Ultrasonic Lamb Wave Inspection of Aircraft Components Using Integrated Optical Fibre Sensing Technology

As continuous health monitoring of aircraft components will provide substantial time and cost savings to the aircraft industry, the use of an integrated sensing technology with autonomous damage assessment is looked for. This study is part of the 6 STREP framework AISHA project, investigating the use of ultrasonic Lamb wave technology for the fast inspection of large shell structures. For the detection of the generated ultrasonic Lamb waves and their alterations due to damage, single mode (SM) optical fibres embedded in a SMARTape and glued on a structural part or embedded in the composite have been used. This paper presents the different ways the optical fibre sensor is implemented in order to detect typical Lamb waves, symmetric and antisymmetric, propagating in a metallic and a composite thin walled structure. One important basic property of Lamb waves is their dispersive nature: the velocity of propagation of a Lamb wave is dependent on its frequency, or to be more specific, on the product of the frequency and the plate thickness. The detection of material damage is based on the fact that the propagation is affected by the presence of damage due to the environmental conditions or caused by mechanical loading like impact or fatigue damage. The influence of these degradation mechanisms upon the signal from the optical fibre sensing the Lamb wave in a composite plate-like structure is investigated. Quantitative correlations between the monitored transient signals of the optical fibre and the type and extent of the damage are established.