The Knoop Hardness Test, using an elongated pyramidal indenter, is a micro-indentation method suitable for thin plastic sheets, thin metal sheets, brittle materials, or materials where the applied load must never exceed 3.6 kgf.
In the Knoop Hardness Test, a load (typically in the range of 25 ~ 3600 gf) is applied to the surface of the test specimen via a pyramidal diamond indenting tool. The resulting indentation is shaped like the diamond tool, with a long-to-short axis ratio of 7:1, and point angles of 130º and 172º respectively. Knoop indentations are about 2.8 times longer and shallower than the Vickers indentations made at the same load. Optical microscope resolution may limit the application of the Knoop hardness test due to the resulting shallow indentions.
The Knoop hardness number is the ratio of the applied load to the area of the indentation, in accordance with the formula:
|HK 508 approximately* =|
|HB (3000)||455||Brinell 10 mm Standard 3000 kgf||80~445|
|HB (500)||>>||Brinell 10 mm Standard 500 kgf||89~189|
|HB (Tungsten 3000)||456||Brinell 10 mm Tungsten 3000 kgf||80~620|
|HB (Indentation)||2.89 mm||Brinell Indentation||6~2|
|HR-15N||85||Rockwell Superficial 15N||69~94|
|HR-15T||>>||Rockwell Superficial 15T||77~93|
|HR-30N||67||Rockwell Superficial 30N||41~85|
|HR-30T||>>||Rockwell Superficial 30T||53~82|
|HR-45N||53||Rockwell Superficial 45N||19~76|
|HR-45T||>>||Rockwell Superficial 45T||28~71|
|Approx. TS||1631 MPa||Tensile Strength (Approx.)||390~2450|
|<<||The hardness value is below the acceptable range of the particular hardness scale.|
|>>||The hardness value is above the acceptable range of the particular hardness scale.|
|###||The hardness value is near the limit (within 15%) of the acceptable range of the particular hardness scale.|
The many hardness tests listed here measure hardness under different experimental conditions (e.g. indenters made in different sizes, shapes, and materials, and applied with different loads) and reduce their data using different formulae. As a result, there is NO direct analytic conversion between hardness measures. Instead, one must correlate test results across the multiple hardness tests.
This calculator is based on hardness data compiled from ASM Metals Reference Book 3rd ed, published by ASM International, and Machinery's Handbook 25th ed, published by Industrial Press. The calculator curve-fits multiple hardness data onto a common polynomial basis and then performs an analytic conversion. The accuracy of the conversion depends on the accuracy of the provided data and the resulting curve-fits, and on the valid ranges spanned by the different hardness tests. Converted hardness values should be used for comparative purposes only.