Through thickness inspection in various structures using ultrasonic wave coupling in an embedded fiber optical sensor

Optical fiber sensors offer several advantages such as small size ability to be multiplexed, immunity to electric and magnetic fields, etc. In addition, another salient feature of fiber optical grating sensors is the ability to be embedded in the structure. The embedding of fiber Bragg grating (FBG) sensors results in a smaller disruption of the planned function of the structural component and allows the placement of the sensor closer to the area most likely to experience deterioration. The FBG sensors have been commonly used for strain and vibration-based structural health monitoring (SHM) but in the last few years, their use for guided waves (GW) based-SHM has been on a rise. This increasing interest is due to the higher sensitivity for GW sensing achieved through the use of FBG sensors in the edge filtering configuration. This study investigates the use of embedded FBG sensors for GW sensing through the thickness of various structures. The effect of different factors such as the depth of the embedding and material properties on the coupling of the waves into the fiber is investigated using a numerical model and experimental results.