Oluschinski: Created page with "{{Language_sel|LANG=ger|ARTIKEL=Zugversuch Einflüsse}} {{PSM_Infobox}} Tensile test influences FORCETOC ==Influencing factors== In conventional tensile testing of plastics, there are numerous influencing factors that can affect the deformation behaviour and the absolute value of the material values. These factors are related to the testing techno..."

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Created page with "{{Language_sel|LANG=ger|ARTIKEL=Zugversuch Einflüsse}} {{PSM_Infobox}} Tensile test influences __FORCETOC__ ==Influencing factors== In conventional tensile testing of plastics, there are numerous influencing factors that can affect the deformation behaviour and the absolute value of the material values. These factors are related to the testing techno..."

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{{Language_sel|LANG=ger|ARTIKEL=Zugversuch Einflüsse}}
{{PSM_Infobox}}
Tensile test influences
__FORCETOC__

==Influencing factors==

In conventional [[Tensile Test|tensile testing]] of [[Plastics|plastics]], there are numerous influencing factors that can affect the [[Deformation|deformation]] behaviour and the absolute value of the [[Material Value|material values]]. These factors are related to the testing technology and conditions as well as the geometry and internal state of the test [[Specimen|specimens]]. These influences also occur when the basic conditions for reproducible characteristic value determination are met, such as comparable chemical and physical structure (see: [[Microscopic Structure|microscopic structure]]), identical geometric conditions and identical testing methodology [1]. The theoretical requirements of the [[Tensile Test|tensile test]] include impact-free load application at a constant [[Crosshead Speed|crosshead speed]], the creation of a [[Uniaxial Stress State|uniaxial load and stress state]] in the test cross-section, a homogeneous and isotropic internal state in the test specimen, and no occurrence of geometric imperfections, such as [[Notch|notches]], as well as no influences from the testing technology. Since these conditions apply to prismatic test specimens and an ideally rigid [[Material Testing Machine|testing machine]], the shoulders of the test specimens represent an imperfection and the testing technique influences the measurement result in the tensile test due to its [[Tensile Test Compliance|compliance]]. These influencing factors can be avoided or minimised by using [[Tensile Test Control|controlled tensile tests]], which, however, are not standardised for plastics, unlike the testing of metallic [[Material & Werkstoff|materials]].

==Influence of specimen shape on strain rate==

In conventional [[Tensile Test|tensile testing]], the actual nominal strain rate d''ε''t/d''t'' across the entire test specimen volume can be calculated from the set [[Crosshead Speed|crosshead speed]] ''v''T ('''Eq. 1''').

{|
|-
|width="20px"|
|width="500px" | <math>v_{T}=\frac{d\varepsilon}{dt} L</math>
|width="50px" |(1)
|}

If the normative strain rate d''ε''/d''t'' in the plane-parallel test specimen section is controlled by using extensometers with an ''L''0 of 75 mm, it can be seen that the set nominal and normative strain rates are different. The causes can be found in the shoulders of the test specimens and the effects of [[Machine Compliance|machine compliance]], as there are different and variable cross-sections ('''Fig. 1''') on the one hand, and deformations of the [[Material Testing Machine|universal testing machine]] on the other.

[[file:Z_einfluesse_1.jpg|350px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Nominal and normative [[Strain Rate|strain rate]] of a test [[Specimen|specimen]] in a tensile test
|}

It can be seen that the normative [[Velocity|test speed]] in the plane-parallel section is theoretically constant, but greater than the nominal strain rate (see '''Eq. 2''').

{|
|-
|width="20px"|
|width="500px" | <math>\varepsilon =\frac{FL}{E_{t}A_{0}L_{0}}</math>
|width="50px" |(2)
|}

The mechanical strain depends on the applied load ''F'' and the [[Elastic Modulus|modulus of elasticity]] ''E''t of the material, but also on the cross-sectional area ''A''0 of the test specimen, which is why the [[Strain Rate Basics|strain rate]] is significantly lower in the shoulder area. For standardised, constantly repeated tests, this deviation is negligible, but for scientific investigations, this difference can distort the [[Material Value|characteristic values]] to be determined. Significant differences can be seen in practical comparisons on plastic test specimens with a nominal strain rate of 1 %/min. While the nominal strain rate does not change due to the constant [[Crosshead Speed|crosshead speed]], the normative strain rate shows a dependence on the strain and only reaches the required value at one point in time (red dot in '''Fig. 2''').

[[file:Z_einfluesse_2.jpg|400px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |Nominal and normative strain rate of a PA 6 test specimen with 20 % GF in a conventional tensile test with constant [[Crosshead Speed|crosshead speed]]
|}

This behaviour is due to the [[Heterogeneity|heterogeneity]] and [[Anisotropy|anisotropy]] of the internal state resulting from differing orientations and [[Tensile Test Residual Stresses Orientations|residual stresses]] originating from the manufacturing process of the test [[Specimen|specimen]]. To minimise the influence of the shoulders on the strain rate distribution, the geometry of the test specimen must therefore be corrected over a reduced length [2, 3]. The geometric data of the test specimen ('''Fig. 3''') is used to calculate the average thickness ''b''m in the shoulder area using the auxiliary variable ''a'' ('''Eqs. 3''' and '''4'''). Knowing the different lengths in the test specimen ('''Eq. 5'''), the corrected or reduced length of the test specimen can then be determined ('''Eq. 6'''). '''Eq. (7)''' is then used to determine the required [[Velocity|test speed]] and set it as the [[Crosshead Speed|crosshead speed]] on the [[Material Testing Machine|universal testing machine]].

{|
|-
|width="20px"|
|width="500px" | <math>a=1+\frac{b}{2r}</math>
|width="50px" |(3)
|}

{|
|-
|width="20px"|
|width="500px" | <math>b_{m}=\frac{l_{m}}{\frac{a}{\sqrt{a^{2}-1}}arc tan \frac{(a+1) tan \left [ \frac{1}{2}arc sin\frac{l_{m}}{r} \right ]}{\sqrt{a^{2}-1}}-\frac{1}{2}arc sin \frac{l_{m}}{r}}</math>
|width="50px" |(4)
|}

{|
|-
|width="20px"|
|width="500px" | <math>L=l_{s}+2l_{m}+2l_{e} </math>
|width="50px" |(5)
|}

{|
|-
|width="20px"|
|width="500px" | <math>L_{red}=b\left [ \frac{l_{s}}{b}+\frac{2l_{m}}{b_{m}}+\frac{2l_{e}}{b_{e}} \right ]</math>
|width="50px" |(6)
|}

{|
|-
|width="20px"|
|width="500px" | <math>v_{T red}=\frac{d\varepsilon }{dt}L_{red}</math>
|width="50px" |(7)
|}

[[file:Z_einfluesse_3.jpg|400px]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px" |Determination of the reduced length of the test specimen for correction of the [[Velocity|test speed]]
|}

An experimental determination of a correction factor is obtained by removing the shoulders of the test specimen, resulting in a prismatic [[Specimen|test specimen]] made of the identical material ('''Fig. 4''').

[[file:Z_einfluesse_4.jpg|150px]]
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px" |Determination of the corrected [[Crosshead Speed|crosshead speed]]
|}

The normative strain rate d''ε''/d''t'' is determined on at least 5 prismatic test specimens and shoulder test specimens at an identical [[Crosshead Speed|crosshead speed]] corresponding to the nominal strain rate d''ε''t/d''t'', using extensometers with identical ''L''0, and the respective mean value is calculated. The correction factor ''k'' is obtained from the quotient of the two strain rates ('''Eq. 8'''), which can then be used to calculate the corrected crosshead speed ('''Eq. 9''').

{|
|-
|width="20px"|
|width="500px" | <math>k=\frac{\frac{d\varepsilon _{1}}{dt}}{\frac{d\varepsilon _{2}}{dt}}</math>
|width="50px" |(8)
|}

{|
|-
|width="20px"|
|width="500px" | <math>v_{Tkorr}=\frac{d\varepsilon }{dt}Lk</math>
|width="50px" |(9)
|}

==Influence of [[Machine Compliance|compliance]] on strain and [[Strain Rate Applications|strain rate]]==

When performing conventional [[Tensile Test|tensile tests]] with constant [[Crosshead Speed|crosshead speed]] in accordance with ISO 527-1 [4], the determination of characteristic values is influenced by unavoidable influencing factors. This is particularly evident in the determination of the [[Elastic Modulus|modulus of elasticity]] ''E''t according to '''Eq. (10)'''. Here, it is assumed that only the pure specimen elongation ΔLP is included in the dimensionless strain. However, due to the external load, the different components of the [[Material Testing Machine|universal testing machine]] are also deformed, which is also known as [[Machine Compliance|machine compliance]].

{|
|-
|width="20px"|
|width="500px" | <math>E_{t}=\frac{\sigma }{\varepsilon }=\frac{FL_{0}}{A_{0}\Delta L_{P}}</math>
|width="50px" |(10)
|}

Here, the deformation of the machine columns and the spindle, the bending of the crosshead and the traverse are included in the measurement signal as Δ''L''F if the traverse path is used as the [[Measured Variable|measured variable]]. The absolute errors are relatively small here. A larger proportion is contributed by the [[Deformation|deformation]] of the force transducer Δ''L''K and, in particular, the slip in the clamping jaws Δ''L''E. The measurement signal Δ''L''M therefore consists of the sum of the individual deformation components Δ''L''M = Δ''L''P + Δ''L''F + Δ''L''K + Δ''L''E and essentially determines the [[Machine Compliance|compliance of the test system]]. Any change in configuration (force transducer (see: [[Electro-mechanical Force Transducer|electro-mechanical force transducer]] and [[Piezoelectric Force Transducer|piezoelectric force transducer]]), extension rods, [[Specimen Clamping|clamping jaws]] or jaw inserts, see: [[Tensile Test Compliance|tensile test compliance]]) changes the compliance value. It is clear from this that the greater the value of the elongation Δ''L'', the smaller the modulus of elasticity becomes. To avoid these measurement effects, the [[Elastic Modulus|modulus of elasticity]] should be measured using strain extensometers or clip-on gauges, as these influencing factors then act outside the measuring length and are not recorded.

If the use of crosshead path measurement is unavoidable, e.g. for tests in a temperature-controlled chamber, the [[Tensile Test Compliance|compliance]] for the test configuration used should be known or determined in order to correct the measured strain. Since the compliance ''K'' also affects the test [[Velocity|speed]], in the case of crosshead path measurement, with knowledge of the modulus ''M'' and the clamping length ''L'', a correction must also be made to the [[Crosshead Speed|crosshead speed]] ('''Eq. 11'''), which takes into account the proportion of self-deformation of the test system.

{|
|-
|width="20px"|
|width="500px" | <math>\frac{d\varepsilon _{t}}{dt}=\frac{V_{T}}{L+A_{0}KM}</math>
|width="50px" |(11)
|}

==See also==

* [[Bend Test – Influences|Bend test – influences]]
* [[Tensile Test True Stress–Strain Diagram|Tensile test true stress–strain diagram]]
* [[Shrinkage Test|Shrinkage test]]

'''Referecnces'''

{|
|-valign="top"
|[1]
|[[Bierögel, Christian|Bierögel, C.]]: Tensile Test 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 978-1-56990-807-5; see [[AMK-Büchersammlung|AMK-Library]] under A 22)
|-valign="top"
|[2]
|DIN 53455 (1981-08): Testing of Plastics – Tensile Test (withdrawn)
|-valign="top"
|[3]
|DIN 53457 (1987-10): Testing of Plastics – Determination of Elastic Modulus by Tensile, Compression and Bend Testing (withdrawn)
|-valign="top"
|[4]
|ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles
|}

[[category: Tensile Test]]

Tensile Test Influences - Revision history