Compression After Impact Test
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Compression after impact (CAI) test – Composite materials testing
General information
Fibre composite materials (FCM), i.e. composites made of different fibres and a thermoplastic or thermosetting plastic matrix, are increasingly being used in a wide variety of industries (see also: fibre-reinforced plastics (FRP)).
Glass fibres or high-strength, high-modulus carbon fibres are the preferred reinforcement materials for thermoset-based long or continuous fibre-reinforced composites, also known as high-performance composites. These lightweight materials can be used in the aerospace industry, medical technology, energy technology for wind turbines, building construction, sports equipment and also in the automotive industry for structural applications.
The mechanical properties depend primarily on the matrix material, the type of fibre, the fibre content, the fibre orientation and the manufacturing process (hot curing of prepregs or cold curing for laminates), whereby the property values of the reinforcement and matrix materials are rarely additive.
When testing the mechanical properties of FRP, it must be taken into account that these materials may already contain damage such as gas bubbles, micropores, foreign inclusions and delaminatiosn due to the manufacturing or processing process. For reproducibility and reliability in determining characteristic values, comprehensive quality assurance of the FRP after the manufacturing process and also during its service life is therefore essential.
In practical use, components made of FRP are often subject to very complex and overlapping stresses, which make dimensioning and design difficult and can usually only be simulated in testing practice for the simplest test specimen geometries. Without taking thermal and media loads into account, static long-term loads (tension, compression and bending) occur in combination with oscillating loads (fatigue), which cannot be recorded simultaneously with material models in approximation methods such as FEM. In addition, different material values are required for the different types of loading [1–5]. If impact loading occur during the use of the components or cracks develop due to notches or defects, the original load assumptions may no longer be valid as a result of material damage and the strength calculation must be reviewed (Fig. 1) [6, 7].
| Fig. 1: | Damage caused by impact of (a) laminates, (b) stringer-top layer composites and (c) defectoscopy of delamination using video thermography according to [8] |
In layered fibre composites such as laminates, impact loading with sufficient energy causes a so-called impact funnel (Fig. 1a), in which the damage caused by delamination, i.e. separation or debonding of the layers, is significantly greater on the inside than on the outside. If there is a stringer under the impact point, it can detach locally from the top layer, resulting in a loss of stiffness and strength (Fig. 1b). In Fig. 1c, video thermography reveals that extensive delamination has occurred across the entire width as a result of the impact load. Such damage patterns are typical of crash loads on structures in the automotive industry or on aircraft components when they are hit by ice or birds during the take-off or landing process.
Requirements of the CAI test
The compression test after impact (Compression After Impact) of laminate-type structures is used to assess the remaining compression strength after an impact in comparison to the compression strength of the undamaged test plate. This test, for which numerous standards and regulations exist, can therefore be classified as a technological test procedure [9]. The standards apply to various technical device specifications [10–16] as well as in-house standards, e.g. from Boeing, Airbus or EADS [17–20], whereby special test systems and test specimen dimensions are used. All test procedures have in common that the compression strength is first tested on undamaged test plates with defined dimensions using a line load q0 (Fig. 2a). Test plates with identical dimensions are then subjected to impact stress with specified and graduated energy contents locally in the centre of the test specimen, whereby the energy variation is achieved by different drop heights and masses of the drop bolt in the instrumented drop weight impact tester (Fig. 2b). Imaging ultrasonic testing is used to determine the damage to the laminates in the depth, x and y directions using B- or C-scan imaging techniques. The damaged test plates are then subjected to the CAI test to determine the remaining compression strength. These tests may also include thermal or medial ageing of the test specimens.
| Fig. 2: | (a) Schematic structure and principle of the CAI test and (b) puncture impact test on a laminate according to [9] |
As a rule, the influence of impact loading is then indicated by the percentage ratio of the residual compressive strength to the compression strength of the laminate materials under investigation, which is used to describe damage tolerance.
Fibre-reinforced materials are sensitive to damage caused by impact loads, which can vary in terms of velocity, stress state and local intensity, meaning that there is no standard test method for the various impact loads (see: impact loading free-falling dart test, pendulum impact tester, high-speed testing). In thermoset matrix materials in particular, shear deformation during impact stressing leads to cracks in the matrix and delamination, whereas thermoplastic FRPs are significantly more damage-tolerant. In addition to the amount of energy absorbed, the residual strength of the composite as a result of damage is of practical interest. The impact energy introduced into the laminate is dissipated over a large volume inside.
This causes delamination between layers with different fibre orientations (Fig. 2b) and, in addition, microcracks run diagonally through the layers. Normally, it is mainly the matrix or the fibre–matrix interface that is damaged, while fibre damage is limited to specific localised areas. After impact stress, the damage that is not visible from the outside but has generated large delamination areas inside the laminate is particularly critical, as it significantly weakens the load-bearing capacity of the laminate. These delaminations can then continue to grow inside the laminate, for example under oscillating loads, and reach critical dimensions.
Test technology and performance of the Compression After Impact test
The Compression After Impact Test (CAI) is used to characterise the damage tolerance of FRP. For thicker laminates, there are various out-of-plane impact tests. The most common is impact loading using an instrumented puncture impact test with precisely adjustable energy. The puncture impact test device must ensure that the bolt is caught after a possible rebound in order to avoid multiple undefined impact loads (Fig. 3a). For the compression tests, a material testing machine with as little compliance as possible is required, with pressure plates installed at the top and bottom. The CAI test device is positioned centrally between these pressure plates with the test plate clamped in place. There are two versions of this CAI tester.
| Fig. 3: | Schematic setup and principle of the free-falling dart test (a), guided CAI test according to [11, 13, 14, 16, 18] (b), and clamped CAI test according to [10, 12, 20] (c) |
The Boeing BSS, DIN, ASTM and SACMA SRM 2R test devices (Fig. 3b) operate with all-round guidance, while the CAI tests according to DIN EN, ISO and Airbus AITM have lateral guidance and the upper and lower parts of the test plate are clamped in place. In both cases, the test specimen is longer than the lateral guide (76.2 x 127 mm² for Boeing and 75 x 125 mm² for Airbus, for example) in order to induce the compressive loading into the plate. Before the CAI tests are carried out, the damage must be analysed with an imaging ultrasonic testing system (e.g. HFUS 2000 from Dr. Hillger, Braunschweig) after the impact damage has been generated with a falling bolt (diameter 16 mm). For this purpose, C-scans of the test specimens are produced in order to determine the damage area As as a function of the impact energy EH, from which the damage resistance of the laminate is then calculated.
As a rule, three to five test panels (101.6 x 152.4 mm²) are produced for the examination of the raw material and per energy level and subjected to impact energies of varying heights, whereby the multi-directional laminate consists of 24 to 48 prepreg layers depending on the weight per unit area. After impact loading and ultrasonic characterisation, the test specimens for the CAI test are cut from these plates.
The design of the pressure testing device (Figs. 3b and c) allows reproducible characteristic values to be determined for quasi-isotropic laminates, even those with low intrinsic strength. The test specimen is subjected to pressure at a speed of 1.3 mm min-1. The compressive strength determined in this way is referred to as the residual compressive strength after impact stress.
Evaluation of the Compression After Impact test
To determine the damage resistance Sw, the residual compressive strength σRM determined in the compression test is plotted as a function of the impact energy EH applied. Knowing the relationship between residual compressive strength and damage area As (Fig. 4), the damage tolerance of a fibre-reinforced plastic can be determined. A fibre-reinforced plastic with a higher residual compressive strength for an identical damage area is considered to be damage-tolerant.
| Fig. 4: | Damage area of the laminate after the puncture impact test using C-Scans for (a) EH = 12 J, (b) EH = 6,3 J und (c) EH = 1,6 J according [21] on medially damaged test plates |
When glass fibre reinforced laminates are subjected to medial exposure to water vapour and lye at 90 °C, it can be seen that, with increasing impact energy, the energy absorption and the damaged area increase significantly compared to the undamaged material, with the undamaged plates being penetrated at 40 J and the medially loaded test specimens already being penetrated at 25 J. Energies up to 14.3 J did not result in any reduction in residual compressive strength (Fig. 5). The areas of damage caused are significantly smaller in the medially displaced test plates than in the initial state, which indicates a plasticising effect.
| Fig. 5: | Residual compressive strength of medially displaced test plates (water vapor, 90 °C) in the CAI test according to [21] |
See also
- Composite materials testing
- Impact loading high-speed testing
- Compression test arrangement
- C-scan technique
- Object raster method
References
| [1] | Erhard, G.: Konstruieren mit Kunststoffen. Carl Hanser, Munich (2008) 4th Edition, (ISBN 978-3-446-41646-8; see AMK-Library under G 59) |
| [2] | Ehrenstein, G. W. (Eds.): Handbuch Kunststoff-Verbindungstechnik. Carl Hanser, Munich (2004) 1st Edition, (ISBN 978-3-446-22668-5) |
| [3] | Stommel, M., Stojek, M.: FEM zur Berechnung von Kunststoff- und Elastomerbauteilen. Carl Hanser, Munich (2011) 1st Edition; (ISBN 978-3-446-42124-0) |
| [4] | Schürmann, H.: Konstruieren mit Faser-Kunststoff-Verbunden. Springer, Berlin Heidelberg (2007) 2nd Edition, (ISBN 978-3-540-72189-5) |
| [5] | Moser, K.: Faser-Kunststoff-Verbund. Springer, Berlin (2013) 3nd Edition, (ISBN 978-3-642-58092-5) |
| [6] | Grellmann, W.: Schlagartige Beanspruchung. In: Grellmann, W., Seidler, S. (Hrsg.): Kunststoffprüfung. Carl Hanser, Munich (2015) 3rd Edition, pp. 158–170, (ISBN 978-3-446-44350-1; see AMK-Library under A 18) |
| [7] | Grellmann, W.: Zähigkeitsbewertung mit bruchmechanischen Methoden. In: Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition, pp. 237–291 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23) |
| [8] | Ridzewski, J.: Herausforderungen an die experimentelle Nachweiskette zur sicheren Auslegung von Compositebauteilen. KKZ-Kolloquium 2012, Merseburg, December 23, 2012 |
| [9] | Altstädt, V.: Prüfung von Verbundwerkstoffen. In: Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition, pp. 527/528 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23) |
| [10] | ISO 18352 (2009-08): Carbon-fibre-reinforced Plastics – Determination of Compression-After-Impact Properties at a Specified Impact-Energy Level |
| [11] | DIN 65561 (1991-05): Aerospce – Fibre-reinforced Plastics – Testing of Multidirectional Laminates – Determination of Compressive Strength after Impact Test (withdrawn; recommended for use: ISO 18352 (2009-08) |
| [12] | DIN EN 6038 (2016-02): Aerospace Series – Fibre Reinforced Plastics – Test Method – Determination of the Compression Strength After Impact |
| [13] | ASTM D 7136/D 7136M (2020): Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event |
| [14] | ASTM D 7137/D 7137M (2017): Standard Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plates |
| [15] | JIS K 7089 (1996-03): Testing Method for Compression after Impact Properties of Carbon Fibre Reinforced Plastics |
| [16] | BSS 7260 Boeing – Type II (2009): Advanced Composite Compression Test |
| [17] | CRAG Method 403 (1988): Compression After Impact |
| [18] | SACMA SRM 2R-97 (1994): Compression after Impact – Properties of Oriented Fibre-Resin Composites |
| [19] | NASA RP 1092 ST-1 (1982): Compression After Impact and Open-Hole Compression Fixture |
| [20] | AITM 1.0010-Airbus Industries Test methods (1994): Fibre Reinforced Plastics – Determination of Compression Strength after Impact-Stress |
| [21] | Walther, H., Bierögel, C., Grellmann, W., Rufke. B.: Einfluss der medialen Auslagerung auf das Impactverhalten glasfaserverstärkter Kunststoffe. In: Grellmann, W., Seidler, S. (Eds.): Deformation und Bruchverhalten von Kunststoffen. Springer Verlag, Berlin (1998), pp. 471–480 (ISBN 3-540-63671-4; see AMK-Library under A 6) |