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

The design of bonded composite repairs to cracked structure requires not on ly accurate stress analysis but also the confidence that the design formula e used are truly conservative. This paper describes a stress analysis of th e double-strap joint based on a modification of Hashins variational method for orthogonally cracked composite laminates in which the principle of mini mum complementary energy has been employed. The modification is essentially to introduce an adhesive layer and to include isotropic as well as orthotr opic 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 f or a parched crack. The expression is shown to yield a truly conservative e stimate, in this case a lower bound, for the equivalent spring constant. (C ) 2001 Published by Elsevier Science Ltd.