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Knoop Hardness

I'd like to know about the constant in knoop's hardness formula. where it from ?

Ulisses Bastos Campregher
- PortoAlegre, RS, Brazil

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Hardness scales are very very interesting. I'm sure you know about the Mohs scale that classifies all mineral compounds from 1 - 10. There is also a modified Mohs scale that adds a .5 to the same classification. The Knoop scale takes these same minerals and gives them numerical values of 1 - 7000. There are about a dozen other scales and a lot of these are just variations of the Mohs scale but the contact stylus is different or the means to measure the indent is different. I have a book coming out soon that covers this subject.

AF Kenton
retired business owner - Hatboro, Pennsylvania

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Hello Ulisses!

I assume you refer to the 14229 number found in ASTM E384, among other sources. This is the constant that relates the projected area of the indentation to the length of the long diagonal, which is what you measure. I believe that Paragraph 3.3 in the ASTM specification covers the derivation fairly well.

I hope this helps!

Lee Gearhart
metallurgist - E. Aurora, New York