Relaxation Plastics

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Relaxation plastics

General

The material value level of plastics is greatly influenced by test conditions such as test speed and test temperature. This behaviour is described by the viscoelastic properties of this group of materials and manifests itself in retardation (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 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.

Fig. 1:

Schematic diagram of the relaxation behaviour of plastics

If a deformation ε₀ is applied to a test 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 σ₀. 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, the relaxation behaviour of a test specimen clamped at both ends with an initial length L₀ is shown in Fig. 2.

Fig. 2:

Schematic diagram of the relaxation behaviour of plastics in a tensile test

If this test specimen is subjected to an elongation ΔL or an elastic strain ε₀, then depending on the degree of deformation, a purely linear-elastic or combined stress consisting of a linear-elastic and 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 Δt. If the traverse of the 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), compressive stress will then occur, as is also observed in tensile tests during the clamping of test specimens. Since 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 phenomena occur.

See also

• Relaxation behaviour determination
• Instrumented hardness measurement, relaxation
• Tensile test overlapping creep relaxation
• MAXWELL model

References

[1]	Höninger, H.: Long-therm 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 978-1-56990-806-5; see AMK-Library under A 22)