Composite Materials Testing

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Composte materials testing – Fundamentals

General

Fibre-reinforced composites are a composite of fibres and matrix, whereby the fibres serve to reinforce the matrix. In such composites, the matrix can consist of a thermoplastic (PP, PA or POM) or thermosetting (EP or UP resin) plastic. The mechanical properties depend primarily on the matrix material, the type of fibre, the fibre content and the fibre orientation.

The property values of the reinforcement and matrix materials are rarely additive. Since fibre-reinforced materials have a heterogeneous structure, the stresses and strains are not only time-dependent but also location- and direction-dependent when subjected to external stress.

The anisotropy of fibres

The anisotropy of this property complicates the dimensioning of components made of fibre composites, requiring special testing methods to describe the directional dependence. In order to make optimum use of the performance potential of the fibres, they are laid unidirectionally (UD), i.e. parallel in layers to the main load directions. Of all conceivable fibre arrangements, unidirectional fibre composites exhibit the lowest degree of anisotropy. Due to the three existing planes of symmetry, this is referred to as an orthotropic material. Isotropic materials are characterised by only two independent material constants. If the modulus of elasticity E and transverse contraction ν are known, then the shear modulus G can be calculated, which is not possible in the case of orthotropic fibre composites. Due to the anisotropy of the fibres and the special requirements in the various industrial sectors, the conventional test methods for plastics can only be applied to fibre composites to a limited extent. For this reason, there are a number of test methods that have been developed specifically for fibre composites and related industrial sectors.

Licensing tests for fibre composites

When testing the mechanical properties of fibre composites, it must be taken into account that these materials may already contain damage (gas bubbles, vacuoles, delamination) due to the manufacturing process. To ensure sufficient reproducibility and reliability in determining characteristic values, comprehensive quality control of fibre composites is therefore necessary after the manufacturing process and during their service life. In addition to material-related quality testing, various sub-component and complete component tests are required for licence as a load-bearing structural component, e.g. in the aerospace or wind power industry.

Fig. 1:

Certification tests depending on the number of test specimens and relative costs [1]

In the aircraft industry, the approval of a new component made of composite materials involves four main stages. Fig. 1 shows a comparison of the number of tests required in the form of a pyramid and the relative costs incurred.

Licensing of components – Component tests

In addition to material-related quality testing, sub-component or complete component tests are required for certification as a component. From bottom to top, the number of test specimens is reduced from over 1000 to one or two test specimens at the coupon test level for component testing. The relative costs incurred are listed on the right. By far the most tests are carried out at coupon test level, e.g. tensile, compression and bending tests as well as short beam tests (see: interlaminar shear strength) in dry and wet conditions. If the materials meet the required specifications, various detailed tests are carried out in the second stage, such as open-hole compression, edge delamination test (EDT) and compression after impact (CAI) (see also: compression test arrangement) tests, before the first sub-components are built and tested. The component test, which leads to a new component made of composite materials, is therefore the most expensive test and accounts for around 70 % of the costs of component development.

The coupon and detailed tests pose a particular challenge for the universal testing machine to be used. Since the deformations that occur under load are very small in fibre composites, high-resolution and precise strain measurement techniques must be used. At the same time, the material testing machine must meet the highest standards in terms of the load frame and the clamping tools used. This applies in particular to machine compliance, which should be as low as possible when testing composite materials. Due to the directional and shear sensitivity of fibre composites, all clamping elements must be precisely aligned with the load line in order to avoid so-called axiality errors such as skewing [2]. To measure skew, ZwickRoell GmbH & Co. KG] uses special measuring devices in its latest generation ‘Allround Line’ testing machine (Fig. 2) that are based on the geometry of the specific test specimens.

Fig. 2:

Universal testing machines from the “Allround Line” series by ZwickRoell GmbH & Co. KG [2]

The alignment of the tensile axes to minimise bending and torsion parts in the material testing machine is achieved using mechanical adjustment devices (alignment fixtures). This is necessary because the testing of composite materials is not only subject to international and national standards such as ISO, ASTM, EN and DIN, but also to specific internal factory regulations (Airbus, EADS, Boeing BSS), which require special testing devices for tensile, compression, bending or shear tests as well as fracture mechanics test methods.

Unidirectional (UD) fibre-reinforced plastic composites have become increasingly important for lightweight constructions. The use of a thermoplastic matrix enables their integration into the injection moulding process, for example, and thus the replacement of metal components. Due to the unidirectional continuous fibre reinforcement in the fibre direction, they meet high requirements for strength and stiffness. Their application along the load paths allows their advantages to be exploited to the full. They can either be applied directly as tape or, after consolidation of the layers, individually adapted to the component, whereby the fibre orientation of the layers is individually adapted to the component [3].

Depending on their use in the component, UD tapes are predominantly subjected to tensile stress. One of the most important properties of UD tapes is therefore their behaviour under tensile stress [4]. When UD tapes are subjected to tensile stress, special phenomena such as splicing and strand-by-strand breakage of UD tapes are observed, which influence the reproducible determination of characteristic values (see also: multiple fracture UD tapes).

In order to describe the complex fracture behaviour of UD tapes and its influence on the failure of the tapes and the laminates made from them in the component, it is necessary to develop and implement special technological investigation methods. These include fracture mechanics methods that can describe crack initiation and crack propagation and are very sensitive to structural changes in plastics [5–7].

See also

• Composite materials testing – Requirements for materials testing machines
• Fracture behaviour of plastics components
• Component testing
• Plastic component

References

[1]	Altstädt, V.: Testing of Composite Materials. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 515/516 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
[2]	https://www.zwickroell.com/de/branchen/composites (Accessed on 6 October 2025)
[3]	Monami, A., Arndt, S., Meyer, S., Lehmann, S., Grellmann, W., Langer, B., Michel, P.: Bewertung der Werkstoffeigenschaften von UD-Tapes für den Leichtbau. Technomer 2017, Chemnitz, November 9 and 10, 2017, Proceedings, pp. 1–7
[4]	Monami, A., Langer, B., Grellmann, W.]: Moderne Methoden der Kunststoffprüfung zur Werkstoffentwicklung und Bauteilprüfung. In: Christ. H.-J. (Eds.), Fortschritte in der Werkstoffprüfung für Forschung und Praxis. Stahleisen, Düsseldorf (2016) 219–224 (ISBN 978-3-514-00830-4; see AMK-Library under M 61)
[5]	Grellmann, W., Langer, B. (Eds.): Deformation and Fracture Behavior of Polymer Materials. Springer Series in Materials Science 247, Springer, Berlin Heidelberg (2017), (ISBN 978-3-319-41877-3; e-Book: ISBN 978-3-319-41879-7; see AMK-Library under A 19)
[6]	Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)
[7]	ISO 527-4 (2023-03): Plastics – Determination of Tensile Properties – Part 4: Test Conditions for Isotropic and Orthotropic Fibre-reinforced Plastic Composites