Oluschinski: Created page with "{{Language_sel|LANG=ger|ARTIKEL=Dehnrate Grundlagen}} {{PSM_Infobox}} Strain rate basics FORCETOC ==Fundamentals of strain rate== The strain rate d''ε''/d''t'' indicates the velocity distribution of the strain according to the type of test in the volume of the test specimen, either infinitesimal or integral within a defined test specimen length. In materials testing, it is as..."

2026-09-07T08:57:55Z

Created page with "{{Language_sel|LANG=ger|ARTIKEL=Dehnrate Grundlagen}} {{PSM_Infobox}} Strain rate basics __FORCETOC__ ==Fundamentals of strain rate== The strain rate d''ε''/d''t'' indicates the velocity distribution of the strain according to the type of test in the volume of the test specimen, either infinitesimal or integral within a defined test specimen length. In materials testing, it is as..."

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{{Language_sel|LANG=ger|ARTIKEL=Dehnrate Grundlagen}}
{{PSM_Infobox}}
Strain rate basics
__FORCETOC__

==Fundamentals of strain rate==

The strain rate d''ε''/d''t'' indicates the velocity distribution of the strain according to the type of test in the volume of the [[Specimen|test specimen]], either infinitesimal or integral within a defined test specimen length. In [[Materials Testing|materials testing]], it is assumed that the applied [[Test Speed|test speed]] in [[Quasi-static Test Methods|quasi-static tests]] is distributed adequately across the test specimen cross-sectional area and length via the load linkage and the clamping device or the supports or bearings. If we consider only the [[Tensile Test|tensile]] or [[Compression Test|compression test]] ('''Figs. 1a''' and '''b'''),

[[File:DehnrateGrundlagen1.JPG]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px"|Measurement of normative strain (a) in tensile testing, (b) in compression testing and (c) in three-point bending testing on plastics
|}

the nominal strain rate can be calculated directly from the [[Crosshead Speed|crosshead speed]] if the clamping length or the pressure stamp distance ''L'' is known, using d''ε''/d''t'' = ''v''T/''L''. However, since the load applied to the test specimen depends on the stiffness ratio between the [[Material Testing Machine|testing machine]] and the [[Specimen|test specimen]], the quality of the [[Specimen Clamping|clamping jaws]] and the linearity of the load line (bending effects), the quality of the drive and the [[Surface|surface]] of the test specimen (slippage) and its geometry, the actual strain rate sometimes deviates significantly from the calculated value. This can be remedied by performing [[Tensile Test Control|strain-controlled tensile]] or [[Compression Test|compression tests]], which are not standardised, however, and determining the integral normative strain and strain rate within the measuring length ''L''0 of strain extensometers.

==Influence of test specimen geometry on strain rate==

A comparison of a tensile test on prismatic test specimens and [[Multipurpose Test Specimen|multipurpose test specimens]] with shoulders for clamping shows how the test specimen geometry affects the velocity profile in the test specimen. '''Figure 2a''' shows that, assuming homogeneous and isotropic [[Material & Werkstoff|material]] behaviour and neglecting clamping effects in the prismatic test specimen, a constant distribution of strain rate is achieved (black curve in '''Fig. 2a'''). In the presence of shoulders in the upper and lower test specimen areas, the changed geometry ''A''0 will result in a lower average strain rate, which is also not constant (red curve in '''Fig. 2a'''). If the tensile test is performed conventionally with a clamping length ''l''E of 100 mm, a comparable constant [[Crosshead Speed|crosshead speed]] ''v''T and strain control, the diagrams shown in '''Fig. 2b''' are obtained. The green curves in '''Fig. 2b''' show the stress–strain behaviour (solid line in '''Fig. 2b''') and the strain rate d''ε''/d''t'' (dashed line in '''Fig. 2b''') for the conventional [[Tensile Test|tensile test]]. Compared to the [[Tensile Test Control|strain-controlled tensile test]] (red lines in '''Fig. 2b'''), there is a higher [[Tensile Strength|tensile strength]] and lower tensile strain at break, as the controlled test provides improved relaxation conditions for the [[Plastics|plastic]]. However, it is essential that there is a constant strain rate between the strain extensometers in the controlled test, whereas in the conventional [[Tensile Test|tensile test]], a comparable nominal and normative strain rate only exists at approximately 2 % strain. Even in integrally controlled tensile tests, if one considers the local strain and speed distribution within the measuring length, there are considerable differences due to the [[Heterogeneity|heterogeneity]] of the test specimen morphology (see: [[Laser Extensometry|laser extensometry]]), which can lead to increased interpretation problems when determining the true strain rate, especially in the case of constricting plastics.

[[File:DehnrateGrundlagen2.JPG]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px"|(a) (a) Schematic influence of the test specimen geometry and (b) effect of conventional and strain-controlled tensile tests on the strain rate of polyamide ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PA) with 20 m.-% GF
|}

==Strain rate of the peripheral fibre in the bending test==

Similar problems exist when determining the peripheral fibre strain rate in the [[Bend Test|bending test]] ('''Fig. 1c'''), whereby differences between the [[Bend Test#The three-point bending test method|three-point]] and [[Bend Test#The four-point bending test method|four-point bending tests]] also occur here. Particularly in the case of significant differences in the tensile and compressive behaviour of [[Plastics|plastics]], displacement of the neutral fibres can occur in the bending test, and misleading measurement effects can arise from the crosshead path measurements [38].

Due to the [[Impact Loading Plastics|impact loading]] in the presence of [[Notch|notches]], a [[Multiaxial Stress State|triaxial]] deformation state occurs in the impact or [[Free Falling Dart Method|free-falling dart test]], which also causes an inhomogeneous multi-axial distribution of the strain rate. Measurements of these velocity distributions are only possible on model materials under idealised conditions using complex testing techniques. In most cases, a theoretical analysis using FEM is used to evaluate the strain rates that occur in the load directions. However, due to the strong effects resulting from stress intensification at [[Notch Sensitivity|sharp notches]] during impact loading, a strong localisation and [[Laser Heterogeneity of Strain Distribution|inhomogeneity in the strain rate distribution]] is caused, which has a very strong effect on the [[Toughness|toughness]] properties of [[Plastics|plastics]].

==Wiki explanations of terms relating to speed==

The Wiki-lexicon "Polymer Testing & Diagnostics" from [[Polymer Service GmbH Merseburg]] (PSM) also explains the following terms in more detail under the heading [[Velocity|‘velocity’]]:

==See also==

* [[Deformation Rate|Deformation rate]]
* [[Deformation Velocity|Deformation velocity]]
* [[Test Speed|Test speed]]
* [[Strain Rate Applications|Strain rate applications]]
* [[Crosshead Speed|Crosshead speed]]

==References==

{|
|-valign="top"
|[1]
|[[Bierögel, Christian|Bierögel, C.]]: Quasi-Static Test Methods. In: [[Grellmann,_Wolfgang|Grellmann, W.]], [[Seidler,_Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 101–143 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see [[AMK-Library]] under A 22)
|}

==Weblink==

* Wikipedia – The Free Encyclopedia: [https://en.wikipedia.org/wiki/Finite_strain_theory Deformation gradient]; https://en.wikipedia.org/wiki/Finite_strain_theory (last accessed on January 4, 2026)

[[Category:Deformation]]
[[Category:Velocity]]

Strain Rate Basics - Revision history