Distributed fiber optic sensing for micro- and macro-crack quantification: an interfacial-fracture-energy-based model

Clarifying the strain transfer between the host material with a crack opening displacement (COD) and the optical fiber after interfacial debonding remains a critical challenge for crack quantification. The interfacial debonding induced a triangular form strain profile around the crack due to the residual shear stresses at the interface. This study investigated the interfacial shear stress-slip relationship between the bare fiber and its coating layer through a plate splitting test. Based on that, we proposed an interfacial-fracture-energy-based analytical model to convert distributed fiber optic strains before and after interfacial debonding to CODs of micro- and macro-cracks. The accuracy of the model under a single crack was validated through plate splitting tests and the experimental data reported in the literature. Furthermore, experimental results from a four-point beam test were utilized to investigate the feasibility of the model under multiple cracks. The model, assisted by the strain superposition strategy, accurately quantified the CODs of multiple cracks before interfacial debonding but failed when interfacial debonding occurred. In addition, we proposed a simplified method based on the linear relationship between the debonded lengths and the corresponding CODs, which facilitates evaluation of the CODs after debonding.