Dominant effect of end distances on guarded failure of composite bolted joints

Composite materials are edging out conventional metal materials and gradually becoming an important composing changing from secondary to primary structures in aircrafts. Their application requires mechanical bolted joints to transfer loads between composite components and other composites or metallic parts. However, due to the anisotropy and brittleness of composites as well as prominent stress concentration around the fastener hole, mechanical joints are generally regarded as weak parts of structures. To exploit full potential of composite materials and achieve an expected guarded bearing failure mode in the design of composite joints, the physical effect of end distances on mechanical behaviors of composite bolted joints is required to be investigated. In current work, in conjunction with the static experiments of single-bolt joints with different end distances, a progressive damage method is introduced to trace the damage process from the onset and propagation up to ultimate failure of the joints. From the experimental and numerical results, for the bolted joints made of T800/X850 carbon/epoxy composites with lay-ups [45/0/-45/0/90/0/45/0/-45/0]_s, the understanding of the effect of the end-distance-to-hole-diameter (E/D) on the mechanical behaviors including the strength, stiffness and failure patterns of single-bolt joints is strengthened: With the increasing E/D, the failure mode of the joints was switched from a predominant cleavage to a complete bearing failure. An intrinsic critical length L_c is presented to characterize the occurrence of the cleavage failure. If the distance between the right verge of the damage area near the hole and the free end of the failed joint, which is marked as L, is no more than L_c, the cleavage failure occurs. A slightly larger L relates to a coupled cleavage and bearing failure. The larger the length L, the smaller is the proportion of cleavage failure. If L is greater than L_c, only bearing failure occurs in the bolted joint. The ratio of E/D, which proportionably reflects the length L, is thus a symbol of failure mode transition in bolted joints. In addition, the initial damage of the joints was slightly affected by the increasing E/D. However, the subsequent damage propagation and the ultimate load were significantly dependent on the E/D, in which the increase amplitude of the ultimate load exhibited descendent. An economic and suitable ratio of E/D≈3 is recommended, which exploits the potential of composite materials farthest as well controls the bearing failure mode in the majority. At the same time, as the end distance increases, the gradually enhanced bearing damage ahead of the fastener hole causes more slippage and shedding of materials in the bearing area and results in larger lateral gaps between laminates. The lateral gaps can heighten the preload of the bolt and introduce an additional bending moment and stunt the further bearing damage in reverse.