Stress analysis of double-strap bonded joints using a variational method

The design of bonded composite repairs to cracked structure requires not only accurate stress analysis but also the confidence that the design formulae used are truly conservative. This paper describes a stress analysis of the double-strap joint based on a modification of Hashin's variational method for orthogonally cracked composite laminates in which the principle of minimum complementary energy has been employed. The modification is essentially to introduce an adhesive layer and to include isotropic as well as orthotropic material as is typically found in bonded repairs. A simple analytical expression is found for the equivalent spring constant for the joint, which is a parameter that is useful in calculating the stress intensity factor for a patched crack. The expression is shown to yield a truly conservative estimate, in this case a lower bound, for the equivalent spring constant.