Oluschinski: Created page with "{{Language_sel|LANG=ger|ARTIKEL=Härte}} {{PSM_Infobox}} Hardness FORCETOC ==General principles== ===Definition=== Hardness is the mechanical resistance that a body opposes to the mechanical indentation of another, generally harder, but in some circumstances equally hard body. Thus, a hard body resists the indentation of a foreign body more than a soft one, from which it can be directly derived that hardness i..."

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{{Language_sel|LANG=ger|ARTIKEL=Härte}}
{{PSM_Infobox}}
<span style="font-size:1.2em;font-weight:bold;">Hardness</span>
__FORCETOC__

==General principles==

===Definition===

Hardness is the mechanical resistance that a body opposes to the mechanical indentation of another, generally harder, but in some circumstances equally hard body. Thus, a hard body resists the indentation of a foreign body more than a soft one, from which it can be directly derived that hardness is also a measure of the wear behavior of [[Material & Werkstoff|materials]]. Hardness testing methods are, by their very nature, comparative methods in which the hardness of two different bodies is determined in comparison to each other, whereby the hardness of the penetrating body, i.e., the [[Indenter|indenter]], is known. On this basis, two fundamentally different groups of hardness testing methods can be identified:

'''Indentation method''': There is no relative movement between the indenter and the [[Surface|surface]] of the test specimen.

'''Scratch method''': There is relative movement between the indenter and the surface of the test specimen.

The above definition and classification principle for hardness testing methods was formulated in a similar way by [[Martens, Adolf|Martens]] [1] in 1898. In it, he describes the [[Mohs, Carl Friedrich Christian|MOHS]] hardness scale as the hardness measurement method widely used at that time.

'''References'''

{|
|-Valign="top"
|[1]
|[https://de.wikipedia.org/wiki/Adolf_Martens Martens, A.]: Handbuch der Materialienkunde für den Maschinenbau. Erster Theil. Materialprüfungswesen, Probirmaschinen und Messinstrumente. Julius Springer, Berlin (1898), Kapitel "Härteprüfung", pp. 238–244
|}

===Types of regulation===

Regardless of the area of application (nano, micro, or macro hardness), there are three different types of control used in [[Instrumented Hardness Testing – Method & Material Parameters|recording hardness measurements]].

Conventional test execution normally involves a loading process up to a specified indentation depth or test load, followed immediately by the unloading process. If the indentation depth ''h'' is selected as the load parameter, this corresponds to the dashed unloading curve in '''Fig. 1''' below.

{| border="0"
|[[File:Hardness-1.jpg]]
|
{| border=0
|-valign="top"
|width="25px"|1 –
|Loading process up to the specified indentation depth
|-valign="top"
|2 –
|Stress relaxation at constant indentation depth over a specified period of time
|-valign="top"
|3 –
|Elastic recovery during the unloading process
|}
|}
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Conventional [[Instrumented Hardness Testing – Method & Material Parameters|recording hardness measurement]] with and without [[Relaxation Plastics|stress relaxation]]
|}
{| border="0"
|[[File:Hardness-2.jpg]]
|
{| border=0
|-valign="top"
|width="25px"|1 –
|Loading process
|-valign="top"
|2 –
|Maximum indentation depth
|-valign="top"
|3 –
|Unloading process
|-valign="top"
|4 –
|Increase in test load
|-valign="top"
|5 –
|Stress relaxation
|-valign="top"
|6 –
|Decrease in test load
|}
|}
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Time curve of indentation depth and test load during stress relaxation
|}

If a holding phase with constant path is inserted between loading and unloading, stress relaxation occurs (see: [[Relaxation Plastics|relaxation plastics]]), which manifests itself in a decrease in the test load, causes the temporal behaviour shown in '''Fig. 2''', and corresponds to path control in terms of control technology.

Since in this case only the path feed is stopped or the position is held after the linear path ramp, this type of control is the easiest test to perform.

If, in contrast, the test load ''F'' is selected as the load parameter, i.e., a constant increase in load per unit of time (load ramp) is applied and the indentation depth generated is recorded, the behaviour shown in '''Fig. 3''' results. If the load is relieved without delay after reaching the specified maximum load, the dashed curve in '''Fig. 3''' is obtained. If the load is regulated to a constant value in time interval 2, the indentation depth increases, i.e., [[Creep Plastics|creep]] occurs, which is documented in the time dependencies of load and indentation depth according to '''Fig. 4'''.

{| border="0"
|[[File:Hardness-3.jpg]]
|
{| border=0
|-valign="top"
|width="25px"|1 –
|Loading process up to the specified test load
|-valign="top"
|2 –
|Kriechvorgang bei konstanter Prüfkraft über eine vorgegebene Zeitdauer
|-valign="top"
|3 –
|Elastic recovery during the unloading process
|}
|}
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px" |Conventional recording hardness measurement with and without creep
|}

{| border="0"
|[[File:Hardness-4.jpg]]
|
{| border=0
|-valign="top"
|width="25px"|1 –
|Loading process
|-valign="top"
|2 –
|Maximum load
|-valign="top"
|3 –
|Unloading process
|-valign="top"
|4 –
|Loadless state
|-valign="top"
|5 –
|Increase in indentation depth
|-valign="top"
|6 –
|Creep at maximum load
|-valign="top"
|7 –
|Decrease in indentation depth
|-valign="top"
|8 –
|Creep back, loadless
|}
|}
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px" |Time curve of indentation depth and test load during creep
|}

===Hardness revaluation===

From the point of view of comparability, it is sometimes desirable or necessary to convert the hardness values determined using a particular method to a different scale, i.e., to reevaluate them. This is usually the case when a hardness tester for the specific method is not available, or when impressions cannot be made on the test specimen after the specific method has been applied, for example, due to space constraints. In addition, it is sometimes necessary to infer the [[Tensile Strength|tensile strength]] from a hardness value, e.g., if no [[Multipurpose Test Specimen|tensile test specimens]] can be taken from the component.

The need for hardness conversion also arises when selecting materials and designing [[Plastic Component|components]] using [[Material Value|characteristic values]] from available databases (see: [[Campus®]] database) without performing tests on the material. Due to the [[Viscoelastic Material Behaviour|viscoelastic deformation behaviour]] of [[Plastics|plastics]], two hardness values determined by different methods can be converted into each other under the following conditions:

* Both hardness values must be determined either under test load or after unloading.
* The same indentation depth–load functions should apply to the [[Indenter|indenters]] under the given geometric dimensions.
* The loading times must be approximately the same.

Instead of identical indentation depth–load functions, similar indentation depth–area functions may also be sufficient for conversion purposes. On this basis, empirical conversion to different hardness scales is possible.

Empirical correlations between [[Ball Indentation Hardness|ball indentation hardness]] HB and [[Alpha ROCKWELL Hardness|α-Rockwell hardness]] as well as [[SHORE Hardness|SHORE A and SHORE D]] are known from the literature.

The following relationship exists between ball indentation hardness and [[Alpha ROCKWELL Hardness|α-Rockwell hardness]] (see: graph for [[Ball Indentation Hardness|ball indentation hardness]]):

{|
|-
|width="20px"|
|width="500px" | <math>H\,=\,\frac{18279}{(150-HR\alpha)^{1.23}}</math>
|}

SHORE A and SHORE D are related to each other as follows (see: [[SHORE Hardness|SHORE hardness]] for a graphical representation):

{|
|-
|width="20px"|
|width="500px" | <math>SHORE\, A\,=\,116{.}1-\frac{1409}{SHORE\, D+12{.}2}</math>
|}

While there are no further conversion options for [[Plastics|plastics]] due to the absence of systematic investigations, there are standard-compliant conversion tables for metallic [[Material & Werkstoff|materials]]. This conversion proves to be very popular for steel and cast steel, and these relationships can also be successfully applied to hardened and cold-formed Cu and Al alloys as well as matrix materials. However, there are significant deviations for cold-worked or high-alloy steels.

Nevertheless, such empirical conversions should always be considered within specific material groups in order to take into account reservations regarding generalized application.

'''References'''

{|
|-Valign="top"
|[2]
|Fett, T.: Zusammenhang zwischen der Rockwell-α-Härte nach ASTM D 785 und der Kugeleindruckhärte nach DIN 53456 für Kunststoffe. Materialprüfung 14 (1972) 151–153
|-Valign="top"
|[3]
|Weiler, W. W.: Härteprüfung an Metallen und Kunststoffen. Ehningen: Expert Verlag Renningen (1990) (ISBN 978-3-8169-0552-3)
|-Valign="top"
|[4]
|Herrmann, K., Kompatscher, H., Polzin, T., Ullner, C., Wehrstedt, A.: Härteprüfung an Metallen und Kunststoffen. Expert Verlag, Renningen (2007) (ISBN 978-3-8169-2550-7; see [[AMK-Büchersammlung|AMK-Library]] under C 31)
|-Valign="top"
|[5]
|Herrmann, K.: Härteprüfung an Metallen und Kunststoffen. Grundlagen und Überblicke zu modernen Verfahren. Unter Mitarbeit von fünf Mitautoren (Polzin, T., Kompatscher, M., Mennicke, R., Ullner, C., Wehrstedt, A.) 2nd revised Edition (2014) (ISBN 978-3-8169-3181-2)
|-Valign="top"
|[6]
|Tobisch, K.: Über den Zusammenhang zwischen Shore A und Shore D Härte. Kautsch. Gummi Kunstst. 34 (1981) 347–349
|-Valign="top"
|[7]
|[[Blumenauer, Horst|Blumenauer, H.]]: Werkstoffprüfung. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (1994) 6th Edition, (ISBN 3-342-00547-5; see [[AMK-Büchersammlung|AMK-Library]] under M 3)
|-Valign="top"
|[8]
|ASTM D 785 (2023): Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials
|}

==Conventional hardness testing methods for plastics and elastomers==

The following conventional hardness testing methods are known in the context of [[Polymer Testing|polymer testing]]:

* [[Barcol Hardness|Barcol hardness]]
* [[BUCHHOLZ Hardness|BUCHHOLZ hardness]]
* [[IRHD Hardness|IRHD hardness]]
* [[KNOOP Hardness|KNOOP hardness]]
* [[Ball Indentation Hardness|Ball indentation hardness]]
* [[ROCKWELL Hardness|ROCKWELL hardness]]
* [[SHORE Hardness|SHORE hardness]]
* [[Vickers Hardness|Vickers hardness]]

Which hardness testing method can or should be used depends, among other things, on the characteristics of the [[Material & Werkstoff|material]] to be tested. Significant differences between the testing methods mentioned exist, for example, in the shape and size of the [[Indenter|indenters]].

==Special testing methods==

Special testing methods include, for example, [[Compression Hardness|compression hardness testing]] (Stauchhärte), [[Scratch Hardness|scratch hardness testing]], [[Ultrasonic Compact Impedance (UCI) Hardness|UCI hardness testing]], and [[Scratch Resistance|scratch resistance testing]].

==Instrumented hardness testing==

If the [[Measured Variable|measured variables]] of load and indentation depth are recorded continuously, i.e., the entire indentation process is recorded, this is referred to in [[Polymer Testing|polymer testing]] and [[Polymer Diagnostic|diagnostics]] as instrumented or recording hardness testing.

The fundamentals of the measurement methodology, the [[Material Parameter|material parameters]], and the special features of the experimental implementation of instrumented hardness testing for [[Instrumented Hardness Measurement – Relaxation|relaxation]] and [[Instrumented Hardness Measurement – Creep|creep]], as well as the temperature control system developed by [[Polymer Service GmbH Merseburg]] in collaboration with [https://www.zwickroell.com/ ZwickRoell GmbH & Co. KG], Ulm, are presented and explained in the WIKI-lexicon “ Polymer Testing & Diagnostics”:

* [[Instrumented Hardness Testing – Method & Material Parameters|Instrumented hardness testing – Method & Material parameter]]
* [[Instrumented Hardness Testing – Creep|Instrumented hardness testing – Creep]]
* [[Instrumented Hardness Measurement – Relaxation|Instrumented hardness measurement – Relaxation]]
* [[Instrumented Hardness Measurement with Tempering|Instrumented hardness measurement with tempering]]
* [[Instrumented hardness measurement – Indentation Depth Measurement with Modified Contact Foot|Instrumented hardness measurement – Indentation Depth measurement with modified contact foot]]

'''References'''

{|
|-valign="top"
|[9]
|Fröhlich, F., Grau, P., [[Grellmann, Wolfgang|Grellmann, W.]]: Performance and Analysis of Recording Microhardness Tests. Phys. stat. sol. (a) 42 (1977) 79–89, DOI: https://doi.org/10.1002/pssa.2210420106
|-valign="top"
|[10]
|[https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.]]: Ermittlung der Härte von Gläsern und Keramiken. Dissertation, Martin-Luther-Universität Halle-Wittenberg (1978) ([https://www.polymerservice-merseburg.de/fileadmin/inhalte/psm/veroeffentlichungen/Diss_Grellmann_Inhaltsverzeichnis.pdf Content as pdf])
|-valign="top"
|[11]
|May, M., Fröhlich, F., Grau, P., [https://de.wikipedia.org/wiki/Wolfgang_Grellmann Grellmann, W.]: Anwendung der Methode der registrierenden Mikrohärteprüfung für die Ermittlung von mechanischen Materialkennwerten an Polymerwerkstoffen. Plaste und Kautschuk 30 (1983) No. 3, pp. 149–153 ([https://www.polymerservice-merseburg.de/fileadmin/inhalte/psm/veroeffentlichungen/May_Anwendung_der_registrierenden_Mikrohaertepruefung.pdf Download as pdf])
|}

'''Weblink'''

* Wikipedia – The Free Encyclopedia: [https://de.wikipedia.org/wiki/Adolf_Martens Martens, Adolf]

[[Category:Hardness]]
[[Category:Surface Testing Technology]]

Hardness - Revision history