Compressive behaviour of CFRP laminates repaired with adhesively bonded external patches

Composite patches can be used to reinforce and repair both cracked composit e and metallic aircraft structures. The repair of a composite structure wit h a composite patch may use mechanical fastening, which often introduces un desirable stress concentrations or adhesive bonding, external or hush patch es. To ensure a reliable and durable bond, various parameters such as the q uality of surface preparation and the design of the composite patch (size, shape, stiffness) are very important. This paper describes the testing of b onded external patch repaired CFRP laminates loaded in compression. It is f ound that the critical failure mechanism is fibre microbuckling in the 0 de grees plies accompanied by matrix cracking and delamination, triggered by F ailures at the adhesive/adherend interface. A three-dimensional finite elem ent analysis is performed to estimate the stress field in the repaired regi on. The calculated stresses are then used with the maximum stress and avera ge stress failure criteria to predict damage initiation, mode and location. Carefully designed external patch repairs can recover more than 80% of the undamaged compressive strength. (C) 1999 Elsevier Science Ltd. All rights reserved.