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Hardness is another property that is closely related to a material's strength. Hardness is a measure of the ability of a material to resist indentation.
Hardness is a popular test because it can be performed non-destructively that is, without breaking the component being tested. Micro-hardness tests, which use a much smaller indenter and lower loads, are also commonly used to evaluate the local hardness.
There is a wide range of different hardness tests that are used. Each test gives a different value for the hardness. Hardness is therefore not a well defined universal materials property, but depends on the method used for measurement.
Hardness tests are based on measuring the size of an indentation formed by a hard indenter under load.
Hardness is defined as the load applied (in kg) divided by the projected area of the indentation.
In practice, the size of the indentation is measured under a microscope, and related to the hardness number (units: kg mm−2) using an equation (often by consulting a table of pre-calculated values).
In the widely-used Vickers hardness test, the indenter is a diamond pyramid.
There is a useful approximate relationship between Vickers hardness and yield strength, which is widely used, but not always strictly correct. This relationship states that:
Vickers Hardness (HV) ~ 0.3 × yield stress (in MPa)
Brinell hardness is another method used to characterize the hardness of a material. Like the Vickers test, it relies on measuring the size of an impression left by an indenter under a known load. The indenter used in the Brinell test is usually a hardened steel ball, and the measured hardness values are different from those determined using the Vickers test. As the indentation geometry changes with indent size in the Brinell test, there is no simple correlation between the Brinell hardness and yield strength.
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