Knoop hardness test

Knoop Hardness Test

Knoop hardness test

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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 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:


L = length of indentation along its long axis
C<sub>p</sub> = 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 g<sub>f</sub>·mm<sup>−2</sup>. The SI unit, pascal, are sometimes used instead: 1&nbsp;kg<sub>f</sub>·mm<sup>−2</sup> = 9.80665 MPa.

The test was developed by Frederick KnoopF. Knoop, C.G. Peters and W.B. Emerson, “A Sensitive Pyramidal-Diamond Tool for Indentation Measurements,” Journal of Research of the National Bureau of Standards, V23 #1, July 1939, Research Paper...
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