Creep Behaviour – Tensile Creep Test

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Creep behaviour – Tensile creep test

General

The creep behaviour of plastics can be determined under tensile, bending and Compression Test|compressive loads]], or by means of instrumented hardness testing. The tensile creep test has become the most important method for the experimental determination of creep or long-term behaviour [1].

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). In rare cases, crosshead displacement measurement may also be used; however, in such cases, the machine compliance of the configuration should be known.

Fig. 1:

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

Conducting the tensile creep test

To achieve the required loading levels, creep test stands or universal testing machines may be used; however, to ensure constant test conditions, these must be equipped with temperature-controlled chambers. For reasons of cost-effectiveness and time efficiency, at least 10 individual test stands should be available, which are operated either with variable loading at a constant temperature or with an identical loading level but at varying temperatures. 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, particularly the load level. On creep test stands, the load stress is applied using mass blocks, in which case a catch device is required (Fig. 1). To determine the strain at specified time points, each test system must be equipped with mechanical or, preferably, non-contact optoelectronic extensometers (video extensometers or laser extensometers), which are read out via an online computer in multiplex mode (Fig. 2).

Fig. 2:

Schematic arrangement of the creep test stand (a) and a universal testing machine for tensile creep tests (b)

Definition of time-dependent creep strain

For the tests, Type 1A or 1B test specimens are used in accordance with ISO 527-1 [2] and ISO 3167 [3], as well as ISO 293 to ISO 295 [4–6]. The test specimens should be prepared in accordance with ISO 2818 [7]. The tensile creep tests are usually carried out in accordance with the standard ISO 899-1 [8] or, in some cases, also with DIN 53 44 [9] and ASTM D 2990 [10]. Following the application of a non-impact load, the test specimen’s strain or elongation is then continuously recorded until failure or a specified time value is reached. The applied load stress is defined in accordance with Eq. (1) via the applied force or the equivalent mass. The time-dependent creep strain is calculated according to Eq. (2).

$\sigma _{t0}={\frac {F}{A_{0}}}$

(1)

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

(2)

Characteristic values of tensile creep test

The recorded creep curves (time–strain lines) are converted, in accordance with the evaluation procedure [8], into the creep diagram, the isochronous stress–strain diagram and the creep modulus curves (see: creep behaviour – determination). From these curves, the characteristic values of the tensile creep test – such as the 1-minute strain, the residual and permanent strain, the tensile creep strength or the time–strain stress, as well as the creep modulus according to Eq. (3) – can then be calculated. An example for polypropylene (abbreviation: PP) for a stress duration of up to 10⁴ h is shown in Fig. 3. Further information on graphical and tabular creep data can be found in [11]. A simplified creep test for assessing the creep behaviour of plastics is the recovery test, which takes considerably less time but does not provide the full range of characteristic values.

$E_{c}(t)={\frac {\sigma }{\varepsilon (t)}}$

(3)

Fig. 3:

Tensile creep behaviour of polypropylene (abbreviation: PP) under various loads [1]: creep curves (a), isochronous stress–strain diagram (b), creep diagram (time – stress diagram) (c) and creep modulus curves (d)

See also

• Creep plastics
• Linear-viscoelastic behaviour
• Viscoelastic material behaviour
• Creep behaviour – Determination
• Tensile test overlapping creep relaxation
• BOLTZMANN's superposition principle
• VOIGT-KELVIN model
• 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)
[2]	ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles
[3]	ISO 3167 (2014-08): Plastics – Multipurpose Test Specimens
[4]	ISO 293 (2023-02): Plastics – Compression Moulding of Thermoplastic Materials
[5]	ISO 294-1 (2017-05): Plastics – Injection Moulding of Test Specimens of Thermoplastic Materials – Part 1: General Principles, and Moulding of Multipurpose and Bar Test Specimens
[6]	ISO 295 (2004-02): Plastics – Compression Moulding of Test Specimens of Thermosetting Materials
[7]	ISO 2818 (2018-12): Plastics – Preparation of Test Specimens by Machining
[8]	ISO 899-1 (2017-09): Plastics – Determination of Creep Behaviour –Part 1: Tensile Creep
[9]	DIN 53444 (1990-01): Testing the Tensile Creep of Plastics (withdrawn)
[10]	ASTM D 2990 (2017): Standard Test Methods for Tensile, Compression, and Flexural Creep and Creep-Rupture of Plastics
[11]	Bierögel, C., Grellmann, W.: Long-term loading test. In: Grellmann, W., Seidler, S. (Eds.): Mechanical and Thermomechanical Properties of Polymers. Landoldt Börnstein. Volume VIII/6A2, Springer, Berlin (2014) pp. 286−331 (ISBN 978-3-642-55165-9; see AMK-Library under A 16)
[12]	DIN EN 12814-3 (2014-07): Testing of Welded Joints of Thermoplastics Semi-finished Products – Part 3: Tensile Creep Test