Static and fatigue testing bolted, bonded and hybrid step lap joints of thick carbon fibre/epoxy laminates used on aircraft structures

Abstract

Fibre reinforced polymer matrix composites are being increasingly used for aircraft structures because of their superior structural performance, such as high strength, high stiffness, long fatigue life, and low density. This study is targeted towards the repair of thicker composite step lap joint configurations. The aim is to compare the static strength and fatigue resistance of a hybrid joint consisting of both bonding and bolting, a purely fastened joint and a purely bonded joint. Parameters such as fastener array, bond strength and the influence of initial defects are investigated.Static tests results revealed there is minimal change in peak strength achieved by a hybrid joint compared to a bonded joint. Fatigue tests showed that hybrid joints have the greatest durability followed by bolted joints and finally bonded joints. The presence of fasteners in a hybrid joint reduces the peel stress and arrests crack propagation in the bondline. The fasteners also act as an adhesive bond failsafe mechanism preventing sudden catastrophic failure particularly for bondlines containing defects. Failure in the various specimen configurations ranged from bearing failure in the bolted joint cases whilst bonded and hybrid joint configurations showed first ply failure followed by net-tension failure as well as adhesive failure.