Delamination detection and characterisation of bridging tractions using long FBG optical sensors

Measurements of non-uniform strains due to the propagation of mode-I delamination and identification of bridging tractions in double cantilever beam (DCB) composite specimens made of uniaxially reinforced AS4/PPS are presented. A fibre Bragg grating (FBG) of 22 or 35 mm is embedded parallel to the reinforcing fibres, during consolidation and above the delamination plane. Strain distributions along the sensor are obtained by optical low-coherence reflectometry and inverse scattering. The specimen’s residual strains in the sensor are used as an initial reference, even when it causes birefringence. Observations and strain measurements indicate fibre bridging across the delamination plane. The measured distribution of strains in the FBG sensor and numerical modeling are used in an inverse procedure to determine the actual distribution of bridging stresses during delamination.