The Effects of Through Thickness reinforcement on the Performance, Damage resistance and Damage Tolerance of CFRP Structures

Carbon fibre reinforced plastics (CFRP) are widely used in aerospace structures because they offer excellent properties, such as resistance against fatigue and corrosion, with significant weight savings. However, when in service, they are subject to impact threats such as dropped tools, runway debris or hail and the induced damage which may not be visible, can lead to significant strength reductions. Currently, individual methods for design, inspection, damage assessment and repair of composite structures are available, used albeit with various levels of confidence. Since a cost-effective, through life, service performance is a primary requirement; a need exists for a comprehensive methodology to allow the damage management of composite aircraft structures, based on an integrated approach towards design, inspection, assessment and repair. The aim of the “DAmage Management Of Composite Structures for Cost Effective Life Extensive Service (DAMOCLES)” programme was to evaluate new materials and structural concepts at the level of a wing box structure and to develop capabilities for damage tolerant design and accurate damage assessment, while taking into account inspection and repair considerations. To allow a significant number of damage tolerance concepts to be assessed, the programme was split into three phases. In the first phase concepts based on an intermediate scale were assessed and to this end four structural elements were developed that incorporated low cost manufacturing and through-thickness enhancement techniques, in the form of z-pins and stitching, in order to optimise a monolithic stiffened structure in terms of weight, performance and damage tolerance. The buckling, damage resistance and damage tolerance performance of the resulting test elements was evaluated in a modular wingbox which simulated in-service loading. The results from phase one of the structural testing are presented and compared with the results from numerical non-linear analyses. Conclusions to date are presented; these show the limitations and structural advantages of through-thickness reinforcement in suppressing delamination growth within composite structures.