In-plane compressive properties

The main difficulty in compression testing of composites is prevention of buckling of the specimen. Numerous methods have been developed for testing composites in compression utilising appropriate instrumentation, to determine compressive modulus, Poisson's ratio, ultimate compressive strength and/or strain-at-failure. Test fixturing must be used to introduce a uniform state of uniaxial stress while minimising stress concentrations and preventing buckling, or artificial restraints. The measured compressive strength for a single material system has been shown to differ when determined by different test methods. Final selection of a compression test method depends on the goals of the testing program and acceptable accuracy required. Other parameters found to be significant contributors to accuracy of the data include fabrication practices, control of fibre alignment, improper and/or inaccurate specimen machining, improper tabbing procedures if tabs are used, poor quality of the test fixture, improper placement of the specimen in the test fixture, improper placement of the fixture in the testing machine, and an improper test procedure.

Compression test methods load the specimen either through shear of the faces or direct compression of the ends or a combination of both. Compressive test methods may also be further classified as having a supported or an unsupported test section to prevent buckling. All of the test methods discussed in this section utilise specimens with unsupported test sections. The following test methods are available to determine compression properties.

ASTM D 3410/D 3410M uses an unsupported gauge length loaded by shear loading
This test method comprises two fixture types, the Celanese (Procedure A - conical wedge grips), and the IITRI (Procedure B - rectangular wedge grips shown in picture). These fixtures use tabbed or untabbed specimens and transfer load via wedge-type grips.

ASTM D 6484, applies a combination of end loading and shear loading to the test specimen. The fixture comprises four blocks clamped in pairs to either end of the test specimen. The surfaces of the fixture blocks in contact with the specimen are roughened, to increase the effective coefficient of friction and hence the shear load transfer. The ratio between shear and end-loading is adjusted by the torque applied to the clamping bolts. Because of the flexibility of this test method, many different types of composite materials may be tested.

ASTM D 5467, uses flexure of a sandwich beam to determine compressive properties. The sandwich beam method comprises a honeycomb-core sandwich beam that is loaded in four-point bending, placing the upper face sheet in compression. The upper and lower face sheets are of the same materials and configuration. The upper face sheet is designed to fail in compression when the beam is subjected to four-point bending. The beam is loaded to failure in bending, resulting in the measurement of compressive strength, compressive modulus and strain-at-failure if strain gauges are applied to the upper surface.

ASTM D 695 was developed for measuring compressive properties of rigid and reinforced plastics and is suitable for many types of composite material. Two I shaped anti-buckling support plates are utilised. These plates are slightly shorter than the specimen so that the ends of the specimen can be loaded. The anti-buckling plates are grooved to reduce the clamping area on the specimen. This test method is generally not adequate for determining the compressive strength of high-modulus composite materials.