Characterization and comparative study of three-dimensional braided hybridcomposites

Cartesian three-dimensional braiding as a method of preforming for hybrid c omposites has been investigated. The fundamental case of a two-sided hybrid 3-D braid was chosen. Hybrid preforms, along with a corresponding set of n on-hybrid preforms for control, were fabricated using a Cartesian braiding method. The preforms were consolidated through a Resin Transfer Molding pro cess and prepared for characterization and mechanical testing. Characteriza tion of the braided hybrid composite specimens included yarn packing and de formation within an assumed unit cell, and measurement of constituent tow f iber volume fraction using digital image analysis. A comparison study of th e elastic performance of Kevlar/epoxy and carbon/Kevlar hybrid composites w as carried out. The tension test results show a near-linear stress-strain r elationship for both specimen types within the range of the applied load. T he tensile modulus for the carbon/epoxy and hybrid composite were found to be 41 GPa and 74 GPa, respectively. In addition, the Poisson ratio of near unity for both specimen types strongly suggests a fiber dominated material response. The difference in hybrid composite transverse strain due to the d iffering constituent fiberous materials is found to be appreciable. It is b elieved that this discrepancy in Poisson contraction, between the carbon an d Kevlar sides of the specimens, causes the propagation of transverse crack s [primarily within the carbon tows] and ultimately leads to catastrophic c omposite failure. Composite ultimate strength and strain to failure were fo und to be 793 MPa and 1.9% for the Kevlar/epoxy sample and 896 MPa and 1.1% for the carbon/Kevlar hybrid. (C) 2000 Kluwer Academic Publishers.