Compressive Strength and Failure Modes of Unidirectional Composites

Abstract

Experimental and theoretical studies are presented on the failuremodes of unidirectional composites subjected to compressive loading parallel to the fiber direction. The various theories which predict microbuckling as a failure mode in compression are reviewed and compared. Nearly perfect model composites consisting of circular rods in an epoxy matrix are employed to obtain experimental data. Compression test data, including different failur modes, are obtained for composites made with brittle, anisotropic fibers and with ductile, isotropic fibers imbedded in matrix materials with Young's moduli ranging from 2 to 460 ksi. Test variables investigated include specimen geometry, fiber diameter, fiber volume fraction, and properties of constituents. It is found that all of the just mentioned parameters influence the compressive microbuckling strength. Moreover, the properties of constituents influence the modes of failure in compression. Elastic and inelastic micro-buckling is found in composites made with low- and intermediate-moduli resins and ductile fibers. For ductile fiber-rigid resin composites, the failure is by compression yielding of the reinforcement. For composites reinforced with anisotropic graphite fibers there exist three different compression failure modes: (1) microbuckling in the shear mode in the case of composites made with low modulus resin; (2) transverse tension failure, including fiber “splitting,” in the case of composites made with intermediate modulus resin; and (3) compression failure of the reinforcement in the case of composites made with high modulus resin. The test results are compared with theoretical predictions.