View source for Compression Test Compliance

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<span style="font-size:1.2em;font-weight:bold;">Compression test compliance</span>
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==Factors influenced by the device system==

In [[Compression Test|compression tests]] to determine the stress–strain behaviour of [[Plastics|plastics]], it is generally not possible to use a mechanical strain transducer (extensometer, strain gauge) to directly measure the strain on the test specimen (10 x 10 x 4 mm<sup>3</sup>) due to the size of the test [[Specimen|specimens]] (see: [[Tensile Test|tensile test]]). This is also the case if, for example, measurements are to be taken in a temperature control chamber or the [[Deformation|deformation]] behaviour of plastics with varying filler contents is to be compared. When performing conventional compression tests with a constant [[Crosshead Speed|crosshead speed]] in accordance with ISO 604 [1], the determination of [[Material Value|material values]] is influenced by various factors. Due to external [[Stress|stress]], the various components of the [[Material Testing Machine|material testing machine]] are also deformed, which is also known as [[Machine Compliance|machine compliance]] and is of great importance for crosshead path measurement.

The deformation of the [[Load Framework|load frame]] (machine beams, spindles, drive slip, bending of the crosshead and traverse) is included in the measurement signal as Δ''L''<sub>F</sub>, although the absolute errors are relatively small here. A larger proportion is contributed by the [[Deformation|deformation]] of the deformation body of the [[Electro-mechanical Force Transducer|electro-mechanical force transducer]] Δ''L''<sub>LK</sub> and, in particular, the setting movements of the extension rod Δ''L''<sub>E</sub>.

==Self-centring and fixed pressure test arrangements==

The quality and storage principle of the [[Compression Test Arrangement|pressure plates]] (see: [[#Weblinks|weblinks]]) significantly determine the measurement result and thus have a significant influence on the characteristic values to be determined. The difference becomes clear when comparing a self-centring and a fixed [[Compression Test Arrangement|compression testing device]] ('''Fig. 1'''). Due to its design, the tolerance in the centring test device is higher, so that settling movements have a greater influence ('''Fig. 1a'''). In addition, self-centring does not occur due to the low test forces on plastics.

[[file:Compression Compliance_1.jpg]]
{| 
|- valign="top"
|width="50px"|'''Fig. 1''': 
|width="600px" |Comparison of a) self-centring and b) fixed compression testing system
|}

==Determination of compliance in the pressure test==

The path measurement signal ΔLM therefore consists of the [[Deformation|deformation]] of the test [[Specimen|specimen]] and the sum of the individual deformation components of the test equipment Δ''L''<sub>M</sub> = Δ''L''<sub>P</sub> + Δ''L''<sub>F</sub> + Δ''L''<sub>L</sub> + Δ''L''<sub>E</sub> and thus essentially determines the compliance of the test system. Any change in the configuration by the user of the [[Material Testing Machine|material testing machine]] (force transducer (see: [[Electro-mechanical Force Transducer|electro-mechanical force transducer]] and [[Piezoelectric Force Transducer|piezoelectric force transducer]]), extension rods) changes the value of the compliance K.

Due to the configuration dependency, the [[Machine Compliance|machine compliance]] is not usually specified by the [[Manufacturer of Material Testing Machines|manufacturer of the universal testing machine]]. If it is, the compliance in mm/kN is usually only the reciprocal of the [[Stiffness|stiffness]] of the [[Load Framework|load frame]] (see also: [[Machine Compliance|machine compliance]]) without additional equipment and often corresponds only to a value calculated using finite element analysis (FEA). Many testing machine manufacturers have therefore integrated software modules into the testing software that allow the compliance and a correction curve to be determined. This means that optimum displacement measurement or positioning accuracy can be guaranteed via the traverse path transducer even without the use of special strain gauges or attachment strain gauges (see: [[Tensile Test#Tensile test, path measurement technique|tensile test path measurement technology]]). However, it should be emphasised that even the best correction curves cannot replace high-precision strain transducers that measure the [[Deformation|deformation]] directly on the test [[Specimen|specimen]].

[[file:Compression Compliance 2.jpg|550px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 2''': 
|width="600px" |Determination of compliance in the compression test
|}

To determine the compliance ''C''<sub>t</sub> in the [[Compression Test|compression test]], a massive test specimen with high compressive stiffness ''E''<sub>c</sub>·''A''<sub>0</sub> and minimal self-deformation at the test force to be used should be selected. Since high test forces occur in the [[Compression Test|compression test]] at very small deformations, there is a risk of destroying the force transducer even when the switch-off threshold is activated. For this reason, a highly rigid spring with a large spring constant is better suited as a test [[Specimen|specimen]]. After centring the test specimen, the compression test is performed at a very low [[Test Speed|test speed]] up to near the nominal load of the [[Electro-mechanical Force Transducer|force transducer]] and then unloaded again while recording the data of the traverse path and the sensor path ('''Fig. 2'''). The corresponding correction curve Δ''L''<sub>T</sub>(''F'') can be calculated from the loading and unloading curve by regression. Subtracting the deformation Δ''L''<sub>F</sub> of the test specimen or the spring then produces the correction curve, which can be used for compliance correction online in the [[Compression Test|compression test]] or offline in post-processing using Excel® or Origin®. Depending on the test load, the inherent deformation component is then subtracted from the measurement signal of the traverse path ('''Fig. 3''').

[[file:Compression Compliance_3.jpg|450px]]
{| 
|- valign="top"
|width="50px"|'''Fig. 3''': 
|width="600px" |Application of the correction curve in the [[Compression Test|compression test]] with traverse path measurement
|}

==See also==

The following terms are explained in more detail in the WIKI lexicon Polymer Testing & Diagnostics:

* [[Stiffness]]
* [[Specimen Compliance|Specimen compliance]]
* [[Machine Compliance|Machine compliance]]
* [[Bend Test Compliance|Bend test compliance]]
* [[Tensile Test Compliance|Tensile test compliance]]


'''References'''

{|
|-valign="top"
|[1]
|ISO 604 (2002-03): Plastics – Determination of Compressive Properties 
|}


'''Additional literature'''

* [[Bierögel, Christian|Bierögel, C.]]: Compression Test on Polymers. In: [[Grellmann, Wolfgang|Grellmann, W.]], [[Seidler, Sabine|Seidler, S.]] (Eds.): Polymer Testing. Carl Hanser, Munich (2022), 3rd Edition, pp.125 – 133 (ISBN 978-1-56990-806-8; e-book: ISBN 978-1-56690-807-5)
* Bierögel, C., [https://de.wikipedia.org/wiki/Wolfgang_Grellmann# Grellmann, W.]: Compression Loading. In: [https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.], [https://de.wikipedia.org/wiki/Sabine_Seidler Seidler, S.]: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume VIII/6A3, Springer, Berlin (2014) pp. 150–163, (ISBN 978-3-642-55165-9; see AMK-Library under A 16)
* Voronko, Y.: Mechanische Eigenschaften von Kunststoffen im Biege- und Druckversuch. Studienarbeit, Martin-Luther-Universität Halle-Wittenberg (2009), (see AMK-Library under A 23)

==Weblinks==

* Compression Platens. Instron/Darmstadt: https://www.instron.com/de/products/testing-accessories/compression-platens-anvils-spherical-seating/
* Compression Platens. Zwick/Roell GmbH & Co. KG/Ulm: https://www.zwickroell.com/de/branchen/werkstoffpruefung-materialpruefung/druckversuch/

[[Category:Compression Test]]
[[Category:Stiffness Compliance]]