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Advanced fibrous composites have a high potential for aircraft structural applications; joint design limits this potential. How to transfer loads efficiently from composite components to supporting structures is a difficult problem. The approach taken in this study is to measure the strengths of adhesive bonded joints, riveted joints, and joints containing both rivets and bonds. The data generated have contributed to defining design allowables and have given a basis for the design of composite parts. A significant variable has been found to be fiber orientation of the adherend. Three orientations of boron epoxy laminates have been bonded to titanium alloy Ti 6Al-4V: unidirectional, isotropic, and modified isotropic. By careful joint design it has been found possible to transfer over 90 percent of the tensile load carrying capability of the laminate through the joint. Both ductile and semirigid adhesives have been tested, with the former giving higher shear stresses at ½ in. overlap distances. Shear stresses of 7000 psi have been measured, which equal metal-to-metal bonds with the same adhesive system. However, when the overlap is increased to 1½ in. the semirigid type will carry approximately 20 percent more load than the ductile type. Adding rivets to bonded joints was found to have no effect on the static strength properties of composite-to-metal joints. This result is unexpected and may be because of nonoptimum design. It is anticipated that fatigue strengths would be improved by the combination joints. Composite components have been designed, built, and tested. The joints did not fail when loaded up to over 4000 psi in shear.
joints, adhesives, adherends, mechanical properties, boron, fiber composites, epoxy laminates, fiber orientation, stress concentration, design, titanium, fatigue strength, flexible resin, L/t, ratio, primer, overlap distance, mechanism of failure, faying surface, evaluation, tests
Dallas, R. N.
Division of Lockheed Aircraft Corp., Burbank, Calif.