Thickness Effects on Distributed Backface Strain Sensing of Adhesively Bonded Joints Under Mixed Mode Fatigue Loading

Adhesively bonded structures often consist of parts with different or varying thicknesses, which can influence their mechanical behavior and performance. Distributed backface strain monitoring using optical fiber sensors offers a promising solution for the real-time structural monitoring of adhesive joints. However, the influence of substrate thickness on the performance of backface strain monitoring remains unexplored. This study investigates the effect of substrate thickness on the performance of distributed backface strain monitoring of adhesive joints under mixed-mode fatigue loading. Numerical analyses and experimental measurements revealed that the deformation of both the substrates and the adhesive layer is highly dependent on substrate thickness, while the crack propagation behavior remains independent of specimen geometry. Stationary and instantaneous backface strain measurements proved effective for monitoring crack growth; however, increased substrate thickness was observed to reduce their precision and accuracy. These findings indicate that, though crack growth behavior is geometry-independent, substrate thickness is a critical parameter for designing backface strain monitoring systems in adhesively bonded structures.