In order to reveal the different damage mechanisms of the 3D–orthogonal C/SiC composite (3D–C/SiC) rudder fins and mental bolt combined structure

and of the carbon cloth stitching C/SiC composite (2D–C/SiC) rudder fins and mental bolt combined structure

interlaminar shear tests and combined bending-torsion loading tests were conducted to investigate the influence of fiber preform configuration on the interlaminar shear and bending-torsion coupled mechanical properties of the 2D–C/SiC composites cut from the low stress area of the corresponding failed rudder fins. The results show that the 2D–C/SiC composites have lower strength property while higher stiffness property compared with the 3D–C/SiC composites. The material test results were compared with the test results of its rudder fins structure

and the reason for the very low strength of the combined 3D–C/SiC composite fins and bolt structure were analyzed. It is show that the applied load on dangerous section of the rudder fins structure is shared by the rudder shaft and the bolt. According to the basic principle in combined structure that the load is distributed based on the stiffness ratio of the components

the load borne by 3D–C/SiC composite rudder shaft with lower density and stiffness is inevitably lower

and the bolt yields early due to its relatively higher load

thus reducing the load-bearing strength of the combined structure.