A preliminary load is applied by a diamond or ball indenter for a short period of time. There are three stages to the Rockwell hardness test. The Rockwell test is probably the most commonly used hardness test today, mainly since it is the fastest and most accurate form of testing. The disadvantages of the Brinell test are that it is slow compared to other methods and is destructive, leaving a large indentation in the test sample. The result of the testing process is a round indentation that can be measured and used along with the applied load to calculate a hardness value. The most frequently used forces range between 500 kgf (typically used for non-ferrous metals) to 3000 kgf (typically used for steel). The testing process involves pressing a carbide ball indenter into the surface of the test material over a set period of time with a constant applied force. The Brinell hardness test was the first standardised test to be widely used, especially on metals. The results vary between a minimum Shore hardness of 0 to a maximum shore hardness of 100 which relates to zero penetration. The indenting instrument used is known as a ‘durometer’, which includes a calibrated spring applying a defined and constant load. They both use a different diameter for the needle-shaped indenter tip and are applied to different types of material. There are two types of Shore scale used - A and D. The Shore hardness test is typically used to qualify and compare the hardness of polymers such as plastics or rubbers. The Shore test involves using a spring-loaded indenting machine to measure the hardness of a material. The results of the Mohs hardness test are not highly accurate as the applied force is not constant and the results are subjective. The testing process involves starting at the soft end of the scale and working through until one of the minerals/materials leaves a permanent indentation. The actual minerals are rarely used, particularly diamond, as it is expensive to procure.įor instance, a fingernail is often used to represent hardness scale 2.5 and a steel file for 6.5. The testing method itself is very basic and involves scratching the surface of the test material with another material similar to those assigned in the scale above. The Mohs scale of mineral hardness is as follows: It is typically used for geological purposes. The Mohs scale consists of values from 1 to 10, which correlate with the ability of the test material to withstand scratching by progressively harder minerals. The Mohs hardness test is one of the earliest attempts at defining and comparing the hardness of mineral materials. This article will look at each type of hardness test independently, comparing the testing method, applications and the type of results obtained. Most hardness tests involve using machinery that indents the material over a certain time period, applying a predetermined force or loading. Which one to apply depends on the type of material to be tested and the equipment available. There are six main hardness tests that can be carried out: Vickers, Rockwell, Brinell, Mohs, Shore and Knoop. Hardness is not a material property, rather a value ascribed to a material as a result of empirical testing. Hardness describes the resistance that a material exhibits to permanent indentation or marking by scratches.
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