Creep Behaviour – Creep Compression Test

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Creep behaviour – Creep compression test

General

The creep behaviour of plastics can be determined under tensile, bend and compressive loading, or by means of instrumented hardness testing. The [Creep Compression Test|creep compression test]] is used for the experimental determination of the creep or long-term behaviour of bearing materials, sealing materials, as well as construction and thermal insulation materials [1], although there is no standard comparable to ISO 899-1 and -2 [2, 3].

Determination of time-dependent compression

The key measurement requirement is to determine the time-dependent strain on the test specimen, which can be measured using mechanical extensometers (clip-on gauges) or optoelectronic strain measurement systems (Fig. 1). Depending on the material under investigation and the relevant manufacturing or product standard, crosshead displacement measurement may also be used; however, the machine compliance of the configuration used should be known.

Fig. 1:

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

Conducting the creep compression test

For the technical implementation of the required load levels, creep test equipment with variable mass blocks or universal testing machines can be used; temperature-controlled chambers should be connected to ensure that test conditions remain constant. For reasons of cost-effectiveness and time efficiency, a minimum of 10 individual test stations should be available, operating either with variable loads at a constant temperature or with identical load levels but varying temperatures. Where universal testing machines are used, the tests must be carried out under force control; otherwise, the simultaneous stress relaxation would alter the test conditions, particularly the load level. To determine the compression 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 interrogated via an online computer in multiplex mode.

Definition of time-dependent compressive creep strain

For the tests, prismatic test specimens in accordance with ISO 7616 [5] or DIN EN 826 [6] are generally used, whereby the slenderness ratio must be taken into account to prevent the test specimens from buckling. Creep compression tests are usually carried out in accordance with the standards ISO 899-1 and -2 [2, 3]. Following the application of the load without impact, the specimen strain or elongation is then continuously recorded until fracture or a specified time is reached. The applied 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)

Derivation of the characteristic values of the creep compression test

The recorded creep compression curves (time-strain lines) are converted, in accordance with the evaluation procedure set out in [2, 3], into the creep diagram, the isochronous compression stress–compression strain diagram and the creep compression modulus curves. The required characteristic values can then be determined from these curves by analogy with the tensile creep test. The creep compression modulus is calculated according to Eq. (3).

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

(3)

Fig. 2:

Creep compression behaviour of polytetrafluoroethylene (abbreviation: PTFE) under various loads [1]: creep compression curves (a), isochronous compressive stress–compressive strain diagram (b), creep diagrams (c) and creep compression modulus curves (d)

An example for polytetrafluoroethylene (abbreviation: PTFE) for a stress duration of up to 10⁴ h is shown in Fig. 2. Further information on graphical and tabular compression creep data can be found in [12]. Specific applications of the creep compression test include investigations into thermal insulation materials used in the construction industry [3–7] and into damping materials made of rubber and elastomers [8, 9]. To predict the service life of pipes and containers made of plastics, creep internal compression tests are carried out in accordance with [10, 11].

• 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
• Creep behaviour –Tensile creep test
• Creep plastics

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 899-1 (2017-09): Plastics – Determination of Creep Behaviour – Part 1: Tensile Creep
[3]	ISO 899-2 (2024-10): Plastics – Determination of Creep Behaviour – Part 2: Flexural Creep by Tree-Point Loading
[4]	DIN EN 1606 (2013-05): Thermal Insulating Products for Building Applications – Determination of Compressive Creep (withdrawn)
[5]	ISO 7616 (1986-08): Cellular Plastics, Rigid – Determination of Compressive Creep under Specific Load and Temperature Conditions
[6]	DIN EN 826 (2013-05): Thermal Insulating Products for Building Applications – Determination of Compression Behaviour (withdrawn)
[7]	DIN 53425 (1965-09): Testing of Rigid Cellular Materials – Time-depending Compression Test under Heat (withdrawn)
[8]	ISO 8013 (2019-07): Rubber, Vulcanized – Determination of Creep Compression or Shear
[9]	DIN 53522-1 (1979-01): Testing of Rubber and Elastomers – Flexing Endurance Test -- Definitions, Apparatus, Preparation of Test Pieces (withdrawn)
[10]	DIN 16887 (1990-07): Determination of Long-therm Hydrostatic Pressure Resistance of Thermoplastics Pipes
[11]	DIN 53759 (1975-02): Testing of Plastics Articles – Determination of the Effect of Internal Pressure on Hollow Objects by Long-time Test an Hohlkörpern (withdrawn)
[12]	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)