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The most frequently used tests for determining material properties are hardness tests. Hardness is the resistance of a material to localized deformation. The term can apply to deformation from indentation, scratching, cutting or bending. In metals, ceramics and most polymers, the deformation considered is plastic deformation of the surface. For elastomers and some polymers, hardness is defined at the resistance to elastic deformation of the surface. The lack of a fundamental definition indicates that hardness is not be a basic property of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and others factors. Hardness measurements are widely used for the quality control of materials because they are quick and considered to be nondestructive tests when the marks or indentations produced by the test are in low stress areas.

There are a large variety of methods used for determining the hardness of a substance. A few of the more common methods are introduced below:-

Mohs Test – One of the first standardized systems of measuring hardness made use of the Mohs scale of hardness, which specifies ten standard minerals arranged in order of their increasing hardness and numbered according to their position. The Mohs scale of hardness has little value for hardness testing of metals but is still widely used in the field of mineralogy.

Brinell Test – The method involves impressing, with a definite load, a hardened steel ball into the material to be tested and calculating a Brinell hardness number from the impression size. A wide range of hardnesses can be tested by varying the size of the ball and the loads imposed, but in the hardness range most frequently tested, a ball 10mm in diameter is impressed into the material under a load of 3000 kgs for 10 seconds to check steel and under a load of 500 kgs for 30 seconds to check nonferrous materials. The numerical value of the Brinel hardness number is obtained by dividing the load in kilograms by the area of the spherical impression in millimetres. In practice, the average diameter of the impression is usually read with a measuring microscope and the Brinell hardness number determined directly from a table.

Rockwell Hardness Test – This is also an impression test, but the hardness number is determined by a differential depth measurement that can be read directly on a dial indicator of the machine used to impose the load.

Vickers Test – The Vickers hardness tester operates on the same principle as the Brinnel instrument but makes use of a diamond penetrator shaped as a four-sided pyramid. The impression made by the penetrator is accurately measured by swinging a microscope into position without moving the test piece in the machine. As in the Brinell method, the Vickers hardness number is the ratio of the force imposed on the indentor to the area of the pyramidal impression.
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