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Previous elastic-plastic analysis of adhesive-bonded joints has covered double-lap, single-lap, scarf, and stepped-lap joint configurations in which the adhesive was considered to be uniform throughout. As part of the Primary Adhesively Bonded Structure Technology (PABST) program, the earlier computer programs for analyzing stepped-lap adhesive-bonded joints and doublers, A4EG and A4EH, were modified to permit the inclusion of variable adhesives as well as nonuniform adherends. This paper presents the results of the modifications. The features of the latest version of the analysis program, A4EI, are described, and examples show the effects of load redistribution around flaws and porosity. These examples are significant in establishing realistic inspection and acceptance criteria for bonded structures. The loss of strength associated with pinched-off edges is illustrated, and the benefits of deliberately thickening the adhesive layer in the immediate vicinity of the ends of the overlap are documented. The changes in shear load transfer due to softening of some of the adhesive by moisture absorption while the remainder either has never been soaked or has dried out locally are discussed. PABST experience and the analyses indicate that bonded structure is more tolerant of bond flaws than was once believed. Other new features of the A4EI program include the provision for applying a running shear load along the length of the joint, as for a wing spar, and a separate calculation for the residual thermal stresses in the absence of external load, when bonding fibrous composites to metal fittings. A modification of the coding logic has shortened run times for the various cases, which now include analysis of internal stresses associated with specified load levels in addition to the elastic and elastic-plastic strengths which could be computed by the earlier programs. Both single and double-bond surfaces can be specified.
adhesive bonding, adhesive joints, stepped-lap joints, stress analysis, structural design, computer programs, bond flaws, environmental effects, adhesive testing, composite materials
Principal engineer scientist, Douglas Aircraft Co., McDonnell Douglas Corp., Long Beach, Calif.