Compressive behaviour of CFRP laminates repaired with adhesively bonded external patches

Composite patches can be used to reinforce and repair both cracked composite and metallic aircraft structures. The repair of a composite structure with a composite patch may use mechanical fastening, which often introduces undesirable stress concentrations or adhesive bonding, external or hush patches. To ensure a reliable and durable bond, various parameters such as the quality of surface preparation and the design of the composite patch (size, shape, stiffness) are very important. This paper describes the testing of bonded external patch repaired CFRP laminates loaded in compression. It is found that the critical failure mechanism is fibre microbuckling in the 0 degrees plies accompanied by matrix cracking and delamination, triggered by Failures at the adhesive/adherend interface. A three-dimensional finite element analysis is performed to estimate the stress field in the repaired region. The calculated stresses are then used with the maximum stress and average 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.