Compression test on long-fiber reinforced composites

The compression test on CFK materials is described as the king of tests. The specimen must be axially loaded in compression with precision during the test. Introducing the load to the specimen is problematic. Flexure and stress peaks can easily lead to erroneous test results. The HCCF compression device was tested and found to be good by a major European aircraft manufacturer in 2010.

The testing is performed in the European aviation industry and by its suppliers on the basis of Airbus standard AITM 1-0008 (2010). With smaller specimen cross-sections, the load introduction can be done by means of simple clamping (shear loading). Large specimen thicknesses require an additional load introduction across exactly processed face sides (combined loading). To avoid stress peaks, the jaws are designed with a small overhang, which is described at Airbus as “Soft load introduction".

Essentially, this test can be performed using parallel closing hydraulic grips. These are also used for tensile tests, but often lack lateral stability. This results in invalid measurements.

The aircraft manufacturer therefore investigated an alternative test arrangement: the “Hydraulic Composites Compression Fixture – HCCF” was developed and patented by IMA Materialforschung und Anwendungstechnik GmbH in Dresden. It is produced and marketed by testing machine manufacturer Zwick in Ulm.

The result was that Zwick confirmed the HCCF compression device is well suited for tests according to AITM 1-0008.

The HCCF is noted for several advantages over existing compression devices for fiber-composite tools.

• Time savings due to significantly simplified handling of the specimens, better control possibilities and the much shorter test time of the series

• Reproducible results through exact alignment, the avoidance of errors sources and a reduction in the number of invalid tests

• Variability of the device, which enables many different specimen geometries to be tested

• Parallel closing hydraulic clamping without moving wedges

The HCCF can also be used for notch compression tests (OHC, FHC) to AITM 1-0008.

Versions that can be used at temperatures of between -60 °C and +150 °C are available for testing over a wider range of temperatures.