Experimental and finite element analysis of pultruded glass-graphite/epoxy hybrids in axial and flexural modes of vibration

The effects of hybridization on the extensional and flexural dynamic properties of pultruded composite materials are reported in this paper. The composite materials considered were made up of unidirectional glass, graphite fibers in an epoxy matrix, and hybrids of glass-graphite/epoxy produced by the pultrusion manufacturing process. The dynamic storage modulus and loss factor of the rectangular and circular cross-section samples were first evaluated using the nondestructive impulse-frequency response vibration technique. The long and slender flat specimens were analyzed in both extensional and flexural modes of vibration in a free-free test configuration, whereas the thicker round specimens were analyzed only in the axial model of vibrations. The properties of the monofiber type glass/epoxy and graphite/expoxy composites were used as input to the finite element model for predicting and experimental validation of various pultruded (and hypothetical) glass-graphite/epoxy hybrid combinations. The modal strain energy method was used for computing the structural loss factors of the hybrid specimens based on the element stiffness matrices and estimated mode shapes in the fundamental mode of vibration. Results of this investigation showed excellent agreement between experimental data and numerical predictions for the dynamic extensional properties of both flat and round specimens. The numerical results for dynamic flexural properties of flat specimens were found to have a similar trend as that obtained from the experimental technique. Small variations in flexural properties could be attributed to the irregular shapes (which were not taken into account in the finite element model) of layers in the hybrid combinations formed during the pultrusion manufacturing process. Results also demonstrated that while the extensional properties were independent of the fiber location and fiber packing geometry, the flexural properties were highly dependent upon these two factors. Previously reported data (which was obtained by testing these hybrid specimens in the flexural model of vibration in a clamped-free boundary condition) were re-analyzed using the free-free configuration and modal strain energy method, yielding more reasonable results.