Study On The Shear Property Of3D Braided Composites With Opening Hole

The characteristics of the three-dimensional (3D) braided composites are the integrity of structure and excellent resistance to delamination performance. The aerospace military and other industries are main application areas. In many cases, the3D braided composites require to be connected with other components. And the3D braided composites need to be drilled and installed the bolts on the specimens to connect with other materials. However, opening hole may affect the structure and mechanical performance of composites, and the effect of opening hole and bolts loading on the performance of composites. The shear performance of3D braided composites is one of the most important parameters for composites, but most studies focus on the sheering property of laminates composites. However, only few studies about3D braided composites are reported at present.The effect of opening hole and bolt loading on the sheering property of3D four and five-directional braided composites through three-point bending shear test is studied in this paper. Four kinds of braided composite specimens are prepared in this paper:braided flat specimens without hole, specimens with a hole but without tightening bolt, specimens with a hole and tightening bolt but tightening and loosening the bolt repeatedly for5and10times respectively. The questions include applying torque on the bolts, three-point bending test after loosening the bolt, the effect of compression caused by tightening bolts on the sheering property of composites and observation of the cracks on the section of composites to analyze the shear failure mode.The results show that the process of drilling holes can cause the decrease of shear strength, but the decrease of strength is only caused by cutting off the continuous fibers at the hole. The compression caused by tightening bolts only brings light damage to the surface, and will not affect the sheering property of the specimens. The failure mode includes several reasons. The common points of four and five-directional braided composites include four aspects. The first is that the cracks distribution region in the longitudinal section is an arc-shape and the number of cracks decreases with increasing depth from the top surface to the inner part in thickness direction of specimens. The second is that the cracks distribution region on the surface is like a rectangle in the specimen without holes while the crack distribution area of specimens with a hole is cambered at the hole and rectangle again with the distance from the hole increasing. The third is that the cracks are more serious in the side surface of specimens. The last is that the cracks on the surface propagate along the fibers braiding angle direction, and the delamination failure is not found. The differentiae of four and five-directional braided composites are that in the inner part of specimens the cracks propagate along the orientation of fiber tows in four-directional specimens but the thickness direction in five-directional specimens.