Linear-viscoelastic Behaviour
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Linear-viscoelastic behaviour
Linear viscoelasticity
The interaction of elastic and viscous behaviour in 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).
Linear viscoelasticity is precisely defined only for the range of infinitesimally small stresses. The 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 ( abbreviation: PVC), for glass fibre reinforced cast resins up to 0.5 %, for polyethylene ( abbreviation: PE) less than 0.1 %, for polycarbonate ( abbreviation: PC) at 23 °C < 1 % and at 130 °C < 0.5 %. However, the actual loads on 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 and retardation behaviour of 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
References
| [1] | Lüpke, T.: Fundamental Principles of Mechanical Behavior. In: Grellmann, W., 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-Library under A 22) |
| [2] | Batzer, M.: Polymere Werkstoffe, Bd. 1: Chemie und Physik. Thieme Verlag, Stuttgart (1984) (ISBN 978-3-648101-1; see AMK-Library under G 17) |