Methods for embedding fiber Bragg grating sensors during material extrusion: Relationship between the interfacial bonding and strain transfer

The embedding of optical fiber sensors called fiber Bragg grating (FBG) sensors into 3D printed polymeric structures for strain measurements has never been studied by in-depth research to understand the link between the matrix-sensor effective strain transfer and interfacial properties. The purpose of this work was to study this link in the case of a FBG sensor with a polyimide (PI) jacket embedded within a 3D printed polylactide (PLA) matrix processed by material extrusion. In particular, printing and surface engineering methods were developed, including the design of a channel to facilitate the sensor embedding in the core of a PLA tensile specimen, and/or solvent welding or chemical bonding to achieve a high level of adhesion between the PI jacket and PLA. The PLA specimen was submitted to cyclic tensile testing, whereas the strain transfer was evaluated as the deviation between digital image correlation (DIC) strain measurements at the specimen surface and the FBG strain mea-surements at the specimen core. The strain transfer was quantitatively discussed based on the intrinsic adhesion from peel testing of model materials and the interfacial porosity fraction detected in 3D by micro-computed x-ray tomography (mu CT). It was found that the strain transfer was mainly related to the interfacial porosity volume fraction that must be minimized when embedding the sensor.