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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Kriechverhalten Ermittlung}} {{PSM_Infobox}} Creep behaviour – Determination __FORCETOC__ ==Fundamentals== The creep behaviour of plastics can be determined under tensile, bend and compressive loading, or by means of instrumented hardness testing. This..."

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{{Language_sel|LANG=ger|ARTIKEL=Kriechverhalten Ermittlung}}
{{PSM_Infobox}}
Creep behaviour – Determination
__FORCETOC__

==Fundamentals==

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]]. This requires the measurement of the time-dependent [[Deformation|deformation]] of the [[Specimen|test specimen]], which, depending on the experimental technique used, may be the elongation, the [[Peripheral Fibre Strain|peripheral fibre strain]] or the compression, as well as the penetration depth. The load applied to the test specimen, expressed as the constant stress ''σ''0, should be adjustable for different load levels ''σ''0i.

==Test systems for determining creep behaviour==

To carry out tests at individual load levels, creep test rigs or simple [[Material Testing Machine|universal testing machines]] can be used; however, to ensure constant test conditions, these should be equipped with temperature-controlled chambers. From an economic and time-related perspective, at least 10 individual test rigs should be available, operated either with variable loading at a constant temperature or with the same [[Stress|stress]] level but at staggered temperatures. When using testing machines, the tests must be carried out under [[Tensile Test Control|force control]], as otherwise the simultaneous occurrence of [[Relaxation Plastics|stress relaxation]] would alter the test conditions. In creep test rigs, the load stress is applied using mass blocks, for which a catch device is required. To determine the strain at specified time points, each test system must be equipped with mechanical or optoelectronic extensometers, which are interrogated via a connected computer in multiplex mode ('''Fig. 1'''). Modern testing software can also acquire deformation data at shorter intervals (variable sampling rate) at the start of the experiments and shortly before the [[Fracture|fracture]] of the test specimen, thereby ensuring a higher data density during these stages of the test.

[[File:Creep_Behaviour_-_Determination_Fig-1.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Schematic illustration of the measurement of time-dependent strain in plastics during a [[Creep Behaviour – Tensile Creep Test|tensile creep test]]
|}

==Analysis of creep experiments==

As the evaluation algorithm is the same regardless of the type of [[Deformation|deformation]] (strain, [[Peripheral Fibre Strain|peripheral fibre strain]], compression or penetration depth), only the general procedure is explained here. The basis for the evaluation of creep experiments is the recording of strain up to a specified time or until [[Fracture|fracture]] of the [[Specimen|test specimen]], which is significantly influenced by the test load and temperature. As, for safety-critical structures, the [[Material & Werkstoff|materials]] used are sometimes subjected to loading for up to 105 hours or more, the creep curves and time-strain curves are plotted on a semi-logarithmic scale ('''Fig. 2a''').

[[File:K_ermittlung_2.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Schematic representation of the creep curves (a) and the isochronous stress–strain diagrams (b) for plastics in a [[Creep Behaviour – Tensile Creep Test|tensile creep test]] [1]
|}

It can be seen that, at a constant temperature and with increasing stress levels, the time–strain lines shift towards higher values. These [[Creep Plastics|creep curves]] form the basis for determining the creep diagrams and the isochronous stress–strain diagrams [1]. In the case of [[Linear-viscoelastic Behaviour|linear viscoelastic]] loading, the creep curves follow a linear trend. If vertical lines are drawn at specified time points in '''Fig. 2a''' (black dots in '''Fig. 2a'''), the pairs of values for ''σ'' and ''ε''(''t'') can be determined for those times. The ''σ''–''ε'' curves determined in this way then correspond to a respective duration of loading and represent the isochronous stress–strain diagrams ('''Fig. 2b'''). The illustration is partly supplemented by the diagram from the short-term tensile test, e.g. at a [[Strain Rate Basics|strain rate]] of 1 %/min (dashed diagram in '''Fig. 2b''').

==Creep diagram and time–stress curves==

By making horizontal cuts in the time–strain line field ('''Fig. 3a''') at defined strain values, one obtains the creep diagrams ''s''(''t''), which consist of the individual time–stress lines for the respective strain ('''Fig. 3b''').

[[File:K_ermittlung_3.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px" |Diagram of creep curves (a) and creep diagrams (b) in the [[Creep Behaviour – Tensile Creep Test|tensile creep test]] [1]
|}

Consequently, during a long test duration at low stress, the same strain is achieved as during a short time at high stress, whereby, in the extreme case, the stress–time line corresponds to the time–fracture line. For design purposes and to describe the time-dependent material behaviour under low [[Stress|stresses]], the creep modulus ''E''c (''t'') ('''Fig. 4''') is often used for [[Plastics|plastics]].

[[File:K_ermittlung_4.jpg]]
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px" |Schematic representation of creep modulus curves in a [[Creep Behaviour – Tensile Creep Test|tensile creep test]] [1]
|}

This long-term modulus is calculated as the ratio of the respective stress to the time-dependent [[Deformation|deformation]], with '''Fig. 4''' showing the decrease in modulus as a function of stress level and duration of loading. The values in '''Fig. 4'' marked with a dot (●) correspond to the short-term modulus obtained from the [[Tensile Test|tensile test]]. In principle, [[Creep Plastics|creep tests]] should be designed such that the [[Specimen|test specimens]] normally withstand a test duration of at least 103 h without [[Fracture|fracture]]. As a guideline, 30 to 50 per cent of the short-term [[Tensile Strength|tensile strength]] is recommended; below this stress level, preferably 6 but at least 4 different stress levels should be specified [1]. This approach increases the information content that can be obtained from creep tests, provided that a consistent evaluation of the test results is carried out in accordance with the functional relationships described. To extrapolate creep data to time scales of ≥ 10 years that are of interest for application purposes, experimentally validated creep curves for measurement times of ≥ 104 h should be available. Such creep experiments are usually only available at room temperature or under standard climatic conditions and for selected [[Plastics|plastics]], meaning that the wide variety of technical variants of [[Fibre-reinforced Plastics|reinforced]] and [[Particle-filled Thermoplastics|filled plastics]] cannot be covered, if only because of matrix modifications. Under [[Standard Atmospheres|standard atmospheres]], extrapolation times of up to two decades can be achieved, although [[Ageing|ageing]] effects, elevated temperatures and additional [[Stress#Tribological stress|stress]] from the medium must be regarded as problematic.

==See also==

* [[Creep Plastics|Creep plastics]]
* [[Instrumented Hardness Measurement – Creep|Instrumented hardness measurement – creep]]
* [[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]]
* [[Stepped Isothermal Method, Tensile Stress|Stepped isothermal method, tensile stress]]
* [[BOLTZMANN's Superposition Principle|BOLTZMANN's superposition principle]]
* [[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)
|}

[[Category:Creep Behaviour Plastics]]

Creep Behaviour – Determination - Revision history