Creep Behaviour – Determination
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Creep behaviour – Determination
Fundamentals
The creep behaviour of plastics can be determined under tensile, bend and compressive loading, or by means of instrumented hardness testing. This requires the measurement of the time-dependent deformation of the test specimen, which, depending on the experimental technique used, may be the elongation, the peripheral fibre strain or the compression, as well as the penetration depth. The load applied to the test specimen, expressed as the constant stress σ0, should be adjustable for different load levels σ0i.
Test systems for determining creep behaviour
To carry out tests at individual load levels, creep test rigs or simple universal testing machines can be used; however, to ensure constant test conditions, these should be equipped with temperature-controlled chambers. From an economic and time-related perspective, at least 10 individual test rigs should be available, operated either with variable loading at a constant temperature or with the same stress level but at staggered temperatures. When using testing machines, the tests must be carried out under force control, as otherwise the simultaneous occurrence of stress relaxation would alter the test conditions. In creep test rigs, the load stress is applied using mass blocks, for which a catch device is required. To determine the strain at specified time points, each test system must be equipped with mechanical or optoelectronic extensometers, which are interrogated via a connected computer in multiplex mode (Fig. 1). Modern testing software can also acquire deformation data at shorter intervals (variable sampling rate) at the start of the experiments and shortly before the fracture of the test specimen, thereby ensuring a higher data density during these stages of the test.
| Fig. 1: | Schematic illustration of the measurement of time-dependent strain in plastics during a tensile creep test |
Analysis of creep experiments
As the evaluation algorithm is the same regardless of the type of deformation (strain, peripheral fibre strain, compression or penetration depth), only the general procedure is explained here. The basis for the evaluation of creep experiments is the recording of strain up to a specified time or until fracture of the test specimen, which is significantly influenced by the test load and temperature. As, for safety-critical structures, the materials used are sometimes subjected to loading for up to 105 hours or more, the creep curves and time-strain curves are plotted on a semi-logarithmic scale (Fig. 2a).
| Fig. 2: | Schematic representation of the creep curves (a) and the isochronous stress–strain diagrams (b) for plastics in a tensile creep test [1] |
It can be seen that, at a constant temperature and with increasing stress levels, the time–strain lines shift towards higher values. These creep curves form the basis for determining the creep diagrams and the isochronous stress–strain diagrams [1]. In the case of linear viscoelastic loading, the creep curves follow a linear trend. If vertical lines are drawn at specified time points in Fig. 2a (black dots in Fig. 2a), the pairs of values for σ and ε(t) can be determined for those times. The σ–ε curves determined in this way then correspond to a respective duration of loading and represent the isochronous stress–strain diagrams (Fig. 2b). The illustration is partly supplemented by the diagram from the short-term tensile test, e.g. at a strain rate of 1 %/min (dashed diagram in Fig. 2b).
Creep diagram and time–stress curves
By making horizontal cuts in the time–strain line field (Fig. 3a) at defined strain values, one obtains the creep diagrams s(t), which consist of the individual time–stress lines for the respective strain (Fig. 3b).
| Fig. 3: | Diagram of creep curves (a) and creep diagrams (b) in the tensile creep test [1] |
Consequently, during a long test duration at low stress, the same strain is achieved as during a short time at high stress, whereby, in the extreme case, the stress–time line corresponds to the time–fracture line. For design purposes and to describe the time-dependent material behaviour under low stresses, the creep modulus Ec (t) (Fig. 4) is often used for plastics.
| Fig. 4: | Schematic representation of creep modulus curves in a tensile creep test [1] |
This long-term modulus is calculated as the ratio of the respective stress to the time-dependent deformation, with Fig. 4' showing the decrease in modulus as a function of stress level and duration of loading. The values in Fig. 4 marked with a dot (●) correspond to the short-term modulus obtained from the tensile test. In principle, creep tests should be designed such that the test specimens normally withstand a test duration of at least 103 h without fracture. As a guideline, 30 to 50 per cent of the short-term tensile strength is recommended; below this stress level, preferably 6 but at least 4 different stress levels should be specified [1]. This approach increases the information content that can be obtained from creep tests, provided that a consistent evaluation of the test results is carried out in accordance with the functional relationships described. To extrapolate creep data to time scales of ≥ 10 years that are of interest for application purposes, experimentally validated creep curves for measurement times of ≥ 104 h should be available. Such creep experiments are usually only available at room temperature or under standard climatic conditions and for selected plastics, meaning that the wide variety of technical variants of reinforced and filled plastics cannot be covered, if only because of matrix modifications. Under standard atmospheres, extrapolation times of up to two decades can be achieved, although ageing effects, elevated temperatures and additional stress from the medium must be regarded as problematic.
See also
- Creep plastics
- Instrumented hardness measurement – creep
- Creep behaviour – Tensile creep test
- Creep behaviour – Creep compression test
- Creep behaviour – Recovery test
- Tensile test overlapping creep relaxation
- Stepped isothermal method, tensile stress
- BOLTZMANN's superposition principle
- VOIGT-KELVIN model
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) |