Residual strain monitoring of out-of-autoclave cured parts by use of polarization dependent loss measurements in embedded optical fiber Bragg gratings

This work presents a novel technique known as polarization dependent loss (PDL) towards the accurate detection of small transverse strains using optical fiber Bragg gratings. The fundamentals of PDL are explained and the benefits discussed. A cure cycle of a cross-ply composite with embedded fibers was monitored using this PDL technique and the data from the experiments are discussed. As shown, differences in transverse strain of up to 500 με will go unnoticed with normal read-out equipment (1 pm resolution), while PDL is capable of accurately detecting much smaller values. Using PDL, it was possible to detect transverse strain build-up due to matrix polymerization, which was found to cause 60 με of difference in transverse strains. A method was proposed combining the benefits of both PDL and amplitude techniques to achieve superior resolution in FBG read-out.