Study On The Bending Property Of 3-D Braided Composites

Three-dimensional braided composites are widely used in the fields of aerospace industry, automobile industry and biomedical engineering due to their excellent mechanical properties, low delimitation tendency, high energy absorbing capability and outstanding fatigue resistance. Researches on the mechanical properties of 3-D braiding composites are mainly focused on their longitudinal properties; however, the transverse mechanical properties are often ignored. This paper mainly studied on the bending properties of 3-D braiding composites, including the longitudinal and transverse bending properties of 3-D braiding stripes and plates.Based on the three-cell model of five-directional braiding, the relation between the braiding parameters were studied. An improved method to calculated the fiber volume fraction, by using of linear density of the beam of fibers, fiber density and the parameters of the braiding (including the width of the braiding, the thickness of the braiding and braiding angle), was developed in this paper, and a good agreement with the experimental results was obtained.Regarding the 3-D braiding five-directional braided composite stripes, experimental results showed that their tenacity increased with the increase of their thickness, and that their bending modulus and strength nearly kept constant at different width and thickness. For the cut composites stripes with different width and thickness, their bending properties only showed a slight decrease, and even a little increase in some case. This was mainly because of the insert of axial yarn which can weaken the nagative impact of cut.The longitudinal and transverse bending properties of composite plates (3-D braiding five-directional braided, and 3-D braiding four-directional braided) were studied. The samples used in this paper were all cut from braiding plate. The bending strength of the composites increased with the increase of the specimen width; but tend to a constant value when the width reached 1.5cm and 2cm.When the braiding angle of composites was 44°, the curve of longitudinal bending load-deflection was obviously linear, but the curve of transverse one was plastic. At the same braided angle and the same volume fraction, when the braiding angle of composites was 44°, it was found that the longitudinal bending modulus of 3-D braiding five-directional braided composites was higher by 95% than 3-D braiding four-directional braided composites, and the bending strength was higher by 98.5%; at same time, the transverse bending modulus of 3-D braiding five-directional braided composites was only slightly lower than 3-D braiding four-directional braided composites, just by 6.3%. Therefore, the mechanical properties of 3-D braiding five-directional braided composites was better than 3-D braiding four-directional braided composites.