Size-dependency of the transverse-tensile failure behavior for triaxially braided composites

Coupon specimens of triaxially braided composites suffer significant out-of-plane tension–torsion coupled deformation along the free edges, known as free-edge effect, under uniaxial tensile/compressive loading. The presence of free-edge effect causes variations in the measured stiffness and strength properties with specimen size. In this study, the size-dependent failure behavior of a single-layer triaxially braided composite under transverse tension load is investigated. Experiments are conducted to characterize the stress–strain response and failure pattern of straight-sided coupon specimens with various widths. Digital image correlation is used to monitor the free-edge-effect–induced tension-torsion coupled deformation. An improved meso-scale finite element model is developed to enable simulation of fiber bundle pull-out failure by introducing a fully covered tow-to-tow interface and plastic failure of the pure matrix zone. The model captures the size-dependent stress–strain response, free-edge effect induced premature failure and ultimate failure phenomena of the braided composite. The experimental and numerical results identify the transition of the failure mechanism from one that is dominated by fiber bundle pull-out to one that is dominated by fiber bundle shear as the specimen width is increased. The numerical simulation results reveal the correlation of free-edge effect with the progressive failure behavior, and they aid in identifying the major stress components that are contributing to the failure. The conclusions provide useful insights for the structural design and architecture optimization of triaxially braided composites.