View source for KNOOP Hardness

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<span style="font-size:1.2em;font-weight:bold;">KNOOP hardness</span>
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==Basics==

The KNOOP hardness test method was introduced into materials testing in 1939 by Knoop, Peters and Emerson. It is one of the classic conventional test methods for evaluating [[Hardness | hardness]], whereby an allocation to the small load or micro range was made from the outset and 10 N was recommended as the highest test load. An asymmetrical four-sided diamond pyramid with a diagonal ratio of 7.114 : 1 is used as the indenter. This Knoop indenter has two different tip angles of 172°30" and 130° respectively. Due to this geometry, the Knoop indenter is also advantageously used to solve application-technical problems with the help of instrumented micro hardness testing (see [[Instrumented Hardness Testing – Method & Material Parameters | Instrumented hardness testing]]).

==Definition of the KNOOP hardness==

In contrast to [[Vickers Hardness | Vickers hardness]], where the indentation surface is used for calculation, KNOOP hardness (abbreviation: HK) is determined with the help of the projection surface of the indentation. HK is calculated using the large indentation diagonals:

{|
|-
|width="20px"|
|width="500px" | <math>HK\,=\,\frac{F}{A}\,=\,\frac{14{,}23\cdot F}{l^2}</math>
|}

with
{|
|-
|HK
|width="15px" | 
|KNOOP hardness in N/mm<sup>2</sup>
|-
|F
|
|test load in N
|-
|A
|
|projected indentation surface in mm<sup>2</sup>
|-
|l
|
|large indentation diagonal in mm
|}

The KNOOP hardness method is particularly suitable for the determination of [[Material Value | characteristic values]] in layers close to the surface and very thin test [[Specimen | specimens]], as the indentation depth of the long diagonal to the short diagonal has a ratio of 1 : 30.514 ('''Fig. 1''').

[[file:Knoop 2 eng.jpg]]
{| 
|- valign="top"
|width="50px"|'''Fig. 1:''' 
|width="600px" |Schematische Darstellung einer Knoop-Härte Prüfung
|}

Due to this penetration depth ratio, extreme demands are made on the evenness and roughness of the surface to be tested, resp. the geometric dimensions of the test [[Specimen | specimen]]. The KNOOP hardness is suitable for the verification of [[Tensile Test Residual Stresses Orientations | residual stress]] and in particular also of structural and morphological anisotropy effects by considering the directional dependence of the determined HK characteristic values.

==Suitability of the KNOOP hardness for the detection of orientations==

'''Figure 2''' shows schematically the influence of the [[Tensile Test Residual Stresses Orientations | orientation]] on the indentation geometry.

[[file:Knoop 3 eng.jpg]]
{| 
|- valign="top"
|width="50px"|'''Fig. 2:'''
|width="600px" |Schematic representation of the influence of orientations on the longitudinal and transverse diagonal of the indentation surface in KNOOP-hardness (top) and determination of the anisotropy of selected [[Plastics | plastics]] (according to Weiler; bottom)
|}

In oriented [[Thermoplastic Material | thermoplastic materials]], the stresses under the indenter are greater in the direction of orientation than perpendicular to it. After relieving the indenter, the shortening of the diagonals of the indenter is pronounced to different degrees, as the strain field is no longer symmetrical.

If the major principal axis is parallel to the [[Tensile Test Residual Stresses Orientations | orientation]], the maximum strain is perpendicular to the preferred direction of the molecular chains, which leads to an enlarged indentation. Thus, higher hardness values are determined in the direction of orientation than perpendicular to it.

If the major axis is oriented perpendicular to the preferred direction of the molecular chains, the maximum strain occurs parallel to the molecular orientation, resulting in a reduced impression. The characteristic values determined with the KNOPP hardness test method prove to be extremely sensitive for detecting anisotropies in materials.

==See also==

*[[Hardness]]
*[[Vickers Hardness | Vickers hardness]]
*[[Instrumented Hardness Testing – Method & Material Parameters | Instrumented hardness testing – Method & Material parameters]]


'''References'''

* Knoop, F., Peters, C., Emerson, W. B.: J. Res. Nat. Bur. Stand. 23 (1939) 39
* ASTM E 384 (2022): Standard Test Method for Microindentation Hardness of Materials
* ASTM D 1474/D1474M (2013; reapproved 2023): Standard Test Method for Indentation Hardness of Organic Coatings
* VDI/VDE 2616 Blatt 2 (2014-07): Härteprüfung an Kunststoffen und Elastomeren
* [[Blumenauer, Horst|Blumenauer, H.]] (Hrsg.): Werkstoffprüfung. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (1994) 6. Auflage, (ISBN 3-342-00547-5; see [[AMK-Büchersammlung | AMK-Library]] under M 3)
* [[Grellmann,_Wolfgang|Grellmann, W.]], [[Seidler,_Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser Munich (2022) 3. Edition, pp. 180/182 (ISBN 978-1-56990-806-8; see [[AMK-Büchersammlung | AMK-Library]] unter A 23)
* Weiler, W. W.: Härteprüfung an Metallen und Kunststoffen. 2. aktualisierte und erweiterte Auflage, Expert Verlag, Renningen (1990), (ISBN 978-3-8169-0552-3)

[[Category:Hardness]]