For the LOV of Advanced Materials: Developing a Multiple-Site Damage Analysis Approach for Fibre Metal Laminates

Fibre metal laminates (FMLs) are comprised of alternating metal layers and composite layers bonded together. This hybrid nature of FMLs leads to superior damage tolerance which is very desirable in the aerospace sector. The fatigue resistant fibres which remain intact in the wake of fatigue cracks in metal layers of FMLs act as a second load path and reduce the stress concentration experienced at the crack tip. This load redistribution between cracked metal layers and intact bridging fibres result in superior fatigue crack resistance and longer critical crack length in FMLs compared to monolithic metal panels. These damage tolerant properties led to the successful application of FMLs as fuselage skin material on the Airbus A380 jet. 

The introduction of Limit of Validity (LOV) requirements placing limits on damage tolerance by the airworthiness regulations initiated new interest in efficient analysis method for predicting multiple-site damage (MSD) growth behaviour. The damage tolerance assessment for FMLs which mainly focuses on the analysis of the evolution of isolated crack growth has been well established. However, this tool is problematic in predicting crack growth in FMLs with multiple-site damage scenario. Therefore, it is crucial to develop analysis methodology for examining multiple-site damage crack growth behaviour in FMLs even they possess superior damage tolerant properties.

This paper proposes an analytical methodology for predicting growth behaviour of MSD cracks in FML structures. Several damage mechanisms are involved in the case of FML structures containing MSD cracks, such as crack interaction, pin-loading effects and load transfer in mechanically fastened FML joint structures. Each of them is addressed in this paper and the generalized methodology is based on the linear elastic fracture mechanics and the principle of superposition.