The potential of guided waves for monitoring large areas of metallic aircraft fuselage structure

The potential for long-range propagation of ultrasonic guided waves through metallic aircraft fuselage structure has been investigated using dispersio n analysis and numerical modelling, validated by experiment. In order to sa tisfy the pressing need for integrated structural health monitoring of agei ng metallic aircraft, it is likely that an active guided wave system based on current technology must feature efficient propagation over distances of at least Im with an attenuation of not more than about 40dB/m. Propagation was examined across free skin, tapering skin, skin loaded with sealant and paint, double skin jointed with either sealant or adhesive, and lap and str inger joints, which together adequately characterise metallic monocoque fus elage construction. Whilst the simple and tapering skins allow long range p ropagation of non-dispersive modes with little reflection at the transition to tapering skin, the attenuation caused by application of a sealant layer generally leaves no viable modes. Guided wave propagation through double s kin features the inevitable generation of twin modes with similar phase vel ocity, which interact with each other during propagation. This interaction crucially determines the efficiency of propagation across narrow joints and effectively precludes propagation across a succession of joints. This work leads to the conclusion that an active aircraft system that relies on guid ed wave propagation of more than Im is not feasible, whereas localised guid ed wave monitoring of structurally significant areas is a more practical ap proach.