Effects of imperfections on the response of composite structures with scarf joints

Abstract

The increased use of carbon fiber reinforced polymer (CFRP) composites as a primary structural material leads to a need for repairs of such structures. One such repair method is the scarf joint, an adhesive joint that allows for the original surface profile of a structure to be maintained. Studies of scarf joints have primarily considered an idealized version of the configuration, whereas there will be imperfections that arise during manufacture and installation of a real scarf joint. Parametric studies that consider scarf joint configurations have generally considered only the linear behavior of the adhesive material, although adhesive materials typically have large nonlinear capability. Finally, in studies of scarf joints, only the stress state in the adhesive tends to be considered. In this work, the stress and strain states in both the adherends and in the adhesive are studied for imperfect scarf joints via finite element analyses. Seven parameters that characterize different imperfections, with two to four values each, are studied and compared to the baseline configuration, which is an idealized scarf joint, for the case of linear behavior of the adhesive. One to two values of each parameter are studied for the case of nonlinear behavior of the adhesive. The parameters investigated are tip bluntness, through-thickness patch offset, mismatched adherend thickness, mismatched scarf angle, adhesive thickness variation, rotated patch adherend, and delamination in the patch. The imperfections are applied to one of the adherends. The resulting strains of the different configurations are presented for the adhesive and adherends. The yield progressions are presented for the nonlinear behavior. Results show that the nonlinear behavior case can be estimated from the results of the linear behavior. The onset of yield occurs at the location of maximum strain and is primarily associated with shear strain. The yield typically occurs at the sharp adherend tip, and failure typically coincides with the location of the onset of yield. The full nonlinear behavior of the adhesive needs to be considered for any complete study of failure. The effects of imperfections are both local and global in nature for the scarf joint configuration. There is an effect on the global strain response as a result of load eccentricity due to asymmetry that leads to bending and other overall equilibrium considerations, while local effects are manifested in the strain fields near the imperfection. The tip bluntness parameter, adherend rotation parameter, and delamination parameter are found to have the most significant effects on the response of the scarf joint configuration in both the adhesive and adherend due to the specifics of the local strain field due to the imperfections. All other parameters have reduced effects on the behavior that are a result of global bending and resulting changes in the overall length of the joint. Key lengthscales are identified for imperfections and for overall joint behavior. Recommendations for further work are presented.