Acoustic emission and distributed backface strain monitoring of crack growth in composite adhesively bonded joints under mode II fatigue loading

The increasing use of composite materials in various structural applications has driven the need for suitable joining techniques. Adhesive bonding answers many of the requirements, as it does not damage the parent material and generally ensures lower stress concentrations. Structural health monitoring techniques for adhesive joints are therefore of growing importance. This work investigates monitoring of fatigue crack growth under mode II loading in a carbon fibre reinforced polymer composite adhesively bonded joint, using acoustic emission and distributed backface strain monitoring via optical fibre sensors. The distributed backface strain sensing technique provides precise localized crack length measurements; using multiple measuring paths, the crack front shape could be reconstructed. Acoustic emission measurements were affected by friction noise coming from the sliding of the two crack surfaces. Time-frequency analysis of signals using continuous wavelet transforms was used to identify and decouple damage and noise content, enabling the correct localization of damage phenomena.