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The Knoop hardness test is a microhardness test – a test for mechanical 
:
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. The SI unit,
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Hardness tests de:Härte#Härteprüfung nach Knoop
The Knoop hardness test is a microhardness test – a test for mechanical hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...
used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramid
A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilat ...
al diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
point is pressed into the polished surface of the test material with a known (often 100g) load, for a specified dwell time, and the resulting indentation is measured using a microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. The Knoop hardness ''HK'' or ''KHN'' is then given by the formula:
:
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. The SI unit, pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
s, are sometimes used instead: 1 kgf·mm−2 = 9.80665 MPa.
The test was developed by Frederick Knoop Frederick may refer to:
People
* Frederick (given name), the name
Nobility
Anhalt-Harzgerode
* Frederick, Prince of Anhalt-Harzgerode (1613–1670)
Austria
* Frederick I, Duke of Austria (Babenberg), Duke of Austria from 1195 to 1198
* Frederic ...
and colleagues at the National Bureau of Standards (now NIST
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
) of the United States in 1939, and is defined by the ASTM
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
E384 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
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
), and the time needed to prepare the sample and apply the indenter.
Variables such as load, temperature, and environment, may affect this procedure, which have been examined in detail.
See also
*Vickers hardness test
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness t ...
* Knoop hardness of ceramics
* Leeb Rebound Hardness Test
* Meyer hardness test
The Meyer hardness test is a hardness test based upon projected area of an impression. The hardness, H, is defined as the maximum load, P_\text divided by the projected area of the indent, A_\text.
:H=\frac .
This is a more fundamental measurem ...
References
{{reflistExternal links
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Hardness tests de:Härte#Härteprüfung nach Knoop