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In Plane Compression

Compression testing of composites is difficult because specimens are susceptible to buckling. This has led to the development of a number of test methods to successfully determine compressive, strain-at-failure, compressive modulus and Poisson's ratio. For all test methods it is critical that a uniform state of uniaxial stress is achieved through test fixturing whilst minimising stress concentrations and preventing buckling, or artificial restraints. The compressive strengths for any single material system will be different when determined by alternative test methods. Consequently selection of a compression test method will depend on the objectives of a testing program and the acceptable accuracy required.

Compression tests load the specimen 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. The following test methods are available to determine compression properties. These methods utilise specimens with unsupported test sections.

ASTM D 6484 - a combination of end loading and shear loading is applied 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. This test procedure allows for significant flexibility and is applicable for many types of composites.

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

ASTM D 5467 - Flexure of a sandwich beam is used to determine compressive properties. The sandwich beam technique comprises a honeycomb-core sandwich beam 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 – this method was developed for measuring compressive properties of rigid and reinforced plastics. Two I shaped anti-buckling support plates are utilised, which are slightly shorter than the specimen so that the ends of the specimen can be loaded. Grooving of the anti-buckling plates helps reduce the clamping area on the specimen. This test method is not recommended for determining the compressive strength of high-modulus composite materials.