Knoop hardness test

Knoop hardness test

The Knoop hardness test is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a microscope. The Knoop hardness "HK" or "KHN" is then given by the formula:

:HK=load(mbox{kgf})} over {impression area (mbox{mm}^2)={P over {C_pL^2

where::"L" = length of indentation along its long axis:"C"p = correction factor related to the shape of the indenter, ideally 0.070279:"P" = load

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kgf·mm-2. SI units (pascals) are sometimes used instead: 1 kgf·mm-2 = 9.80665 MPa.

The test was developed by F. Knoop and colleagues at the National Bureau of Standards (now NIST) of the USA in 1939, and is defined by the ASTM D-1474 standard.

The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

Sample values

See also

* Vickers hardness test, another microhardness test
* Knoop hardness of ceramics

External links

* [http://www.efunda.com/units/hardness/convert_hardness.cfm?cat=Steel&HD=HK efunda]
* [http://www.lib.umich.edu/dentlib/Dental_tables/Knoophard.html Dental hardness tables]


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