How a Hardness Tester Works
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Hardness is the resistance of a material to permanent deformation. Quality assurance and quality control personnel and engineers use hardness testers to qualify parts for manufacturing and assembly. Hardness testers indicate the hardness of a surface area of an object, or the resistance a surface has to external pressure or puncturing force. In manufacturing and engineering procedures the size of the indentation is then logged and recorded by quality assurance and quality control personnel. The degree of indentation is measured using the Rockwell hardness scale system. Modern hardness testing devices automate the hardness test. Read the indicator screen (typically, displayed in DIN ISO units) on the hardness tester to determine the hardness of the test material. Hardness is read and recorded in a number of different measurements that can be converted using a conversion table. The scale systems commonly used in the engineering industry include Rockwell, Mohs, Brinell and Vickers. The uses for hardness testers are myriad, and include measuring the strengths of materials like metals and plastics. Knowing the strength a material has to resist deformation can help engineers build products which resist the Bauschinger effect (plastic deformed in one direction results in deformation in other directions), which destroys the structure of a material product.
Set the indicator on the hardness tester to the reading position. Begin loading the hardness tester. The indenter (a penetrating cone of metal) is positioned to make contact with the test material at a 120-degree angle. Preset the level of pressure to be applied to the test material (known as the preload). Release the indenter so that it makes impact with the test material. Record the depth of the indentation for comparing with the next test. For the next test, apply pressure at a different area on the surface of the test material. Maintain this pressure for a preset time. It is referred to as dwell time and is usually several seconds. The result is an indentation or mark defacing the test material to some degree. Compare the difference between the two indentations. The difference is the measured hardness of the test material.
Total force is the addition of more force applied to the preload force on the test material. This extra force is held for a time before returning the force level back to the preload setting. The preload setting is applied for a fixed time. To get the reading for the total force (some authorities refer to it as the “Major Load”), remove the indenter, and then compare the difference between the depth of the indentation created by the additional force and the depth created by the preload setting.
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