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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Kriechverhalten Zeitstandbiegeversuch}} {{PSM_Infobox}} Creep behaviour – Flexural creep test __FORCETOC__ ==General== The creep behaviour of plastics can be determined under tensile, bend and compressive stress, or by means of Instrumented Hardness Measurement – Creep|instrumented hardness meas..."

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{{Language_sel|LANG=ger|ARTIKEL=Kriechverhalten Zeitstandbiegeversuch}}
{{PSM_Infobox}}
Creep behaviour – Flexural creep test
__FORCETOC__

==General==

The [[Creep Plastics|creep behaviour]] of [[Plastics|plastics]] can be determined under [[Tensile Test|tensile]], [[Bend Test|bend]] and [[Compression Test|compressive stress]], or by means of [[Instrumented Hardness Measurement – Creep|instrumented hardness measurement]]. The [[Creep Behaviour – Flexural Creep Test|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 [[Stress|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 [[Specimen|test specimen]], which can be recorded using mechanical extensometers (flexometers) or optoelectronic strain measurement systems ([[Video Extensometry|video extensometers]], [[Laser Extensometry|laser extensometers]]) ('''Fig. 1'''). However, crosshead displacement measurement can also be used; in this case, however, the [[Machine Compliance|machine compliance]] of the configuration used should be known.

[[File:Kriechen_Zeitstand_1.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Schematic illustration of the measurement of the time-dependent deflection of [[Plastics|plastics]] in a [[Creep Behaviour – Flexural Creep Test|flexural creep test]] using a time-dependent testing equipment
|}

==Conducting the flexural creep test==

To generate the required stress levels, creep test machines or [[Material Testing Machine|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 [[Tensile Test Control|force control]], as otherwise the [[Relaxation Plastics|stress relaxation]] that occurs would alter the test conditions, particularly the load. In principle, [[Flexural Creep Test|flexural creep tests]] can be carried out in a [[Bend Test#The three-point bending test method|three-point]] or [[Bend Test#The four-point bending test method|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 [[Testing|tests]], prismatic [[Specimen|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 Test|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 [[Bend Test#The three-point bending test method|three-]] and [[Bend Test#The four-point bending test method|four-point bending methods]] to determine the long-term properties of [[Plastics|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|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)'''.

{|
|-
|width="20px"|
|width="500px" | <math>\sigma _{f0}=\frac{3\,F\,L}{2\,b\,h^2}</math>
|width="50px" |(1)
|}

{|
|-
|width="20px"|
|width="500px" | <math>\varepsilon _{t}=\varepsilon(t)=\frac{6\,s_t\,h}{L^2}100 \ \%</math>
|width="50px" |(2)
|}

==Characteristic values of the flexural creep test==

By analogy with the [[Tensile Creep Test|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 [[Material Value|characteristic values]] of the [[Flexural Creep Test|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

{|
|-
|width="20px"|
|width="500px" | <math>E_t(t)=\frac{F\,L^3}{4\,b\,h^3s_t(t)}</math>
|width="50px" |(3)
|}

[[File:Creep_Bahaviour_-_flexural_Fig-2.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |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 ([[Plastics – Symbols and Abbreviated Terms|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|plastics]] can be found in [9].

==See also==

* [[Creep Behaviour – Determination|Creep behaviour – Determination]]
* [[Creep Behaviour – Tensile Creep Test|Creep behaviour – Tensile creep test]]
* [[Creep Behaviour – Creep Compression Test|Creep behaviour – Creep compression test]]
* [[Creep Behaviour – Recovery Test|Creep behaviour – Recovery test]]
* [[Tensile Test Overlapping Creep Relaxation|Tensile test overlapping creep relaxation]]
* [[VOIGT-KELVIN Model|VOIGT-KELVIN model]]

==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 178 (2026-03): Plastics – Determination Flexural Properties (Draft)
|-valign="top"
|[3]
|ISO 293 (2023-02): Plastics – Compression Moulding of Test Specimens of Thermoplastic Materials
|-valign="top"
|[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
|-valign="top"
|[5]
|ISO 295 (2004-02): Plastics – Compression Moulding of Test Specimen of Thermosetting Materials
|-valign="top"
|[6]
|ISO 2818 (2018-12): Plastics – Preparation of Test Specimens by Machining
|-valign="top"
|[7]
|ISO 899-2 (2024-10): Plastics – Determination of Creep Behaviour – Part 2: Flexural Creep by Tree-Point Loading
|-valign="top"
|[8]
|DIN 54852 (1986-09): Testing of Plastics – Determination of Flexural Creep of Plastics by Three-Point Loading and Four-Point Loading (withdrawn)
|-valign="top"
|[9]
|[[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/6A3, Springer, Berlin (2014) pp. 286−331
|}

[[Category:Creep Behaviour Plastics]]

Creep Behaviour – Flexural Creep Test - Revision history