Creep Behaviour – Flexural Creep Test

Sprachauswahl/Language selection

Dieser Artikel ist auch auf Deutsch verfügbar Kriechverhalten Zeitstandbiegeversuch

A service provided by
Polymer Service GmbH Merseburg
Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de
Our further education offers: https://www.psm-merseburg.de/weiterbildung
PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg

Creep behaviour – Flexural creep test

General

The creep behaviour of plastics can be determined under tensile, bend and compressive stress, or by means of instrumented hardness measurement. The flexural creep test can also be used for the experimental determination of creep or long-term behaviour [1]. This test is used specifically to investigate long-term loading in structural and machine components, in order to assess their time-dependent deformation and strength behaviour.

Determination of the time-dependent deflection

The fundamental measurement requirement is to determine the time-dependent deflection s(t) of the test specimen, which can be recorded using mechanical extensometers (flexometers) or optoelectronic strain measurement systems (video extensometers, laser extensometers) (Fig. 1). However, crosshead displacement measurement can also be used; in this case, however, the machine compliance of the configuration used should be known.

Fig. 1:

Schematic illustration of the measurement of the time-dependent deflection of plastics in a flexural creep test using a time-dependent testing equipment

Conducting the flexural creep test

To generate the required stress levels, creep test machines 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 stations should be available, which are operated either with variable loading at a constant temperature or with an identical load level but at varying temperatures. When using universal testing machines, the tests must be carried out under force control, as otherwise the stress relaxation that occurs would alter the test conditions, particularly the load. In principle, flexural creep tests can be carried out in a three-point or four-point bending configuration, although the technically simpler and standardised three-point bending test is usually preferred (Fig. 1).

Definition of time-dependent creep-induced peripheral fibre strain

For the tests, prismatic test specimens measuring 80 x 10 x 4 mm³ are used in accordance with ISO 178 [2], whilst standards ISO 293 to ISO 295 [3–5] must be considered. The test specimens should be prepared in accordance with the requirements of ISO 2818 [6]. Today, flexural creep tests are usually carried out in accordance with ISO 899-2 [7] using the three-point bending method. In contrast, the withdrawn standard DIN 54852 [8] permitted the use of the three- and four-point bending methods to determine the long-term properties of plastics. Following the application of a non-impact load, the specimen deflection s_t(t) is then continuously recorded at the time intervals specified in the standard [7] until fracture or a defined time value is reached. The applied stress is defined in accordance with Eq. (1) via the applied force or the equivalent mass. The time-dependent creep periphal fibre strain is calculated from the deflection st according to Eq. (2).

$\sigma _{f0}={\frac {3\,F\,L}{2\,b\,h^{2}}}$

(1)

$\varepsilon _{t}=\varepsilon (t)={\frac {6\,s_{t}\,h}{L^{2}}}100\ \%$

(2)

Characteristic values of the flexural creep test

By analogy with the tensile creep test, the recorded semi-logarithmic creep curves (time–strain lines) are converted, in accordance with the evaluation procedure [7], into the creep diagrams, the isochronous stress–strain diagram and the creep modulus–time curves. From these curves, the characteristic values of the flexural creep test can then be calculated in accordance with standard [7]. In the case of three-point bending, the creep modulus is given by

$E_{t}(t)={\frac {F\,L^{3}}{4\,b\,h^{3}s_{t}(t)}}$

(3)

Fig. 2:

Flexural creep behaviour of PVC under various loads [1]: Creep curves (a), isochronous flexural stress-peripheral fibre strain diagram (b), creep diagram (c) and flexural creep modulus curves

An example of polyvinyl chloride (abbreviation: PVC) for a service life of up to 10⁴ h is shown in Fig. 2. Further information on graphical and tabular flexural creep data for plastics can be found in [9].

See also

• Creep behaviour – Determination
• Creep behaviour – Tensile creep test
• Creep behaviour – Creep compression test
• Creep behaviour – Recovery test
• Tensile test overlapping creep relaxation
• 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)
[2]	ISO 178 (2026-03): Plastics – Determination Flexural Properties (Draft)
[3]	ISO 293 (2023-02): Plastics – Compression Moulding of Test Specimens of Thermoplastic Materials
[4]	ISO 294-1 (2017-09): Plastics – Injection Moulding of Test Specimen of Thermoplastic Materials – Part 1: General Principles, and Moulding of Multipurpose and Bar Test Specimen
[5]	ISO 295 (2004-02): Plastics – Compression Moulding of Test Specimen of Thermosetting Materials
[6]	ISO 2818 (2018-12): Plastics – Preparation of Test Specimens by Machining
[7]	ISO 899-2 (2024-10): Plastics – Determination of Creep Behaviour – Part 2: Flexural Creep by Tree-Point Loading
[8]	DIN 54852 (1986-09): Testing of Plastics – Determination of Flexural Creep of Plastics by Three-Point Loading and Four-Point Loading (withdrawn)
[9]	Bierögel, C., Grellmann, W.: Long-term loading test. In: Grellmann, W., Seidler, S. (Eds.): Mechanical and Thermomechanical Properties of Polymers. Landoldt Börnstein. Volume VIII/6A3, Springer, Berlin (2014) pp. 286−331