Creep Plastics

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Creep plastics

General

The properties of plastics are greatly influenced by the test speed and the test temperature. This behaviour is described by the viscoelastic properties of these materials and, under service conditions, manifests itself as creep and stress relaxation even at room temperature; these effects are even more pronounced at higher temperatures. For plastic components under service conditions, creep means that dimensional stability and dimensional accuracy – and thus ultimately the functionality of these components – are adversely affected.

Fundamentals of Creep

The creep behaviour of plastics is illustrated, in simplified terms, by the behaviour shown in Fig. 1.

Fig. 1:

Schematic representation of the creep behaviour of plastics

If a load σ₀ is applied to a test specimen or a component and subsequently maintained for a defined period, the test specimen or component initially responds with a sudden increase in deformation to the value ε₀. Depending on the holding time t of the load and the test temperature T, a time-dependent deformation then occurs, which is referred to as creep ε(t). In the case of tensile stress (see: tensile test), the creep behaviour of a test specimen clamped at one end with an initial length L₀ is illustrated in Fig. 2.

Fig. 2:

Diagram illustrating the creep behaviour of plastics in a tensile test

If this test specimen is subjected to a load of mass m, then, depending on the magnitude of the load, a purely linear-elastic deformation or a combined deformation comprising both linear-elastic and linear-viscoelastic components occurs spontaneously. By definition, creep refers to the increase in strain under a constant load; consequently, when the load is maintained for a period of time Δt, a time-dependent strain ε(t) occurs. If the mass is removed, spontaneous recovery occurs and only the time-dependent strain remains.

Viscoelastic Material Behaviour

Viscoelasticity is a time-dependent form of elasticity that arises from the delayed return of macromolecules to equilibrium. Consequently, it takes a certain amount of time for the material to return to its initial length through creep processes.

This reversible Tensile Test#Tensile test, stress-strain diagram|stress–strain behaviour]] is characteristic of linear viscoelastic material behaviour and is reflected in the closed hysteresis loop with the cycle 1-->2-->1 (Fig. 3a). However, when the load is increased, pronounced non-linear viscoelastic material behaviour (see: elasticity) may occur, which is defined by an irreversible characteristic that depends on time, temperature and load. In this case, an open load hysteresis curve with the cycle 1→2→3 (Fig. 3b) is produced, which is why the deformation in Fig. 2 does not return to the origin [1].

Fig. 3:

Schematic representation of the linear-viscoelastic behaviour (a) and the non-linear viscoelastic stress–strain behaviour of plastics (b)

See also

• Elasticity
• Linear-viscoelastic behaviour
• Viscoelastic material behaviour
• Relaxation plastics
• Tensile test overlapping creep relaxation
• VOIGT-KELVIN model
• Creep behaviour – Flexural creep test
• Creep behaviour – Determination
• BOLTZMANN's superposition principle
• Instrumented hardness measurement, creep
• Stepped isothermal method, tensile stress
• Creep behaviour – recovery test

References

[1]	Höninger, H.: Long-term static behavior. In: Grellmann, W., 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)