Optimization of Single-Strap Composite Joints

Adhesive joints are increasingly being utilized in joining primary structural components made of fiber reinforced polymer composites (FRP). While adhesively bonded joints transfer loads by means of shear stresses the eccentricity of the load path found in such joints results in the lateral deformation of the joint assembly and the creation of bending moments at the end portions of the adhesive layer. In the case of the single-strap joint, the magnitude of such bending moments can render the joint structurally inefficient. That said, in many practical situations, the single-strap joint leads to be the only feasible joint configuration; therefore, the understanding of its characteristics is of paramount importance. A detailed analytical investigation of the deformations of single-strap joints was carried out to better understand the dependence of edge moments on various parameters influencing the joint capacity and performance. Accurate expressions were also developed for evaluating the magnitude of the bending moment and shear forces at the ends on the adhesive layer. A complete solution is provided that can accurately predict the magnitude of the edge forces in both balanced and unbalanced single-strap joints. The edge forces obtained from the solution can be used as boundary conditions for the complete analysis of shear and peel stresses in the adhesive layer. The analytical expressions of the bending moment and shear forces can also be used to determine the upper and lower bounds of the magnitudes of the edge forces. These limits show that the efficiency of a single-strap joint can be easily made comparable to that of the commonly used single-lap joint. It will also be shown that on the other hand, a carelessly designed single-strap joint can be nothing more than a so-called "built-in stress concentration". The integrity of the analytical expressions was also verified by geometrically nonlinear finite element analysis. The results obtained from the proposed solution showed better agreement to the finite element results than those obtained from the currently available solutions cited in the literature.