Direction-dependent mechanical properties of a triaxially braided quasi-isotropic carbon/epoxy composite

Triaxially braided fabrics with equal volume fractions of fibers oriented at 60-degree intervals allow for the quick and easy hand-layup of quasi-isotropic (QI) composite laminates, in comparison to unidirectional tape and biaxial-woven fiber architectures. The material system for this study, T700S/MTM45-1, is the first material used in full-scale testing to demonstrate lightweight aircraft fuselage shielding for open-rotor engine applications. The objective of the current investigation is to measure the in-plane tensile modulus, Poisson’s ratio, failure strain, and tensile strength of a triaxially braided QI T700S/MTM45-1 carbon/epoxy laminate. Straight-sided specimens were cut from panels with the axial tow of the fabric oriented at angles of 0º, 30º, 45º, 60º, and 90ºrelative to the loading direction. Results were compared to previously published test results for composites consisting of the same fibers and fiber architecture but with a different (PR520) epoxy resin. Test results showed a considerable variation of strength with in-plane orientation. The initial elastic modulus and ultimate strength of the T700S/MTM45-1 material were also analyzed using classical laminated-plate theory (CLPT) and ply-discount (PD) to account for matrix cracking. As a predictive model, the quasi-laminar CLPT/PD model accurately predicted modulus but over-predicted strength. Using the model to back-calculate the best fiber-direction ply-level properties in the axial and bias directions of the braid indicated that significant adjustments of the modulus and strength can be implemented to match the initial experimental modulus and strength for nearly all specimen orientations. Reductions in these properties relative to the equivalent unidirectional tape properties were greatest in the plies representing the bias tow plies, indicating a greater degree of undulation in the bias directions due to the manufacturing process.