FBG-based real-time evaluation of transverse cracking in cross-ply laminates

Transverse cracking is one of the most common initial failure modes of composites and has detrimental effects on the structural integrity. In this paper a novel real-time non-destructive testing method is proposed to evaluate the transverse cracking in cross-ply laminates. The FBG sensors were embedded in the cross-ply specimens [02/90n]s and [90n/02]s, n = 2, 3, 4, at the interface along the fiber direction. The Digital Image Correlation measurement was applied in the tensile testing to determine the density of the transverse cracks. The FBG spectrum was distorted as the crack density increased and its width at a quarter of the peak w1/4 showed a high correlation with the crack density. A numerical method based on spectrum reconstruction using the Transfer Matrix Method was then introduced to investigate the relations between spectrum distortion and transverse cracking. The reconstructed spectrum has an accurate reproduction of the reflective spectrum of FBG. The results demonstrate that the spectrum distortion attributes to the non-uniform distributions of the longitudinal strain due to the transverse cracks and w1/4 can be applied as an effective indicator to evaluate the transverse crack density. The demodulation method using the FBG spectrum can be used to real-time detect the internal transverse cracks of composites.