Investigation of birefringence-dependent spectral distortion of embedded fiber Bragg grating affected by residual strains of composite forming

Abstract In-situ forming monitoring of fiber-reinforced plastics is important for optimizing product quality, with fiber Bragg gratings (FBGs) being extensively employed for this purpose. Inconsistent strains generated during the forming process may introduce spectral distortion in the embedded sensors, negatively affecting measurement accuracy. In particular, the correlation between the spectral birefringence of FBGs and the transverse strains in composites has not been comprehensively investigated. This study proposes a novel in-situ forming monitoring technique employing a phase-shifted FBG (PSFBG) with narrow transmission peaks to achieve high-precision and polarization-dependent spectral responses. The forming process of carbon-fiber-reinforced thermoplastics with embedded PSFBGs was comprehensively investigated to elucidate the underlying causes of spectral shift and birefringence. The axial strain and transverse strain difference detected by the PSFBGs experience ascent and descent during heating, and they experience a plateau and a linear change with temperature during cooling. The correlation between the PSFBG-detected strains and the thermal state transition of the composite, manifested as knee points near the glass and liquid-to-liquid transition temperatures, was elucidated based on micromechanical analysis. These findings facilitate the development of FBG-based in-situ multidirectional strain monitoring techniques for composite forming processes.