Compression Test Compliance
Druckversuch Nachgiebigkeit | A service provided by |
|---|
|
| Polymer Service GmbH Merseburg |
| Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de |
| Our further education offers: https://www.psm-merseburg.de/weiterbildung |
| PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg |
Compression test compliance
Factors influenced by the device system
In compression tests to determine the stress–strain behaviour of 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 mm3) due to the size of the test specimens (see: tensile test). This is also the case if, for example, measurements are to be taken in a temperature control chamber or the deformation behaviour of plastics with varying filler contents is to be compared. When performing conventional compression tests with a constant crosshead speed in accordance with ISO 604 [1], the determination of material values is influenced by various factors. Due to external stress, the various components of the material testing machine are also deformed, which is also known as machine compliance and is of great importance for crosshead path measurement.
The deformation of the load frame (machine beams, spindles, drive slip, bending of the crosshead and traverse) is included in the measurement signal as ΔLF, although the absolute errors are relatively small here. A larger proportion is contributed by the deformation of the deformation body of the electro-mechanical force transducer ΔLLK and, in particular, the setting movements of the extension rod ΔLE.
Self-centring and fixed pressure test arrangements
The quality and storage principle of the pressure plates (see: 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 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.
| Fig. 1: | 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 of the test specimen and the sum of the individual deformation components of the test equipment ΔLM = ΔLP + ΔLF + ΔLL + ΔLE and thus essentially determines the compliance of the test system. Any change in the configuration by the user of the material testing machine (force transducer (see: electro-mechanical force transducer and piezoelectric force transducer), extension rods) changes the value of the compliance K.
Due to the configuration dependency, the machine compliance is not usually specified by the manufacturer of the universal testing machine. If it is, the compliance in mm/kN is usually only the reciprocal of the stiffness of the load frame (see also: 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 path measurement technology). However, it should be emphasised that even the best correction curves cannot replace high-precision strain transducers that measure the deformation directly on the test specimen.
| Fig. 2: | Determination of compliance in the compression test |
To determine the compliance Ct in the compression test, a massive test specimen with high compressive stiffness Ec·A0 and minimal self-deformation at the test force to be used should be selected. Since high test forces occur in the 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. After centring the test specimen, the compression test is performed at a very low test speed up to near the nominal load of the force transducer and then unloaded again while recording the data of the traverse path and the sensor path (Fig. 2). The corresponding correction curve ΔLT(F) can be calculated from the loading and unloading curve by regression. Subtracting the deformation ΔLF of the test specimen or the spring then produces the correction curve, which can be used for compliance correction online in the 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).
| Fig. 3: | Application of the correction curve in the compression test with traverse path measurement |
See also
The following terms are explained in more detail in the WIKI lexicon Polymer Testing & Diagnostics:
References
| [1] | ISO 604 (2002-03): Plastics – Determination of Compressive Properties |
Additional literature
- Bierögel, C.: Compression Test on Polymers. In: Grellmann, W., 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., Grellmann, W.: Compression Loading. In: Grellmann, W., 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/