In contrast, the hardness tester integrated into a test stand offers advantages in maintaining the correct angle and in accurate positioning of the test points. Material samples and parts with suitable dimensions should therefore be tested on the test stand if possible.

Accessories for the hardness tester
When comparing different hardness testers, it makes sense to pay attention to the accessories included in the scope of delivery and also to the optional accessories. For example, if suitable calibration plates are included, the user can calibrate the hardness tester himself. For the models with removable data memory and data interface, the storage medium and data cable are often included in the scope of delivery. In most cases, suitable PC software for convenient evaluation is also available for these instruments. Optional accessories include tripods, test stands or adapters for mounting on convex or concave surfaces to facilitate centring and alignment during the measurement.

Hardness testers should be calibrated regularly. This ensures that the measurement results have the expected accuracy. With the hardness tester, calibration is performed by measuring on suitable test discs or test blocks of known hardness. Many calibration laboratories offer calibrations according to ISO or DAkkS for various hardness testers and issue calibration certificates for the measurements performed.

When inquiring with calibration service providers, it is essential to specify which method, e.g. Leeb, Rockwell, Barcol, and at which numerical values the hardness tester is to be calibrated. If the measured values of the calibration are outside the instrument-specific accuracy, the instrument should be adjusted. During adjustment, the hardness tester is set back to the specified accuracy.

The term hardness is used to describe the resistance of materials to penetration by foreign bodies. The term strength, on the other hand, is used to describe resistance to deformation and separation. For some materials, over certain ranges there is a close correlation between hardness and strength. For this reason, the strength of many metals and of concrete and masonry units can be determined using the hardness tester.

The harder the surface of a material is, the more difficult it is to penetrate with the hardness tester. For this reason, the test methods and the test specimens for hardness testing have been adapted to different material groups and further developed. For some of the test methods, a handy hardness tester is sufficient to measure and display the meaningful values. More complex methods require stationary test benches and additional equipment to evaluate the test specimens after loading.

Some part of the tests take place under laboratory conditions on precisely adjustable hardness testing machines. However, testing methods have also become established in which the compact hardness testers can be used quickly and easily directly in the incoming goods department, during product development and for spot checks during the production. Many mobile hardness testers are designed in such a way that the hardness can also be tested on very large or permanently installed parts.

Hardness testers for on-site measurements usually determine hardness by penetration depth or rebound height. Many methods for testing the indentation resistance vary mainly in the size and shape of the indenter and the amount of force applied. In some methods, the indentation created by the indentation of the test specimen must be measured under a microscope. In the other test methods, the hardness tester used for indentation can evaluate the indentation depth or rebound energy directly and display it as a numerical value.

The hardness data obtained must be marked with the abbreviation of the test method and, if necessary, further information on the test conditions. If this is not done, the numerical values can only be used for internal comparisons where the same hardness testers are always used.

The eddy current method can also be used for mechanical differentiation between hardened and unhardened parts. The material surface is scanned without contact with the test probe, which induces eddy currents on the part to be tested. The resulting electromagnetic fields are then evaluated. For this purpose, the eddy current hardness testing equipment must be trained with hardened and unhardened samples.

Since there are so many methods and variants for testing hardness, it is essential to ensure that the method used to determine the value is correctly indicated when specifying hardness. Except for internal control measurements where the same hardness tester is always used, it must always be apparent how testing was performed. There are significant differences in the values when Shore D and not Shore A was measured.

The measured values of certain frequently used methods can, where there are sufficiently many confirmed comparative values, also be given as hardness values according to other methods or as tensile strength. However, this only applies to materials for which the verified comparative values are available. For this purpose, the values are read from material-specific tables.

EN ISO 18265, for example, contains tables for the conversion between Vickers, Brinell, Rockwell hardness and tensile strength for six different metallic materials. Tested values are thus available for unalloyed and low-alloy steels and cast steels, quenched and tempered steels, cold-work steels, high-speed steels, hard metals and non-ferrous metals and alloys.

There is also a direct correlation between the Shore A and Shore D tests frequently used for plastics and elastomers. Under certain conditions, the measured values can therefore also be transferred into values of the other variable.

Some digital hardness testers offer the selection of several units for the display of the results. When using the hardness tester, it is essential to check which test method is used and whether the correct comparison values are stored for the material being tested.