Experimental Research On Dynamic Shear Failure Of Braided Composite Pin

C/C composites are regarded as the ideal thermal structure material owning to its high temperature mechanical properties, high thermal conductivity and low bulk density. But, it is difficult to fabricate a large-scale aircraft structure due to the limit of braiding technology and densification technology. So, mechanical joints are necessary for assembling an aircraft structure in engineering. To our knowledge, most of aircraft structure failure occur at joints, which consists of the failure of connecting members (such as bolts, pins and rivets) and connected members (such as aircraft skins). In this thesis, the failure of pins is studied:First of all, a composites pin cutter is designed and fabricated according to the standard of ASTM (American Society for Testing and Materials). Then, shearing experiments are implemented with different loading rates, different shear band widths and different shear directions. Finally, the effects of loading rates, shear band widths and shear directions on mechanical property of C/C composite pins are summarized as follows:1) Some effects of loading rates on failure are observed through shearing experiments on type A and type B C/C composite pins. On one hand, the shear strength of two types of C/C composite pins increases with the increase of loading rates. On the other hand, the shear stiffness of type A C/C composite pins decreases slightly with loading rate increasing.2) Some effects of shear band widths on failure are concluded. The results show that both of shear strength and shear stiffness decrease with the shear band width increasing. And, different types of failure are found via fracture analysis. With the increase of shear band widths, more carbon fibers are pulled out with longer fibers on the fracture surface.3) According to shearing experiments under different loading directions, the shear strength is unrelated to shear direction, while the shear stiffness is greatly related to shear direction. With shear direction varying from 0° to 45°, the shear stiffness decreases with the increase of loading angle. While shear direction varying from 45° to 90° the shear stiffness vibrates at a low level without obvious trend. Additionally, the failure mode under 0° loading direction differs from that under 90° loading direction. On 0° loading direction, the load-displacement curve appears like hikkside with fibers in a layer fracturing continuously. But on 90° loading direction, the load-displacement curve appears like serration with fibers in a single layer damaging simultaneously.