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The Knoop hardness test is a microhardness test – a test for mechanical 
:
where:
: ''L'' is the length of indentation along its long axis
: ''C''p is the correction factor related to the shape of the indenter, ideally 0.070279
: ''P'' is the 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, is sometimes used instead: 1 kgf⋅mm−2 = 9.80665 MPa.
The test was developed by Frederick Knoop and colleagues at the National Bureau of Standards (now
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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 plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...
used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramid
A pyramid () is a structure whose visible surfaces are triangular in broad outline and converge toward the top, making the appearance roughly a pyramid in the geometric sense. The base of a pyramid can be of any polygon shape, such as trian ...
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. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...
point is pressed into the polished surface of the test material with a known (often 100 g) load, for a specified dwell time, and the resulting indentation is measured using a microscope
A microscope () is a laboratory equipment, 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 ...
. 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'' is the length of indentation along its long axis
: ''C''p is the correction factor related to the shape of the indenter, ideally 0.070279
: ''P'' is the 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, is sometimes used instead: 1 kgf⋅mm−2 = 9.80665 MPa.
The test was developed by Frederick Knoop 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 s ...
) of the United States in 1939, and is defined by the ASTM
ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...
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 (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
), 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 scale, Brinell method to measure the hardness of materials. The Vickers test is often easier to use than ot ...
* Knoop hardness of ceramics
* Leeb rebound hardness test
The Leeb Rebound Hardness Test (LRHT) invented by Swiss company Proceq SA is one of the four most used methods for testing metal hardness. This portable method is mainly used for testing sufficiently large workpieces (mainly above 1 kg).
It m ...
* Meyer hardness test
References
{{reflistExternal links
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Hardness tests de:Härte#Härteprüfung nach Knoop