Distributed fibre optic monitoring of mode I fatigue crack propagation in adhesive bonded joints and comparison with digital image correlation

•Backface strain measured with optical fibres could locate the crack-tip in the adhesive under fatigue crack propagation.•The combination of optical fibres and digital image correlation could identify the length of the process zone under fatigue.•Results of the optical fibres were confirmed by comparison with DIC and visual evaluation. The feasibility of applying Optical Backscatter Reflectometry as a backface strain monitoring method for bonded joints subjected to mode I fatigue crack propagation was studied. Visual evaluation and Digital Image Correlation methods were applied to track the crack-tip position and the onset of plasticisation in the adhesive, respectively. The methods were compared to understand the relationship between the changes in the optical fibre measurements and the crack propagation. The absolute maximum strain values gave important information about the crack propagation stage (stable or high velocity) within the adhesive and fast-rate crack propagation, which resulted in a sudden increase in strain values. Finally, the strain peak position was found to be a valuable indicator of crack tip position.