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{{Language_sel|LANG=ger|ARTIKEL=Relaxation Kunststoffe}}
{{PSM_Infobox}}
Relaxation plastics
__FORCETOC__

==General==

The [[Material Value|material value]] level of [[Plastics|plastics]] is greatly influenced by test conditions such as [[Test Speed|test speed]] and test temperature. This behaviour is described by the [[Viscoelastic Material Behaviour|viscoelastic properties]] of this group of materials and manifests itself in retardation ([[Creep Plastics|creep]]) and stress relaxation even at room temperature under operating conditions. These effects are even more pronounced at higher temperatures. Under the operating conditions of [[Plastic Component|plastic components]], relaxation means that the required preloads of joined parts are reduced, which has a negative long-term effect on the dimensional accuracy and thus the functionality of such components.

==Fundamentals of relaxation==

The stress relaxation of plastics can be simplified using the diagram in '''Fig. 1'''.

[[File:Relaxation_2.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Schematic diagram of the relaxation behaviour of plastics
|}

If a [[Deformation|deformation]] ''ε''0 is applied to a test [[Specimen|specimen]] or component and then held for a defined period of time, the specimen or component initially reacts with a spontaneous increase in stress to the value ''σ''0. Depending on the holding time ''t'' of the load and the temperature ''T'', a time-dependent decrease in stress then occurs, which is referred to as stress relaxation ''σ''(''t''). In the case of tensile [[Stress|stress]], the relaxation behaviour of a test [[Specimen|specimen]] clamped at both ends with an initial length ''L''0 is shown in '''Fig. 2'''.

[[File:Relaxation_3.jpg|550px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Schematic diagram of the relaxation behaviour of plastics in a [[Tensile Test|tensile test]]
|}

If this test specimen is subjected to an elongation Δ''L'' or an elastic strain ''ε''0, then depending on the degree of deformation, a purely linear-elastic or combined stress consisting of a linear-elastic and [[Linear-viscoelastic Behaviour|linear-viscoelastic]] part occurs spontaneously. By definition, relaxation means the decrease in stress under constant deformation, which is why a time-dependent stress drop ''σ''(''t'') occurs when the deformation state is maintained over a period of time Δ''t''. If the traverse of the [[Material Testing Machine|material testing machine]] is returned to its initial state, spontaneous stress relief occurs. Due to stress relaxation and the constraint of the clamping jaws (see: [[Specimen Clamping|specimen clamping]]), compressive stress will then occur, as is also observed in [[Tensile Test|tensile tests]] during the clamping of test specimens. Since [[Linear-viscoelastic Behaviour|viscoelasticity]] represents a time-dependent elasticity based on the delayed equilibrium adjustment of the macromolecules, it takes a certain amount of time for the load-free initial state to be restored through recovery processes. If the clamping jaws are opened immediately after the test is completed, the load is restored immediately, but slight [[Creep Plastics|creep phenomena]] occur.

==See also==

* [[Relaxation Behaviour Determination|Relaxation behaviour determination]]
* [[Instrumented Hardness Measurement, Relaxation|Instrumented hardness measurement, relaxation]]
* [[Tensile Test Overlapping Creep Relaxation|Tensile test overlapping creep relaxation]]
* [[MAXWELL Model|MAXWELL model]]

'''References'''

{|
|-valign="top"
|[1]
|Höninger, H.: Long-therm Static Behavior. In: [[Grellmann, Wolfgang|Grellmann, W.]], [[Seidler, Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 167–177 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-806-5; see [[AMK-Büchersammlung|AMK-Library]] under A 22)
|}

[[Category:Deformation]]

Relaxation Plastics - Revision history