View source for Instrumented Hardness Measurement – Creep

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<span style="font-size:1.2em;font-weight:bold;">Instrumented hardness measurement – creep</span>
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==General information==

Unlike metallic materials, [[Plastics|plastics]] exhibit a time dependence of their mechanical properties even at room temperature, which is referred to as [[Viscoelastic Material Behaviour|viscoelasticity]]. Depending on the absolute level of [[Stress|stress]] or deformation, a distinction is made between [[Linear-viscoelastic Behaviour|linear viscoelastic]] and non-linear viscoelastic deformation (see: [[Elasticity|elasticity]]). In the case of long-term static stress, these time-dependent [[Deformation|deformations]] are of considerable importance for practical applications, depending on the temperature and degree of stress. These plastic-specific deformation phenomena are referred to as [[Creep Plastics|creep]] (retardation) or [[Relaxation Plastics|stress relaxation]].

==Fundamentals of creep==

Under constant static loading or stress, a spontaneous linear-elastic deformation is followed by a time-dependent increase in deformation, known as [[Creep Plastics|creep]], which depends on the temperature and the level of stress ('''Fig. 1'''). If the [[Stress|stress conditions]] are limited to the linear viscoelastic range, the linear-elastic deformation will initially return without any time delay when the load is removed, and then the [[Creep Deformation|creep deformation]] will completely creep back (recreep) in a time-dependent manner. This reversibility does not occur in non-linear viscoelastic deformation due to the occurrence of initial [[Micro-Damage Limit|microscopic damage processes]].

{| border="0"
|[[File:Kriechen_1a.jpg]]
|
{| border=0
|-valign="top"
|width="25px"|1 –
|unloaded state
|-valign="top"
|2 –
|elastic deformation
|-valign="top"
|3 – 
|creep
|-valign="top"
|4 – 
|elastic re-deformation
|-valign="top"
|5 – 
|creeping back
|}
|}
{| 
|- valign="top"
|width="50px"|'''Fig. 1''': 
|width="600px" |Schematic representation of creep in plastics
|}

Depending on the load level and the test temperature, irreversible creep processes are observed which, under long-term stress, lead to creep failure or impermissible deformations, thereby causing dimensional deviations and loss of [[Component Testing|component functionality]], with the absolute magnitude of these processes being largely determined by the type of [[Plastics|plastics]] used.

To investigate such creep processes, [[Tensile Creep Test|tensile creep]], [[Flexural Creep Test|flexural creep]] and [[Creep Compression Test|creep compression tests]] are normally used, which are based on the generation of [[Uniaxial Stress State|uniaxial stress states]] in the [[Specimen|test specimen]]. The aim is to record the multi-parameter relationship between stress, strain and time as well as temperature, which is documented in the stress–strain–time diagram and should include test times > 10<sup>4</sup> h for dimensioning purposes.

==Instrumented macrohardness measurement==

If no [[Specimen|test specimens]] are available for the plastic to be characterised (materials used in electronics, microsystems and medical technology) and results on the creep tendency of a material are required quickly, [[Instrumented Hardness Testing – Method & Material Parameters|instrumented macro hardness measurement]] can also be used for this application ('''Fig. 2''').

[[File:InstrHardMeasure-Creep_Fig-2.jpg|650px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 2''': 
|width="600px" |Schematic arrangement of the instrumented macrohardness for creep experiments
|}

For this purpose, the [[Material Testing Machine|universal testing machine]] applies a constant load ''F''<sub>0</sub> with a ramp function and then maintains it at a constant level for a specified time using force control (see [[Tensile Test Control|tensile test control]]). Load control is necessary because relaxation occurs at the same time, which would reduce the test load acting on the [[Indenter|indenter]]. Depending on the [[Indenter|indenter]] used, the amount of force and the stress state caused, a linear elastic deformation or indentation depth initially occurs. Under the effect of the constant test load, an increase in the indentation depth is then recorded, which documents the [[Creep Plastics|creep behaviour]] of the [[Material & Werkstoff|material]]. After the holding time has ended, the load is reduced to zero in a load controlled manner, causing the elastic indentation portion to spontaneously return and subsequently a time-dependent creep of the indentation depth to occur.

With the connected temperature control chamber in the range from -100 °C to +100 °C, the test temperature can also be varied, ensuring a wide range of applications for instrumented macrohardness testing.

==See also==

* [[Instrumented Hardness Testing – Method & Material Parameters|Instrumented hardness test – method & material parameters]]
* [[Instrumented Hardness Measurement – Relaxation|Instrumented hardness measurement – relaxation]]
* [[Instrumented Hardness Measurement – Indentation Depth Measurement with Modified Contact Foot|Instrumented hardness measurement – Indentation depth measurement with modified contact foot]]
* [[Hardness]]

==References==

* [[Bierögel, Christian|Bierögel, C.]], Schöne, J., [https://researchgate.net/profile/Ralf-Lach Lach, R.], [[Grellmann,_Wolfgang|Grellmann, W.]]: Bewertung des temperatur- und zeitabhängigen Verhaltens von Thermoplasten und Elastomeren mittels der instrumentierten Makroeindringprüfung. In: [https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.] (Ed.): Neue Entwicklungen in der Werkstoffprüfung – Herausforderung an die Kennwertermittlung. "Werkstoffprüfung 2011", December 1 and 2, 2011, Berlin, Proceedings pp. 285–292 (ISSN 1861–8154; ISBN 978-3-9814516-1-0; see [[AMK-Library]] under A 13)
* Lach, R., Schöne, J., Bierögel, C., [https://de.wikipedia.org/wiki/Wolfgang_Grellmann Grellmann, W.]: Instrumented Macroindentation Techniques for Polymers and Composites – Mechanical Properties, Fracture Toughness and Time-Dependent Behaviour as a Function of the Temperature. Macromolecular Symposia 31 (2012) 125–131; [https://doi.org/10.1016/j.polymertesting.2013.09.016 https://doi.org/10.1016/j.polymertesting.2013.09.016]

[[Category:Hardness]]
[[Category:Creep Behaviour Plastics]]