Scarf repairs to highly strained graphite epoxy structure

Scarf joints representative of repairs to graphite/epoxy (gr/ep) honeycomb structure, typical of that used in the F/A-18 horizontal stabilator, have b een investigated using mechanical tests and finite-element (FE) modelling. The load capacity of such scarf repairs is marginal when tested under hot/w et conditions compared to the required design ultimate strain of 5200 mu ep silon An analysis of the scarf joint using FE methods predicts that high st resses will occur at ply drop-offs and at the top of the scarf and good cor relation is observed between the FE models and experimental results. Detail ed predictions of the shear stress distribution within the joint have also been obtained from the FE models. These show that the shear stress in the a dhesive is not uniform over the length of the scarf, as would be the case f or isotropic adherends, but rather varies with position by up to 250%, due to the varying longitudinal compliances of the plies within the gr/ep adher ends. Experimental evidence indicating the occurrence of stress relaxation or creep within the scarf joints is also reported. An F/A-18 stabilator con taining a scarf repair has been loaded satisfactorily to design limit load without failure. External patches have also been shown to be effective as t emporary repairs. (C) 1999 Aeronautical and Maritime Research Laboratory. P ublished by Elsevier Science Ltd. All rights reserved.