Compression Testing of High-Strength Composite Laminates

摘要

High compression strength and high stiffness composites are attractive for a wide variety ofapplications. The testing of these materials is not always straightforward and may require specialconsideration for appropriate specimen design, even while following American Society for Testingand Materials (ASTM) standards. Specimen gage lengths must be chosen sufficiently short toguarantee that material failure occurs prior to buckling, yet must be long enough to ensure that auniform state of stress and strain exists in the gage section as not to invalidate the data reductionscheme. In this paper, we employ finite-element modeling and simulation of a combined loadingcompression (CLC) test method for high stiffness and strength composite materials characterization.Emphasis is placed on specimen geometry design in terms of buckling and the determination of stressdecay lengths (SDLs) for uniform stress. The materials of interest include a continuous aluminafiber-reinforced aluminum matrix system (MMC) and a graphite/boron fiber reinforced epoxy matrixsystem (HYBOR). Both unidirectional and cross-ply laminated specimens with various percentagesof 0 degree plies are investigated. A finite-element model composed of the tabbed specimen, testfixture, and an end plate is developed and used to evaluate SDLs of specimens. A well-knownanalytic expression is used to assess buckling in terms of specimen geometry. As expected, numericalresults show that SDLs varied linearly with the product of the specimen thickness and anisotropy. Itwas found that the HYBOR specimens exhibited much longer SDLs than the MMC specimens due totheir high anisotropy. Consequently, HYBOR laminates with a high percentage of 0 degree pliesrequired an increase in gage section thickness to preclude buckling while ensuring stress uniformityin the gage section. This situation was not found in any of the MMC specimen configurationsinvestigated due to low SLDs inherent in the material.