Round Specimen
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Round-specimen – Polymer testing
General information
In materials testing, round specimens or so-called flat tensile specimens or prismatic test specimens with defined dimensions are usually used in tensile and compression tests to determine the tensile or compressive properties as well as the modulus of elasticity E and Poisson's ratio μ [1, 2]. Regardless of the test specimen shape, tensile test specimens (see: multipurpose test specimen) are usually equipped with shoulders on both sides, which serve to ensure optimal clamping conditions and minimize measurement errors in strain measurement (see: tensile test, path measurement technique). At the same time, due to their larger cross-section, the shoulders also prevent the test specimen from fracture at the edges of the clamping device (wedge clamp or parallel clamp) or from failing within the clamping device. However, such shoulders are not necessary for compression tests on metallic or polymeric materials, where the test specimens can also have round [3] or prismatic cross-sections [4].
Tensile testing of metallic materials
In tensile tests on metallic materials (steel, aluminium, brass, etc.), so-called proportional bars are preferred, which have a specified ratio of length to width of the test specimen [1, 5, 6]. These round tensile test specimens (Fig. 1) or proportional test specimens are mostly used to characterize forged, drawn, cast, or pressed semi-finished products, whereas so-called flat tensile specimens with a prismatic cross-section are used for sheets or plates.
| Fig. 1: | Round tensile test specimens of varying geometry with proportional dimensions |
The short and long proportional bars have a length/diameter ratio of L0 = 5 d0 and L0 = 10 d0, respectively, although special forms of these test specimens are also known. Flat tensile specimens with a high width-to-thickness ratio and a thickness d < 3 mm are referred to as non-proportional test specimens, while those with d ≥ 3 mm are referred to as proportional, since the flow process above 3 mm thick plates or semi-finished products is proportional to the thickness. Other test specimen geometries differ not only in terms of their length/diameter ratio but also in terms of the design of the shoulders of the specimens [7] (Fig. 2).
| FIg. 2: | Round tensile test specimens with different geometries and shoulder designs: (a) threaded head specimen, (b) shoulder head specimen, and (c) cylindrical head specimen [7] |
Whereas tensile tests on metallic materials used to be performed at a constant crosshead speed, similar to the practice used for plastics, today strain-controlled tensile tests, and less frequently force-controlled tests, are mostly used to determine the tensile properties of metals (see also: tensile test control).
Tensile testing of plastics
In polymer testing, especially tensile and compression testing, round test specimens are rather the exception, as manufacturing problems hinder the economic requirements for test specimen production in the context of quality assurance. The reason for this is that such test specimens can only be turned from sufficiently thick extruded or cast profiles, as is possible, for example, with extruded polyvinyl chloride (abbreviation: PVC) or cast polyamide (abbreviation: PA) molds. Compared to commercial products (plates, injection-molded parts), however, a different orientation and internal stress state can be observed in round test specimens, which causes a non-comparable characteristic value level and is caused by the unusual thicknesses and cooling conditions of the semi-finished products.
In-house efforts have also been made in the past to investigate the behavior of round test specimens in comparison to prismatic test specimens (Fig. 3). The test specimens in Figs. 3a and b were turned from PVC and cast polyamide semi-finished products, whereby special manufacturing conditions must be observed [6]. When using adapted clamping jaws, these test specimens can be used for tensile and compression tests under quasi-static and static loading due to their shoulder shape, with lugs provided for attaching clip-on strain gauges. The test specimens in Figs. 3b and c are predestined for strain-controlled tests due to their design, as the minimum diameter, which is always located in the center of the test specimen, can be used as the control variable [9–12].
| Fig. 3: | Round tensile test specimens with different geometries and shoulder designs in polymer testing: (a) turned tensile and compression test specimen, (b) turned parabolic test specimen, and (c) injection-molded round test specimen |
However, due to the pressure in the injection mould and the demoulding of the test specimen, injection moulding seams with so-called webbing occur in the injection-moulded test specimens (Fig. 3c), which lead to significant disturbances in the deformation behaviour. The flow conditions of the melt, in particular the shear flow, in the mould for the round test specimens are not comparable to those of a prismatic test specimen and cause a completely different orientation state. The residual stresses in the round test specimen are also not comparable due to the limited holding pressure and sometimes cause cavities or pores in the centre of the test specimen.
The characteristic values determined in the tensile test are therefore not comparable for the different test specimen geometries and are not representative for the low thicknesses of plastic components. Due to the high time and cost involved in manufacturing round test specimens, the technically irrelevant test specimen thicknesses and the deviating characteristic value levels, these were unable to prevail over prismatic test specimens. In addition, the regulated tensile test is only used for scientific studies and is not standardised. Due to these problems, the prismatic test specimen in the form of the so-called multipurpose test specimen has become established in polymer testing. Data collections on the characteristic values of thermoplastics and their composites are based on this type of test specimen and the specifications in the relevant regulations and standards [13–16].
| Fig. 4: | Prismatic multipurpose test specimen and proportional reductions |
See also
References
| [1] | Blumenauer, H. (Ed.): Werkstoffprüfung. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart, 6th Edition (1994) (ISBN 3-342-00547-5; see AMK-Library under M 3) |
| [2] | Schmidt, W., Dietrich, H.: Praxis der mechanischen Werkstoffprüfung. Expert Publishing, Renningen-Malmheim, (1999) (ISBN 3-8169-1612-0; see AMK-Library under M 5) |
| [3] | ISO 4506 (2018-02): Hardmetals – Compression Test |
| [4] | ISO 604 (2002-03): Plastics – Determination of Compressive Properties |
| [5] | DIN EN 10002 (2001-12): Metallic Materials – Tensile Testing – Part 1: Method of Testing at Ambient Temperature (withdrawn; replaced by ISO 6892-1 (2019-11)) |
| [6] | DIN 50125 (2022-08): Testing of Metallic Materials – Tensile Test Pieces |
| [7] | ISO 6892-1 (2019-11): Metallic Materials – Tensile Testing – Part 1: Method of Test at Room Temperature |
| [8] | Bierögel, C.: Preparation of Specimen. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 15–38 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [9] | Janzen, W., Ehrenstein, G. W.: Bemessungsgrenzen von glasfaserverstärktem PBT bei schwingender Beanspruchung. Kunststoffe 81 (1991) 3, pp. 231–226 |
| [10] | G’Sell, C., Jonas, J. J.: Determination of the Plastic Behaviour of Solid Polymers at Constant True Strain Rate. J. of Mat. Science 14 (1979) 3, 583–591 DOI: https://doi.org/10.1007/BF00772717 |
| [11] | Ghosh, A. K.: Tensile Instability and Necking in Materials with Strain Hardening and Strain-Rate Hardening. Acta Metallurgica 25 (1977) 12, 1413–1424; https://doi.org/10.1016/0001-6160(77)90072-4 |
| [12] | Weis, E.-M., Wilke, W.: Structure and Mechanical Behaviour of Short Glass Fibre-Reinforced Ethylene-Tetrafluoroethylene Copolymers. J. of Mat. Science 27 (1992) 5, 1876–1882 DOI: https://doi.org/10.1007/BF01107215 |
| [13] | ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles |
| [14] | ISO 527-2 (2025-06): Plastics – Determination of Tensile Properties – Part 2: Test Conditions for Moulding and Extrusion Plastics |
| [15] | DIN EN ISO 3167 (2014-11): Plastics – Multipurpose Test Specimens |
| [16] | Bierögel, C.: Tensile Test on Polymers. In: Grellmann, W., 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-Library under A 22) |