Compression tests are used to characterize various compressive properties such as the compression modulus and compressive strength. They are performed on fiber-reinforced composites with uni-directional fibers, in the 0° or 90° direction, multi-directional laminates, fibers, fabrics, and matrix materials made of synthetic plastics or thermoplastics.
In addition to tests on undamaged laminates, methods for characterizing the influence of damage in the form of a hole are used. Notched compression tests such as open hole compression (OHC) and filled hole compression (FHC) tests using an inserted screw or bolt simulates the conditions of a composite structure with a screw or rivet.
The compression tests place high demands on specimen preparation, the lab staff who perform the tests, and on the testing technology itself. When a load is introduced to the laminate, shear stress peaks and bending strain must be avoided in order to characterize the actual properties of the material.
Various test methods have been developed in the last years that each demonstrated a different strength.
This method is based on ASTM D695 and has be developed further in various standards.
One advantage is the relatively simple test arrangement. The tool requires only a support block that is positioned exactly vertical.
In this method, the specimen is loaded between two compression platens in the longitudinal direction. A support block prevents the material from prematurely failing through buckling.
The test consists of the following two parts:
- A specimen without cap strips is used to measure the compression modulus and the strain is measured with a strain gauge.
- The specimen is reinforced by cap strips to measure the compressive strength. This prevents the material from prematurely failing at the end loading points.
The method is described in ISO, ASTM, EN, DIN, and other standards. As a rule, easily reproducible compression modulus values can be achieved if the test is performed correctly, whereby large scatters often occur when determining the compressive strength.
Measurement of the compression modulus:
If specimen strain values are small, reproducible compression modulus values can be determined. If strain values are large, specimen splaying at the end loading points imposes limitations for the test method.
Applying cap strips strengthens the specimen at the end loading points on the front side, causing the specimen to break in the required measurement path. Initial delaminations occur prior to complete failure of the specimen and are visible in the diagram. This method requires very precise processing of the specimen's front surface.
In this procedure, compressive force is applied by gripping the specimen, that is via frictional contact. If the specimen is long enough, homogeneous stress distribution is achieved in the measurement path.
- One advantage to this is the good axial guiding of the specimen during the test, as well as the omission of front-side end loading and the associated need for processing the front side.
- The method was standardized in the 1980s in ASTM D3410 as the Celanese testing tool with conical clamping elements and further developed in various other standards.
- With the Celanese tool, deviations in specimen thickness lead to unwanted linear support of the gripping elements. DIN 65375 and prEN 2850 offer modified Celanese tools with flat wedges to solve the problem posed by the specimen thickness.
- IITRI in the USA developed a similar tool with column guides, which nowadays replaces the old Celanese compression tool in ASTM. This test fixture—like its predecessor model—uses a wedge clamping principle.
- The wedge jaws are first aligned to the specimen outside the compression fixture and then placed into the compression fixture.
- The gripping pressure is generated with a wedge action during the test via the axial force, resulting in wedge movements that could lead to bending in the specimen.
- IMA Dresden developed and patented the HCCF hydraulic compression tool.
- It offers excellent accessibility, simple handling, and fixed jaws, which remain precisely aligned with one another even during the test procedure.
- Parallel hydraulic specimen clamping is stick-slip free, which is not the case with wedge-based principles. Use of a clip-on extensometer is possible.
This procedure is suitable for the testing of fiber composites under the higher loads that occur when specimens have larger cross-sections. Part of the compressive force is loaded via specimen clamping; the rest on the ends of the specimen. The length of the specimen is matched precisely to the length of the jaws and is greater than that in simple end loading compression tests. Very high requirements are placed on the processing of specimen ends, as in the end loading procedure.
- ASTM D6641 and Method 2 according to ISO 14126 describe a mechanical testing tool comprising four elements connected to each other by guide columns. The clamping force is generated with 8 screws, which are tightened with a torque key. The test device with integrated specimen is placed between two compression plates in the testing machine.
- The procedure was developed further and stipulated in the Airbus standard AITM 1.0008 edition 2010, which describes both shear loading and combined loading. The application of clamping force is described as a hydraulic parallel clamping principle. Studies show that better break images 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 HCCF test fixture developed and patented by IMA Dresden fulfills the requirements of ASTM D6641. With this compression tool, the clamping force is applied hydraulically and can be set via the pressure.
- For labs that regularly perform compression tests, this test tool offers excellent accessibility from the front, simplified handling, and facilitates easy cleaning.
- The HCCF test fixture meets the requirements of AITM 1.0008. It was tested and approved by Airbus in Bremen.
- Notched compression test:
This test characterizes the influence of a hole on the compression strength of a laminate. The result is usually presented as a notch factor, which gives the ratio of damaged to undamaged specimen. According to Airbus AITM 1.0008 , this test is performed with the fixtures of the compression test. ASTM and Boeing standards use a 300 mm-long specimen, which is to be gripped by a hydraulic parallel clamping tool with the aid of a support device.
- Compression test on bolted laminates:
This test uses the same specimen as the notch tensile test, and the hole is closed with a threaded connection.