Investigation On Bending Property Of Stitching Joint 3D Braided Composites

Along with continuous expansion of three-dimensional (3-D) braided composites' application fields, some preforms with complex shapes can't be made by single three-dimensional braiding technology. Joint is necessary to meet the need of engineering design and structural integrity. Stitching joint 3-D braiding technology availably solved the limitation of three-dimensional braiding in making preforms with complex shapes. The study on the bending property of stitching joint 3-D braided composites is made in this paper. Stitching joint 3-D braided composites with various braiding angle, lap length and stitching density are experimentally studied to reveal the effect of these parameters on the bending property.According to different lap length, different stitching density and different braiding angle, the thesis designed and made several groups of 3-D 5-directional stitching joint braided composites. And unstitched joint and 3-D integral braided composites were made for contrast. All 3-D braided glass fiber reinforced epoxy resin composites were prepared by Resin transfer molding (RTM) process. Results indicate that the bending strength and modulus of stitching joint 3-D braided composites are lower than that of 3-D integral braided composites. The bending strength is lower about 30%～50%, and the bending modulus is lower 10%～30%. However, compared with that of the unstitched joint braided composites, the bending strength is higher about 20%～50%, and the bending modulus is higher 5%～15%. The bending strength and modulus are influenced by lap length and stitching density greatly. The results show that the bending property increases with the lap length increasing. When the braiding angle of composites is 35°, the bending strength of the composites with lap length 70mm decreases as stitching density increases and the bending strength of the composites with lap length 25mm, 40mm, and 55mm increases as stitching density increasing. When the braiding angle is 20°, the composites with lap length 25mm, 40mm, and 55mm show the same trend, whereas the bending strength of the composites with lap length 70mm increases at the beginning stage, and then falls with the stitching density increasing. The braiding angle has a great influence upon the bending property of the composites with lap length 70mm. For the composite with middle and high stitching density, the bending strength of the composites with braiding angle 20°is higher than that of the composites with braiding angle 35°. However for the composites with low stitching density, the result is completely contrary.Bending stress status and destruction mechanism of stitching joint 3-D braided composites were also analyzed. There are two kinds of bending destruction modes and which are also influenced by lap length and stitching density. One is the delaminating breakage at the joint plane, and the other is the permanent distortion at the loaded position, which is the same as the destruction mode of the integral braided composites.