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Encyclopedia > 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: In materials science, hardness is the characteristic of a solid material expressing its resistance to permanent deformation. ... Geometric shape created by connecting a polygonal base to an apex A pyramid is a geometric shape formed by connecting a polygonal base and a point, called the apex, by triangular faces. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ... A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ...

where:

L = length of indentation along its long axis
Cp = correction factor related to the shape of the indenter, usually about 0.07
P = load

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kg·mm-2. SI units (pascals) are sometimes used instead. The pascal (symbol Pa) is the SI unit of pressure. ...


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. As a non-regulatory agency of the United States Department of Commerce’s Technology Administration, the National Institute of Standards (NIST) develops and promotes measurement, standards, and technology to enhance productivity, facilitate trade, and improve the quality of life. ... 1939 was a common year starting on Sunday (link will take you to calendar). ...


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 micrometres), and the time needed to prepare the sample and apply the indenter.


Sample values

Material HK
Gold foil 69
Quartz 820
Silicon carbide 2480
Diamond 8000

General Name, Symbol, Number Gold, Au, 79 Chemical series transition metals Group, Period, Block 11 (IB), 6, d Density, Hardness 19. ... For other uses of this word, see Quartz (disambiguation). ... Silicon carbide (SiC) or moissanite is a ceramic compound of silicon and carbon. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ...

See also

External links

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

  Results from FactBites:
 
NationMaster - Encyclopedia: Hardness (392 words)
Hardness is a measure of dissolved mineral salts, mainly calcium and magnesium carbonates, in water.
Hardness has a direct affect on the cellular functions of fish, plants and microorganisms and although fish from hard water areas do not seem to suffer from softer than natural conditions the reverse does not apply.
Hardness is expressed in degrees but the units differ form country to country and test kit measurements vary depending on whether total hardness or carbonate hardness is being determined.
Vickers hardness test (493 words)
The Vickers hardness test was developed in the early 1920s as an alternative method to measure the hardness of materials.
The hardness number can be converted into units of Pa, but should not be confused with a pressure, which also has units of Pa. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not a pressure.
The Vickers hardness test uses a diamond, with the shape of square-based pyramid with an angle of 136° between opposite faces as an indenter (22° between the indenter face and surface).
  More results at FactBites »

 
 

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