Mechanical fasteners are commonly used in today’s composite aircraft adhesive joints. The primary purpose for using mechanical fasteners was to provide redundancy to the adhesive joints because of the uncertainties associated with an adhesive only joint. Therefore, the use of a fastener in a conventional composite hybrid joint was mainly a part of a fail-safe design. Acting as a safety backup, the fastener did not contribute tensile strength to the joint until the adhesive joint failed. When only being used for redundancy purposes in case of an adhesive joint failure, the existing fastener became simply an added weight to the aircraft structure. In this research, a proposed new design was incorporated into the composite hybrid joint where two different types of attachments were used in order to provide alternate load paths to redirect load to the fastener and utilize the fastener to provide strength to the joint once the joint was loaded. Two types of attachments were used: a stepped attachment and a curved attachment. The attachments would reduce the amount of load induced in the adhesive joint and increase the overall strength of the hybrid joint. Experiments were conducted with both the conventional hybrid joint and the new hybrid joint design with two different attachments to assess effectiveness of the new design, the more efficient attachment type, and whether the added weight of the attachments could be justified by the improvements in the overall strength of the joint. Each type of hybrid joints consisted of 15 specimens. A total of 45 specimens were prepared for this study. The specimens were tested with a MTS Systems Corporation testing apparatus with a 22 kip (±100kN) load cell. The ultimate tensile load data gathered were used to compare the strength of the different hybrid joints. From the tensile testing data collected of the three different types of specimens, the hybrid joints with attachments showed a significant improvement in ultimate tensile strength compared to the conventional hybrid joints. Conventional hybrid joints had an average ultimate tensile strength of 5859.99 lbf. Hybrid joints with curved attachments had an average ultimate tensile strength of 10617.62 lbf, which was 81.19% higher than the conventional hybrid joints. Hybrid joints with stepped attachments had an average ultimate tensile strength of 10342.14 lbf which was 76.49% higher than conventional hybrid joints. Hybrid joints with curved attachments were also found to be more efficient compared to hybrid joints with stepped attachments and improved the ultimate tensile strength 2.66%
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