Flexural performance of three-dimensional braided open-hole composite with different hole diameter and preform size

Three dimensional (3-D) braided composite is one element of the structures which must connect with other structural elements, to form load transmission path during its service in aerospace, transport and engineering structures. Bolted joints are usually the lower-cost option because of its simplicity, low-cost tooling, and inspection requirements. Secondary bending induced by minor eccentricity of the loads occurs in many types of joints and can cause serious problems. In this study, flexural performance of 3-D braided open-hole composite (3DBOHC) considering open-hole diameter and preform size is systematically studied. Three-point bending test combined with a high-speed camera observation was conducted. The results show that the damage initiates in the top surface of 3DBOHC. Failure mode gradually changed from fiber breakage into interface debonding, when increasing the braiding angle. The reduction of composite strength becomes less sensitive to drilling process when braiding angle increased. The strength reduction of 3DBOHC with hole diameter of 1.5 is 13.8%, 10.3% and 5.8% for 10°, 20° and 30°, respectively. The increase in modulus and strength achieved by increasing preform size is higher for larger braiding angle 3DBOHCs. When the preform size increased from 3 ⅹ 9 to 3 ⅹ 17, the modulus increase for 10°20°and 30° 3DBOHCs is 25.4%, 42.5% and 43.2%, respectively. The strength increase for 10°20°and 30°3DBOHCs is 20.3%, 26.3% and 30.2%, respectively. This research provides a comprehensive insight for the design and application of 3-D braided composite joints.