The objective of the mechanical testing of composite is the determination of mechanical parameters such as strength and stiffness that will be later used on the design of a composite structure. Referring to the classical laminate theory, 5 elastic properties (E1, E2, nu12, nu23 and G12) are necessary to calculate the elastic behaviour of a long fibre reinforced composite. This can be reduced to 4 parameters (E1, E2, nu12 and G12) in case of thin laminate composite in plane stress condition. These parameters are usually determined by testing flat coupons in tensile and shear modes.
For the ultimate strength and failure prediction of a composite, further tests on compression, shear and interlaminar strength are required for determining the proper parameters of the failure criterion model.
The test methods outlined in this section merely represent a small selection available to our customers. However, taking into account the terms and conditions of the corresponding standard (DIN, ASTM, EN, ISO,…), we perform measurements using only calibrated test equipment.
For tensile tests the fiber-reinforced plastic samples are prepared with tabs to prevent a break in the restraint. For the tensile measurement either a video extensometer or a plug-on extensometer available in various sizes is used. For the different power ranges we use force sensors. Mechanical parameters are determined and the thermal behavior is investigated in the temperature range between -269°C and 600°C.
Determination of interlaminar energy release rate
Here, the interlaminar energy release rate G1C is determined , which describes the required total energy for the production of a defined crack growth under normal stress . The tensile force required for crack propagation is recorded on the traverse and a force-displacement diagram is created. The tests are carried out between -196°C and 600 °C taking into account DIN 65563.
For measuring the deflection and bending strength of fibre reinforced plastics we offer 3-point as well as 4-point bending tests. The mechanical parameters are determined between -269°C and 600°C by use of a moving coil extensometer or cross head movement of the machine.
Compression tests: modified Celanese
For our compression tests we employ the modified Celanese fixture which is most widely used. The according ASTM standard, which was also the first one specifically written for the compression testing of composite materials, is ASTM D 34101. The so-called Celanese compression test method was developed by the Celanese Corp in the early 1970s and was improved by the so called modified Celanese. This method required the use of a tabbed specimen of precise thickness, gripped between split cones. The specimen has to be prepared accurately and the test has to be performed very carefully, for otherwise the results are likely to be erratic. The tests are carried out between -269°C and 600 °C
Two Rail Shear tests
The Two-Rail Shear Test Method is described in ASTM Standard D 4255. This test method determines the in-plane shear properties of high-modulus fiber-reinforced composite materials. Laminates are clamped between two pairs of loading rails. When loaded in tension the rails introduce shear forces in the specimen. Application of this test method is limited to continuous fiber or discontinuous-fiber-reinforced polymer matrix composites in the following material forms:
- Laminates composed only of unidirectional fibrous laminae, with the fiber direction oriented either parallel or perpendicular to the fixture rails.
- Laminates composed only of woven fabric filamentary laminae with the warp direction oriented either parallel or perpendicular to the fixture rails.
- Laminates of balanced and symmetric construction, with the 0° direction oriented either parallel or perpendicular to the fixture rails.
- Short-fiber-reinforced composites with a majority of the fibers being randomly distributed.
In-Plane shear test by tensile test of a ±45° Laminate
For our analysis of polymer matrix composite materials we use the standard ASTM D3518 / D3518M – 94(2007). The described test method determines the in-plane shear response of polymer matrix composite materials reinforced by high modulus fibers. A uniaxial tension test of a ±45° laminate is performed in accordance with Test Method D 3039. The in-plane shear stress in the material coordinate system is directly calculated from the applied axial load, and the related shear stress is determined from longitudinal and transverse normal strain data obtained by the extensometers.
Inter-laminar shear strength testing (ILSS)
The inter-laminar shear strength of carbon fibre reinforced plastics and unidirectional laminates are determined in a 3-point bending test. The resistance to interlaminar shear stress parallel to the layers of the laminate is measured in accordance with DIN EN 2563. This test provides information about the quality of the resin-fiber bond.
Compression strength on tubes
To determine the compression strength and the compressive stress of fibre reinforced plastics, pressure tests are conducted. Furthermore, we determine the compression and the E-modulus by use of a video extensometer (depending on the sample dimension).
Flexural strength on large laminates
We can perform 3-point or 4-point bending tests on large fibre-reinforced plates. This means that advantageously even large components can be tested as a whole.
Tensile, compression, shear, bending and ILSS tests on fibre reinforced plastics can be conducted in our cryostat in liquid helium temperature close to absolute zero (down to 4 degrees Kelvin). These tests are performed on materials that are specifically designed for application in space technology.
For thin films and fabrics creep tests are carried out at temperatures up to 100 °C.
Tests with specific sample dimensions
For a variety of sample shapes, we can construct individual jigs and thus conduct tests which are custom-tailored and meet specific customer desires and requirements.
Test on composite rings
To determine the strength of composite rings, we use the standard ASTM D 2290. The facility in use prevents bending moments as far as possible. Furthermore, we modified the design so that a plug-on extensometer for determination of the elongation can be applied.
Single lap shear test
Single-lap specimens are economical, practical, and easy to make. Special specimens and test methods have been developed that yield accurate estimates of the true shear strength. These methods can eliminate or minimize many of the deficiencies of the single-lap specimens, but are more difficult to make and test.