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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Zugversuch Gleichmaßdehnung}} {{PSM_Infobox}} Tensile test uniform elongation (elongation without necking) __FORCETOC__ ==Uniform elongation and necking== In ductile materials with a yield point, a distinction is made between the global deformation areas ‘uniform elongation’ and ‘necking elongation’. Uniform s..."

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{{Language_sel|LANG=ger|ARTIKEL=Zugversuch Gleichmaßdehnung}}
{{PSM_Infobox}}
Tensile test uniform elongation (elongation without necking)
__FORCETOC__

==Uniform elongation and necking==

In [[Ductility Plastics|ductile]] [[Material & Werkstoff|materials]] with a [[Yield Stress|yield point]], a distinction is made between the global deformation areas ‘uniform elongation’ and ‘necking elongation’.

Uniform strain is characterised, assuming a [[Plane Stress and Strain State|plane stress state]] in the test [[Specimen|specimen]], by the fact that the longitudinal strain occurring under [[Uniaxial Stress State|uniaxial]] loading in the [[Tensile Test|tensile test]] is proportional to the resulting transverse strain. If local necking occurs as a result of reaching the yield point or the [[Tensile Strength|tensile strength]], the cross-section decreases disproportionately and the area of necking deformation begins, whereby this effect is associated with different [[Deformation|deformation]] ranges in metals and [[Plastics|plastics]].

[[file:Z_Gleichmass_1.jpg|500px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Deformation areas in ductile steels ((red = uniform strain (elongation without necking), blue = necking elongation))
|}

In the case of carbonaceous ferritic-pearlitic construction steel, a local stress maximum occurs after the linear-elastic and non-linear-elastic deformation range, which is also referred to as the upper [[Yield Stress|yield strength]] ''R''eH. This [[Material Parameter|parameter]] marks the beginning of [[Deformation#Plastic deformation|plastic deformation]] and is followed by a spontaneous decrease in force or stress. The subsequent singularities in the stress–strain behaviour are characterised by dislocation movements (Lüders lines) and the minimum corresponds to the lower yield strength ''R''eL. As a result of work hardening, the ''σ''–''ε'' curve rises to its maximum, the [[Tensile Strength|tensile strength]] ''R''m. At this point, the [[Specimen|tensile test specimen]] begins to neck down with a strong localisation of the shear strain. This distinct position is quantified using the parameter ''A''G as the difference between the plastic elongation ''L''pm and the initial gauge length ''L''0, relative to ''L''0 according to '''Eq. (1)''', and is therefore identical to the plastic strain ''ε''m:

{|
|-
|width="20px"|
|width="500px" | <math>A_{g}=\frac{L_{pm}-L_{0}}{L_{0}}100 \ \%</math>
|width="50px" |(1)
|}

==Deformation behaviour using polyamide 6 as an example==

In the case of a [[Ductility Plastics|ductile]] [[Polymer|polymer]] [[Material & Werkstoff|material]] such as polyamide 6 ([[Plastics – Symbols and Abbreviated Terms|abbreviation]]: PA 6), a local stress maximum also occurs after the linear-elastic, [[Linear-viscoelastic Behaviour|linear-viscoelastic]] and [[Elasticity|non-linear-viscoelastic]] [[Deformation|deformation]], which is also referred to as the yield strength ''σ''y ('''Fig. 2''').

[[file:Z_Gleichmass_2.jpg|500px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Deformation areas in ductile plastics ((red = uniform elongation (elongation without necking)), blue = necking elongation))
|}

This yield strength ''σ''y marks the beginning of plastic deformation and is accompanied by a spontaneous decrease in force or stress. Up to the [[Yield Stress|yield point]], uniform strain dominates due to the proportionality of longitudinal and transverse strain. This is followed by the so-called cold flow region, which is characterised by an increase in macromolecule orientation and a simultaneous decrease in entropy.

==Formation of necking==

Macroscopically, this process is recognisable by the formation of two necking fronts, which move towards the shoulders of the test [[Specimen|specimen]] as [[Deformation|deformation]] increases ('''Fig. 3'''). Due to the localisation of the strain fronts and the simultaneous disproportionate reduction in cross-section, this area is also referred to as necking strain. The increasing orientation in the already stretched areas is also reflected in an increase in [[Density|density]]. If, under the given test conditions, the necking fronts reach the shoulders of the test specimen, the hardening zone begins, which is associated with an increase in stress. If the orientation potential is not yet exhausted, especially at low [[Test Speed|test speeds]], further partial yield points may occur ('''Fig. 2'''), which manifest themselves in new cross-sectional reductions ('''Fig. 3''').

[[file:Z_Gleichmass_3.jpg|500px]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px" |Deformation phases of a ductile plastic (see '''Fig. 2''')
|}

This effect only occurs at low [[Strain Rate Basics|strain rates]] close to the equilibrium state, as this allows sufficient time for the [[Material & Werkstoff|material]] to respond to the external [[Stress|stress]] with rearrangement and orientation processes. At high [[Test Speed|test speeds]], a [[Fracture Types|brittle fracture]] with very low strain at break occurs due to a lack of [[Relaxation Plastics|relaxation conditions]].

==See also==

* [[Tensile Test Event-related Interpretation|Tensile test event-related interpretation]]
* [[Tensile Test True Stress–Strain Diagram|Tensile test true stress–strain diagram]]
* [[Laser Cross-Unit|Laser cross-unit]]
* [[Peripheral Fibre Strain|Peripheral fibre strain]]

'''References'''

* [[Bierögel, Christian|Bierögel, C.]]: Tensile Tests on Polymers. In: [[Grellmann, Wolfgang|Grellmann, W.]], [[Seidler, Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 106–123 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-806-5; see [[AMK-Büchersammlung|AMK-Library]] under A 22)

[[category:Tensile Test]]

Tensile Test Uniform Elongation - Revision history