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{{Language_sel|LANG=ger|ARTIKEL=Kriechverhalten Rückfederungsversuch}}
{{PSM_Infobox}}
Creep behaviour – Recovery test
__FORCETOC__

==General==

The [[Creep Plastics|creep behaviour]] of [[Plastics|plastics]] can be determined under [[Tensile Test|tensile]], [[Bend Test|bend]] and [[Compression Test|compressive loading]], or by means of [[Instrumented Hardness Measurement – Creep|instrumented hardness testing]]. The [[Tensile Creep Test|long-term tensile test]] has become the most important method for the experimental determination of creep or long-term behaviour [1]. A simplified variant of the tensile creep test for determining the creep tendency of plastics is the rebound or recovery test, which requires the same technical equipment as the time-dependent [[Creep Behaviour – Tensile Creep Test|tensile creep test]] ('''Fig. 1''') [2].

==Determination of time-dependent strain==

The key measurement requirement is to determine the time-dependent strain on the [[Specimen|test specimen]], which is measured using mechanical extensometers ([[Strain Gauge|clip-on gauges]]) or optoelectronic strain measurement systems ('''Fig. 1''').

[[File:K_Rueckfeder_1.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Schematic illustration of the measurement of time-dependent strain in plastics during a tensile creep test using a creep testing equipment
|}

==Conducting the recovery test==

To apply the required load, creep test rigs or [[Material Testing Machine|universal testing machines]] can be used; however, these must be equipped with temperature-controlled chambers to ensure constant test conditions. When using universal testing machines, the tests must be carried out under [[Tensile Test Control|force control]], as otherwise the simultaneous [[Relaxation Plastics|stress relaxation]] would alter the test conditions, in particular the set load. On creep test rigs, the applied stress is achieved by means of mass blocks ('''Fig. 1'''). To measure strain continuously, each test system must be equipped with mechanical or, preferably, non-contact optoelectronic extensometers (see also: [[Video Extensometry|video extensometers]] or [[Laser Extensometry|laser extensometers]]), which record the data via an online computer in multi-channel mode ('''Fig. 2'''). For the investigations, [[Specimen|test specimens]] of type 1A or 1B are used in accordance with ISO 527-1 [3] and ISO 3167 [4]. The recovery test continues until time ''t''2, at which point the test specimen is unloaded again, in accordance with the conditions of the [[Creep Behaviour – Tensile Creep Test|tensile creep test]] [5], although the loads acting on the test specimen are generally smaller ('''Fig. 2'''). During the test, the strain is recorded continuously in accordance with '''Eq. (1)''', as the test durations are shorter in this case.

[[File:K_Rueckfeder_2.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Functional principle of the recovery test
|}

{|
|-
|width="20px"|
|width="500px" | <math>\varepsilon _{t}=\varepsilon (t)=\frac{\Delta L_{0}(t)}{L_{0}} 100\;%</math>
|width="50px" |(1)
|}

Once the load is removed, the magnitude of the linear-elastic [[Deformation|deformation]] returns to its original state spontaneously. From this point onwards, the [[Entropy Elasticity|entropy-elastic]] recovery of the magnitude ''ε''(''t'') takes effect; this is caused by the reversible deformation of chains and chain segments. The resulting residual deformation, on the other hand, corresponds to a strain caused by irreversible flow, the sliding of chain segments or the formation of [[Crack|microcracks]] (see also: [[Fracture Formation|crack formation]]).

==Recovery behaviour of polycarbonate==

'''Figure 3''' shows, by way of example, an investigation into the recovery behaviour of polycarbonate.

[[File:K_Rueckfeder_3.jpg|500px]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px" |Recovery behaviour of polycarbonate ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PC) [2]
|}

This makes it possible to predict how a [[Plastics|plastic]] material will reverse existing or accumulated [[Deformation|deformations]] over time during an unloading phase. The time-dependence of the recovery is determined by what is known as the creep compliance or the reciprocal creep modulus ''C''(''t'') ('''Eq. 2''') [2].

{|
|-
|width="20px"|
|width="500px" | <math>\frac{\sigma }{E}(t)=\sigma \left [ C(t)-C(t_1-t_2) \right ]</math>
|width="50px" |(2)
|}

==See also==

* [[Linear-viscoelastic Behaviour|Linear-viscoelastic behaviour]]
* [[Viscoelastic Material Behaviour|Viscoelastic material behaviour]]
* [[Creep Behaviour – Determination|Creep behaviour – Determination]]
* [[Tensile Test Overlapping Creep Relaxation|Tensile test overlapping creep relaxation]]
* [[BOLTZMANN's Superposition Principle|BOLTZMANN's superposition principle]]
* [[VOIGT-KELVIN Model|VOIGT-KELVIN model]]
* [[Instrumented Hardness Measurement – Relaxation|Instrumented hardness measurement, relaxation]]
* [[Stepped Isothermal Method, Tensile Stress|Stepped isothermal method, tensile stress]]
* [[Creep Plastics|Creep plastics]]
* [[Creep Behaviour – Tensile Creep Test|Creep behaviour – Tensile creep test]]

==References==

{|
|-valign="top"
|[1]
|Höninger, H.: Long-term static behavior. In: [https://de.wikipedia.org/wiki/Wolfgang_Grellmann Grellmann, W.], [https://de.wikipedia.org/wiki/Sabine_Seidler Seidler, S.] (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 167–177 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
|-valign="top"
|[2]
|Schmiedel, H.: Langzeitverhalten bei ruhender Beanspruchung. In: Schmiedel, H. (Eds.): Handbuch der Kunststoffprüfung. Carl Hanser, Munich (1992), pp. 111–118 (ISBN 3-446-16336-0; see AMK-Library under A 3)
|-valign="top"
|[3]
|ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles
|-valign="top"
|[4]
|ISO 3167 (2014-08): Plastics – Multipurpose Test Specimens
|-valign="top"
|[5]
|ISO 899-1 (2017-09): Plastics – Determination of Creep Behaviour – Part 1: Tensile Creep
|}

[[Category:Creep Behaviour Plastics]]

Creep Behaviour – Recovery Test - Revision history