Creep Behaviour – Recovery Test

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Creep behaviour – Recovery test

General

The creep behaviour of plastics can be determined under tensile, bend and compressive loading, or by means of instrumented hardness testing. The long-term tensile test has become the most important method for the experimental determination of creep or long-term behaviour [1]. A simplified variant of the tensile creep test for determining the creep tendency of plastics is the rebound or recovery test, which requires the same technical equipment as the time-dependent tensile creep test (Fig. 1) [2].

Determination of time-dependent strain

The key measurement requirement is to determine the time-dependent strain on the test specimen, which is measured using mechanical extensometers (clip-on gauges) or optoelectronic strain measurement systems (Fig. 1).

Fig. 1:

Schematic illustration of the measurement of time-dependent strain in plastics during a tensile creep test using a creep testing equipment

Conducting the recovery test

To apply the required load, creep test rigs or universal testing machines can be used; however, these must be equipped with temperature-controlled chambers to ensure constant test conditions. When using universal testing machines, the tests must be carried out under force control, as otherwise the simultaneous stress relaxation would alter the test conditions, in particular the set load. On creep test rigs, the applied stress is achieved by means of mass blocks (Fig. 1). To measure strain continuously, each test system must be equipped with mechanical or, preferably, non-contact optoelectronic extensometers (see also: video extensometers or laser extensometers), which record the data via an online computer in multi-channel mode (Fig. 2). For the investigations, test specimens of type 1A or 1B are used in accordance with ISO 527-1 [3] and ISO 3167 [4]. The recovery test continues until time t₂, at which point the test specimen is unloaded again, in accordance with the conditions of the tensile creep test [5], although the loads acting on the test specimen are generally smaller (Fig. 2). During the test, the strain is recorded continuously in accordance with Eq. (1), as the test durations are shorter in this case.

Fig. 2:

Functional principle of the recovery test

$\varepsilon _{t}=\varepsilon (t)={\frac {\Delta L_{0}(t)}{L_{0}}}100\;\%$

(1)

Once the load is removed, the magnitude of the linear-elastic deformation returns to its original state spontaneously. From this point onwards, the entropy-elastic recovery of the magnitude ε(t) takes effect; this is caused by the reversible deformation of chains and chain segments. The resulting residual deformation, on the other hand, corresponds to a strain caused by irreversible flow, the sliding of chain segments or the formation of microcracks (see also: crack formation).

Recovery behaviour of polycarbonate

Figure 3 shows, by way of example, an investigation into the recovery behaviour of polycarbonate.

Fig. 3:

Recovery behaviour of polycarbonate (abbreviation: PC) [2]

This makes it possible to predict how a plastic material will reverse existing or accumulated deformations over time during an unloading phase. The time-dependence of the recovery is determined by what is known as the creep compliance or the reciprocal creep modulus C(t) (Eq. 2) [2].

${\frac {\sigma }{E}}(t)=\sigma \left[C(t)-C(t_{1}-t_{2})\right]$

(2)

See also

• Linear-viscoelastic behaviour
• Viscoelastic material behaviour
• Creep behaviour – Determination
• Tensile test overlapping creep relaxation
• BOLTZMANN's superposition principle
• VOIGT-KELVIN model
• Instrumented hardness measurement, relaxation
• Stepped isothermal method, tensile stress
• Creep plastics
• Creep behaviour – Tensile creep 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)
[2]	Schmiedel, H.: Langzeitverhalten bei ruhender Beanspruchung. In: Schmiedel, H. (Eds.): Handbuch der Kunststoffprüfung. Carl Hanser, Munich (1992), pp. 111–118 (ISBN 3-446-16336-0; see AMK-Library under A 3)
[3]	ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles
[4]	ISO 3167 (2014-08): Plastics – Multipurpose Test Specimens
[5]	ISO 899-1 (2017-09): Plastics – Determination of Creep Behaviour – Part 1: Tensile Creep