View source for Linear-viscoelastic Behaviour

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<span style="font-size:1.2em;font-weight:bold;">Linear-viscoelastic behaviour</span>
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==Linear viscoelasticity==

The interaction of elastic and viscous behaviour in [[Plastics | plastics]] and their dependence on time and temperature can only be described comprehensibly if one limits oneself to the area of linear-viscoelastic behaviour (see also: [[Viscoelastic Material Behaviour | viscoelastic material behaviour]]).

Linear viscoelasticity is precisely defined only for the range of infinitesimally small [[Stress | stresses]]. The [[Material & Werkstoff | material]] properties should therefore only be dependent on time, but not on the level of mechanical stress. This behaviour is referred to as purely linear-viscoelastic material behaviour.

==Validity limits for plastics==

The range of validity includes only small strains of a few tenths to a maximum of 2 %, e.g. for polyvinyl chloride ([[Plastics – Symbols and Abbreviated Terms | abbreviation]]: PVC), for glass fibre reinforced cast resins up to 0.5 %, for polyethylene ([[Plastics – Symbols and Abbreviated Terms | abbreviation]]: PE) less than 0.1 %, for polycarbonate ([[Plastics – Symbols and Abbreviated Terms | abbreviation]]: PC) at 23 °C < 1 % and at 130 °C < 0.5 %. However, the actual loads on [[Plastics | plastics]] in use are only partially within this range.

In [1], Lüpke specifies practical validity limits of less than 1 % for solid polymers and up to 100 % for polymer melts [2].

==Mechanical analogue models==

Linear-viscoelastic behaviour can be represented by the combination of linear-elastic and linear-viscous processes. In classical mechanics, mechanical analogy models are used for a better description. A spring is used for the elastic behaviour and a damper for the viscous behaviour. If these basic elements are connected in series, the MAXWELL model is obtained and if they are connected in parallel, the VOIGT-KELVIN model is obtained, with which the [[Relaxation Plastics | relaxation]] and retardation behaviour of [[Plastics |plastics]] can be described in a simplified manner.

The description of linear-viscoelastic behaviour is based on 3 principles:

* BOLTZMANN's superposition principle
* Correspondence principle
* Time-temperature-superposition principle

==See also==

* [[Elasticity]]
* [[Energy Elasticity | Energy elasticity]]
* [[Ductility Plastics | Ductility plastics]]
* [[Viscoelastic Material Behaviour | Viscoelastic material behaviour]]

'''References'''

{|
|-valign="top"
|[1]
|Lüpke, T.: Fundamental Principles of Mechanical Behavior. In: [[Grellmann,_Wolfgang|Grellmann, W.]], [[Seidler,_Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser Munich (2022), 3rd Edition, pp. 75–85 (ISBN 978-1-56990-806-8; e-book ISBN 978-1-56990-807-5 see [[AMK-Büchersammlung | AMK-Library]] under A 22)
|-valign="top"
|[2]
|Batzer, M.: Polymere Werkstoffe, Bd. 1: Chemie und Physik. Thieme Verlag, Stuttgart (1984) (ISBN 978-3-648101-1; see [[AMK-Büchersammlung | AMK-Library]] under G 17)
|}

[[Category:Deformation]]