Fatigue Crack Growth Monitoring in Composite Bonded Lap Joints by a Distributed Fibre Optic Sensing System and Comparison with Ultrasonic Testing

In this paper, the back-face technique is exploited to monitor fatigue crack growth (FCG) in a composite, single lap adhesive bonded joint, using distributed sensing by Optical Backscatter Reflectometry (OBR). Some preliminary results are presented, indicating that, by measuring accurately the strain profile in the overlap region, the correlation between the minimum peak of the strain profile and the position of the crack tip can be exploited for monitoring the structural health of joints. The proposed structural health monitoring technique was validated on the basis of the results obtained by a non-destructive technique using phased array ultrasonic testing (PAUT). The comparison between the two methods yielded encouraging results, suggesting that, thanks to its distributed sensing capabilities, the OBR technology could allow for improving the back face (BF) technique, as well as any other strain field-based measurement technique, for the health monitoring of adhesive joints.