The ASTM D6641 and Airbus 1.0008 standards describe the combined loading compression test.
The combined loading compression method is ideally suited for tests on composite compression specimens with larger specimen cross-sections and for compression tests with high-strength unidirectional carbon-fiber-reinforced composite laminates. Part of the compression force is applied along the gripping area via shear forces, the rest via the specimen end faces. The length of the specimen is precisely adapted to the length of the jaws and is longer than the length used for regular end loading compression tests. Because of the partial force application via the specimen end faces, preparation of the end faces is subject to strict requirements.
The ISO 14126 standard also describes the combined loading compression test on composites (Method 2).
ASTM D6641 and ISO 14126 Method 2 describe a mechanical compression test kit that consists of four elements connected by guide columns. The clamping force is generated with 8 screws, which are tightened with a torque wrench. For the test, the test fixture with installed specimen is inserted in the testing machine between two compression platens.
The method was developed further and stipulated in the Airbus standard AITM 1.0008 edition 2010, which describes both shear loading and combined loading compression. The application of clamping force is described as a hydraulic parallel clamping principle. Studies have shown that better compression tests are achieved when clamping peaks at the transition between free clamping length and clamping area are avoided by the structural design of the jaws. In the airbus standard, this is described as soft load introduction.
The patented hydraulic compression test kit HCCF, originally developed by IMA Dresden and available through ZwickRoell, meets the ASTM D6641 requirements. With this compression test kit, the specimen is gripped via manually operated compression actuators. The gripping pressure can thereby be steplessly adjusted in a wide range.
The HCCF test fixture meets the Airbus AITM 1.0008 requirements and has been tested and approved by Airbus for this purpose.
For laboratories that regularly perform compression tests, this test fixture offers excellent accessibility from the front, simplified handling, and facilitates easy cleaning.
In contrast to all other common compression test kits, the HCCF is installed in the testing machine’s load string and does not have to be removed for every new specimen. The tested specimen simply has to be taken out and the next specimen can be secured in the grips.
To determine the validity of the compression test by quantifying superimposed bending deformations (percent bending), the combined loading compression method also requires a strain measurement to be carried out with separately measuring strain gauges applied centrally on both sides. The signals of the strain gauges are then averaged for determination of the compressive strain.
With sufficient free gauge length in the center of the specimen (≥12.7 mm), the ZwickRoell HCCF compression test kit can be used with a two-sided measuring clip-on extensometer instead of the strain gauges, due to the easy accessibility.