Application of sensor technologies for local and distributed structural health monitoring

The work presented in this paper deals with the application of different sensor technologies for fatigue crack damage monitoring of metallic helicopter fuselages. A test programme has been conducted, consisting of seven fatigue crack propagation tests on aluminium panels (skin with riveted stringers) representative of the rear fuselage of a helicopter. Electrical crack gauges and comparative vacuum monitoring sensors have been used locally to monitor propagating cracks. A network of optical fibre Bragg gratings is presented as a valid possibility for distributed monitoring (based on strain field dependence on damage), alternative to consolidated electrical resistance-based strain gauges. A Smart Layer based on piezoelectric transducers that emit and receive Lamb wave signals has also been analysed in this paper for distributed monitoring. Two damages have been considered: a skin crack artificially initiated on a panel bay and a skin crack propagating from a rivet hole after stringer failure. Damage sensitivity has been evaluated and compared among the considered technologies, thus providing useful recommendations for each considered system, together with advantages and drawbacks concerning their suitability for on-board installation and monitoring. The damage index sensitivity to operative condition, load in particular, is verified and compared with damage effect for distributed networks. A finite element model able to describe any selected feature sensitivity to the monitored damage is also presented as a useful tool for the optimization of the structural health monitoring system design process.