Knoop hardness test - wikipedia Knoop hardness test Angles of a Knoop hardness test indenter The Knoop hardness test / kəˈnuːp / 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 ( often 100g ) load, 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 : H K = load ( kgf ) impression area ( mm 2 ) = P C p L 2 ( displaystyle HK = ( ( ( \textrm ( load ) ) ( ( mbox ( kgf ) ) ) ) over ( ( \textrm ( impression area ) ) ( ( mbox ( mm ) ) ^ ( 2 ) ) ) ) = ( P over ( C_ ( p ) L ^ ( 2 ) ) ) ) where : L = length of indentation along its long axis C = 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 kg mm. The SI unit, pascals, are sometimes used instead : 1 kg mm = 9.80665 MPa. The test was developed by Frederick Knoop and colleagues at the National Bureau of Standards ( now NIST ) of the United States in 1939, and is defined by the ASTM E384 standard. 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 micrometre ), and the time needed to prepare the sample and apply the indenter. Variables such as load, temperature, and environment, may affect this procedure, which have been examined in detail. Comparison between the Mohs and the Knoop scales. Sample values Material HK Dentin 68 Gold foil 69 Tooth enamel 343 Quartz 820 Silicon carbide 2480 Diamond 7000 See also Vickers hardness test Knoop hardness of ceramics Leeb Rebound Hardness Test Meyer hardness test References Jump up ^ `` Microhardness Test '', Surface Engineering Forum Jump up ^ F. Knoop, C.G. Peters and W.B. Emerson ( 1939 ). `` A Sensitive Pyramidal - Diamond Tool for Indentation Measurements ''. Journal of Research of the National Bureau of Standards. 23 ( 1 ) : 39 -- 61 ( Research Paper RP1220 ). doi : 10.6028 / jres. 023.022. Jump up ^ Czemuska, J.T. ( 1984 ). Proc. Br. Ceram. Soc. 34 : 145 -- 156. Missing or empty title = ( help ) External links efunda Dental hardness tables Retrieved from `` https://en.wikipedia.org/w/index.php?title=Knoop_hardness_test&oldid=780416619 '' Categories : Hardness tests Hidden categories : Pages with citations lacking titles Talk About Wikipedia Català Čeština Deutsch Español Français Italiano 日本 語 Polski Português Svenska Edit links This page was last edited on 14 May 2017, at 23 : 40. About Wikipedia

Question: knoop hardness test is used to measure the hardness of
Answer: very brittle materials or thin sheets