View source for Creep Behaviour – Creep Compression Test

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<span style="font-size:1.2em;font-weight:bold;">Creep behaviour – Creep compression test</span>
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==General==

The [[Creep Plastics|creep behaviour]] of [[Plastics|plastics]] can be determined under [[Tensile Test|tensile]], [[Bend Test|bend]] and [[Compression Test|compressive loading]], or by means of [[Instrumented Hardness Measurement – Creep|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 [[Specimen|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|machine compliance]] of the configuration used should be known.

[[File:Creep_-_Comression_Test_Fig-1.jpg|500px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 1''': 
|width="600px" |Schematic illustration of the measurement of time-dependent strain in [[Plastics|plastics]] during a [[Creep Compression Test|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 [[Material Testing Machine|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 [[Tensile Test Control|force control]]; otherwise, the simultaneous [[Relaxation Stress|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 Extensometry|video extensometers]] or [[Laser Extensometry|laser extensometers]]), which are interrogated via an online computer in multiplex mode.

==Definition of time-dependent compressive creep strain==

For the [[Testing|tests]], prismatic [[Specimen|test specimens]] in accordance with ISO 7616 [5] or DIN EN 826 [6] are generally used, whereby the [[Slenderness Ratio|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|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)'''.

{|
|-
|width="20px"|
|width="500px" | <math>\sigma _{t0}=\frac{F}{A_{0}}</math>
|width="50px" |(1)
|}

{|
|-
|width="20px"|
|width="500px" | <math>\varepsilon _{t}=\varepsilon (t) = \frac{\Delta L_{0}(t)}{L_{0}}100 \ \%</math>
|width="50px" |(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 [[Material Value|characteristic values]] can then be determined from these curves by analogy with the [[Tensile Creep Test|tensile creep test]]. The creep compression modulus is calculated according to '''Eq. (3)'''.

{|
|-
|width="20px"|
|width="500px" | <math>E_{c}(t)=\frac{\sigma}{\varepsilon(t)}</math>
|width="50px" |(3)
|}

[[File:Creep_-_Comression_Test_Fig-2.jpg]]
{| 
|- valign="top"
|width="50px"|'''Fig. 2''': 
|width="600px" |Creep compression behaviour of polytetrafluoroethylene ([[Plastics – Symbols and Abbreviated Terms|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 ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PTFE) for a stress duration of up to 10<sup>4</sup> 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|elastomers]] [8, 9]. To predict the service life of pipes and containers made of [[Plastics|plastics]], [[Creep Behaviour – Creep Internal Compression Test|creep internal compression tests]] are carried out in accordance with [10, 11].

* [[Linear-viscoelastic Behaviour|Linear-viscoelastic behaviour]]
* [[Viscoelastic Material Behaviour|Viscoelastic material behaviour]]
* [[Creep Behaviour – Determination|Creep behaviour – Determination]]
* [[Tensile Test Overlapping Creep Relaxation|Tensile test overlapping creep relaxation]]
* [[BOLTZMANN's Superposition Principle|BOLTZMANN's superposition principle]]
* [[VOIGT-KELVIN Model|VOIGT-KELVIN model]]
* [[Instrumented Hardness Measurement – Creep|Instrumented hardness measurement – Creep]]
* [[Stepped Isothermal Method, Tensile Stress|Stepped isothermal method, tensile stress]]
* [[Creep Behaviour – Recovery Test|Creep behaviour – Recovery test]]
* [[Creep Behaviour –Tensile Creep Test|Creep behaviour –Tensile creep test]]
* [[Creep Plastics|Creep plastics]]

==References==

{|
|-valign="top"
|[1]
|Höninger, H.: Long-term static behavior. In: [[Grellmann,_Wolfgang|Grellmann, W.]], [[Seidler,_Sabine|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) 
|-valign="top"
|[2]
|ISO 899-1 (2017-09): Plastics – Determination of Creep Behaviour – Part 1: Tensile Creep 
|-valign="top"
|[3]
|ISO 899-2 (2024-10): Plastics – Determination of Creep Behaviour – Part 2: Flexural Creep by Tree-Point Loading 
|-valign="top"
|[4]
|DIN EN 1606 (2013-05): Thermal Insulating Products for Building Applications – Determination of Compressive Creep (withdrawn) 
|-valign="top"
|[5]
|ISO 7616 (1986-08): Cellular Plastics, Rigid – Determination of Compressive Creep under Specific Load and Temperature Conditions 
|-valign="top"
|[6]
|DIN EN 826 (2013-05): Thermal Insulating Products for Building Applications – Determination of Compression Behaviour (withdrawn) 
|-valign="top"
|[7]
|DIN 53425 (1965-09): Testing of Rigid Cellular Materials – Time-depending Compression Test under Heat (withdrawn) 
|-valign="top"
|[8]
|ISO 8013 (2019-07): Rubber, Vulcanized – Determination of Creep Compression or Shear 
|-valign="top"
|[9]
|DIN 53522-1 (1979-01): Testing of Rubber and Elastomers – Flexing Endurance Test -- Definitions, Apparatus, Preparation of Test Pieces (withdrawn) 
|-valign="top"
|[10]
|DIN 16887 (1990-07): Determination of Long-therm Hydrostatic Pressure Resistance of Thermoplastics Pipes 
|-valign="top"
|[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) 
|-valign="top"
|[12]
|[[Bierögel, Christian|Bierögel, C.]], [https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.]: Long-term loading test. In: [https://de.wikipedia.org/wiki/Wolfgang_Grellmann Grellmann, W.], [https://www.researchgate.net/profile/Sabine-Seidler 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) 
|}

[[Category:Creep Behaviour Plastics]]