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Fiber reinforced composites are weakened when bolt or rivet holes pierce the laminate to form mechanical joints. Premature failures occurring in the joint area have been a well recognized problem. In this investigation, unidirectional (all 0 deg), bidirectional (±45 deg), and tridirectional (0 deg, ±45 deg) graphite fiber/epoxy resin composites were fabricated to form a series of double-lap bolted joint models. These were tested to measure the joint strength effectiveness of transversely isotropic boron film colaminated locally at 6 and 12 volume percent levels. Results show that the addition of film plies to the highly directional fiber laminae increases the joint strength and stiffness up to 200 percent, the chief contribution of the boron film being high bearing strength and resistance to shear distortion. A novel test method for bolted lap joints was developed in which the bolt head pressure was maintained constant by means of preloaded springs, thus eliminating the ambiguity resulting from an inconstant contribution of the bolt clamping friction to the total joint strength. The strength weight effectiveness of boron film reinforcement in this particular application is demonstrated.
composite materials, fibers, laminates, reinforcement (structures), graphite composites, epoxy resins, boron, bolted joints, lap joints, bearing strength, stiffness, shear, isotropy, failure, aircraft structures, fasteners
Program manager, Advanced Materials and Composites, National Research Corporation, Cambridge, Mass.