Stress Redistribution in Skin/Flange Assemblies

A prestressing procedure is proposed for reduction of stress concentrations at the leading edges of adhesive bondlines in composite skin/ ange assemblies. A Ž niteelementanalysisof a speciŽ c geometryof such an assembly is presented,whichaccounts for nonlinear viscoelastic deformation of the adhesive. Design diagrams based on elastic analysis are constructed for evaluation of prestress forces that reduce or completely eliminate adhesive stress concentrations caused by either tension stresses or bending applied to the skin. Many composite structures contain skin/ ange assemblies that provide attachment and support for stringers and other parts of the structure. Adhesive joints which are typically used in such applications are exposed to large stress concentrations at the leading edges of the bondline and the adjacent tapered end of the laminated  ange. Joint failures often originate at these locations, as shown in numerous experiments by Kevin O’Brien’s group at NASA-Langley [1, 2]. The presentwork shows that the stress concentrationscontributingto such joint failures can be reduced or eliminated altogetherby certain prestress forces applied to the skin/ ange assembly prior to and then removed after adhesive cure. Prestress applicationcan be accomplished by simple Ž xtures, as described in the Ž rst section. Since a stressed adhesive can undergononlinearviscoelasticdeformationover time, we then discuss the relevantconstitutive relations for the adhesive, and details of the Ž nite-element analysis [3]. As expected, the nonlinear viscoelastic deformation is shown to reduce the adhesive stress concentrations. Next, a speciŽ c skin/ ange assembly is selected and subjected to prestress followed by adhesive cure, prestress release, and subsequent mechanical loading by tension and bending applied to the skin. Design diagrams based on elastic analysis are constructed for evaluation of prestress forces that reduce or completely cancel adhesive stress components induced at the leading edges of the bondline by selected skin loads. PRESTRESSED SKIN/FLANGE ASSEMBLY Figure 1 shows a section of unit width b of a typical skin/ ange geometry considered herein. Extending together with the skin in the x2 direction, the  ange serves to attach a The authors appreciate Ž nancial support of this work by the Ship Structures and Systems S&T Division of the OfŽ ce of Naval Research. Dr. Yapa D. S. Rajapakse served as programmonitor. The authorswould also like to acknowledge the CRC Press LLC, for permission to reproduce conference material. Address correspondenceto George J. Dvorak, 5012 JEC Rensselaer Polytechnic Institute, Troy, NY 12180-3590,USA. E-mail: [email protected]