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A diamond indenter, in the form of a rhombic-based pyramid with angles between opposite edges faces respectively equal to 172.5° and 130° at the vertex, is forced into the surface of a test piece followed by measurement of the long diagonal, d, of the indentation remaining in the surface after removal of the test force.
HK = 0.102 ⋅ F / Ap = 1.451 ⋅ F / d2 ... Knoop hardness
This method, developed by Knoop, Emerson and Peters, was presented for the first time ever in 1939.
DIN EN ISO 4545:2006 1 - 4 Metallic materials - Knoop hardness test