Monitoring of Crack Opening Displacement of Steel Structure by PPP- BOTDA-Distributed Fiber Optical Sensors: Theory and Experiment

• An explicit COD expression was derived considering the elastoplastic behavior of the optical fiber coating. • The presented theoretical COD computations account for the inclination angle between the sensor axis and crack length. • The PPP- BOTDA-Distributed fiber optical sensor demonstrates high accuracy and sensitivity of COD monitoring. Crack opening displacement (COD) is an essential parameter in fracture analysis of steel structures, and a novel methodology was developed for measurement of COD using high-resolution distributed fiber optical sensors. Based on crack-induced strain transfer mechanism, a theoretical model was firstly proposed to explicitly determine COD of steel element by strain singularity considering both the elastic and elastoplastic behavior of the optical fiber coating. The presented theoretical COD equations consider the inclination angle between the sensor axis and crack length as well. An experimental program of a pre-cracked pure bending steel element was then conducted to confirm the feasibility of the proposed monitoring approach. The PPP-BOTDA (Pre-Pump-Pulse Brillouin optical time domain analysis) with 1-cm-level spatial resolution was used in distributed measurement of strain and determination of COD. Estimated CODs were also compared to those measured using traditional electronic digital display micrometer. The results indicate that the inclination effect of the optical fiber is slight on the COD within the inclination angle range of 30°. This proposed approach demonstrates great promise for high accuracy and sensitivity of COD monitoring applications.