Experiments on distributed fiber optic sensing for fatigue crack growth monitoring of composite adhesively bonded single lap joints

In this work, the Optical Backscatter Reflectometry (OBR) distributed sensing technique is used to monitor fatigue crack growth in composite, single lap adhesive bonded joints subject to constant amplitude (CA) fatigue loading. OBR allows for measuring the strain along the entire fibre with numerous, distributed sensing locations interrogated simultaneously, transforming an ordinary optical fibre into a high spatial-resolution strain sensor. Particularly, the structural health monitoring (SHM) approach exploited in this research work is the Back Face (BF) strain technique. The BF strain along the overlap is characterized by a negative peak, whose position varies if a crack propagates in the adhesive. This constitutes the basis for a structural health monitoring method, whose results in this paper are referenced with measurements by phased array ultrasonic testing.