Joint Strength Optimization of Adhesively Bonded Patches

Aircraft face damage from impact with objects or birds or due to ageing that leads to fatigue cracks. The conventional methods of repairing aircraft metallic structures generally include the use of a plate joined by screws or rivets. Although these methods are efficient in the short term, they introduce stress concentrations leading to the initiation of new cracks that are difficult or impossible to detect by non-destructive methods. For these reasons, it is necessary to develop new methods to improve the behaviour of the structure (especially for long term) and its manufacture cost. One of the solutions that have been studied by the aeronautical industry is the use of patches bonded with structural adhesives. However, adhesively bonded patches have problems of stress concentration at the edges where crack initiation is prone to occur. This problem can be reduced by the use of a taper and a spew fillet at the end of the patch and by the use of a mixed adhesive technique where a ductile adhesive is placed at the edges of the patch. Double strap specimens from 3mm thick 6063-T6 aluminium alloy sheet were analysed. Aluminium and straps (or patches) with an internal taper, an adhesive spew fillet, and dual adhesives were experimentally tested. The results obtained were explained by a finite element analysis. A taper angle is beneficial only for the brittle adhesive. The use of two adhesives is advantageous for the taperless configuration.