Tensile Test Overlapping Creep Relaxation
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Tensile test overlapping creep relaxation
Stress–strain behaviour without superimpositions
The characteristic value level of plastics depends largely on the test speed and temperature. These viscoelastic properties of plastics are evident in the creep and relaxation effects of these materials, which influence the dimensional stability of plastic components or the durability of the connection in force-bound constructions. In tensile tests on plastics, these effects are superimposed by the rapid stressing of this test. Assuming a hypothetical plastic that shows no tendency to creep or relax, the stress-strain behaviour shown in Fig. 1 would be recorded in the tensile test, whereby only the tensile strength σm and the elongation at break εb (see: tensile strength) are used here to compare the material behaviour.
| Fig. 1: | Stress–strain behaviour in tensile testing without creep and relaxation effects |
Influences due to creep and relaxation
The influence of creep behaviour superimposed on the tensile test can be seen in Fig. 2a. As a result of creep during the tensile test, the elongation at break increases compared to the material in Fig. 1, since the elongation is composed of εb and the time-dependent elongation ε(t).
| Fig. 2: | Stress–strain behaviour in tensile testing with creep (a) and relaxation effects (b) |
It can be seen from this that the slower the test speed in the tensile test, the stronger this effect is. If the tensile test is superimposed by pure relaxation behaviour, the stress–strain behaviour shown in Fig. 2b results. As a result of the stress reduction occurring simultaneously in the tensile test due to relaxation, the tensile strength is reduced by the time-dependent stress component σ(t), whereby the elongation at break itself is not affected. In a real tensile test, both components occur simultaneously, so that the tensile strength and elongation at break both show significant changes (Fig. 3) with increasing stress time or reduced strain rate.
| Fig. 3: | Stress–strain behaviour in tensile testing |
In general, the following statements can be made regarding the influence of test conditions [1]:
- As the test temperature increases (Fig. 4a), the behaviour changes from predominantly brittle to ductile.
- At higher test speeds or strain rates, the stress–strain behaviour changes from predominantly ductile to brittle (Fig. 4b). Both test influences have opposite effects, which is specifically exploited in the time–temperature shift principle.
| Fig. 4: | Influence of test speed and test temperature in tensile testing |
Figure 5 shows a practical example of tensile tests with varying test speeds on a short glass fibre reinforced polyamide 6 material (see also: short-fibre reinforced composites). The crosshead speeds correspond in part to the values specified in ISO 527-2 [2]. It can be seen that in this comparatively small speed interval, the tensile strengths double and the elongation at break decreases by approx. 25 %.
| Fig. 5: | Influence of test speed in tensile testing of PA6 with 30 wt.-% GF |
Quasi-static mechanical tests, such as tensile, bending or compression tests, as well as hardness measurements, always represent a superposition of rapid stress with relaxation and retardation effects in plastics, whereby the absolute proportion is influenced by the test conditions.
See also
References
| [1] | Bierögel, C.: Tensile Tests on Polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 106–123 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-806-5; see AMK-Library under A 22) |
| [2] | ISO 527-2 (2025-06): Plastics – Determination of Tensile Properties – Part 2: Test Conditions for Moulding and Extrusion Plastics |