Micro-surface crack detection is important for the health monitoring of civil structures. The present literature review shows that micro-surface cracks can be detected by the Brillouin scattering process in optical fibers. However, the existing reports focus on experiment research. The comparison between theory and experiment for Brillouin-scattering-based optical sensors is rarely reported. In this paper, a distributed optical fiber sensor for monitoring micro-surface cracks is presented and demonstrated. In the simulation, by using finite element methods, an assemblage of a three-dimensional beam model for Brillouin optical time domain analysis (BOTDA) was built. The change in Brillouin frequency (distributed strain) as a function of different cracks was numerically investigated. Simulation results indicate that the amplitudes of the Brillouin peak increase from 27 με to 140 με when the crack opening displacement (COD) is enlarged from 0.002 mm to 0.009 mm. The experiment program was designed to evaluate the cracks in a beam with the length of 15 m. Experimental results indicate that it is possible to detect the COD in the length of 0.002~0.009 mm, which is consistent with the simulation data. The limitations of the proposed sensing method are discussed, and the future research direction is prospected.

A new method for conversion of the distributed strain measurements to crack opening displacements (CODs) in structural elements is introduced. A hybrid approach involving theoretical and experimental strain distribution at the crack location was employed to formulate the conversion equation. The objective was to find a simple deterministic equation for practical field application that quantifies crack opening displacements directly from distributed strain measurements. The viability of the method was accomplished by experiments involving a 15-m steel beam with prefabricated flaws. A Brillouin scattering–based optical fiber sensor system was employed for distributed measurement of strains along the length of the 15-m-long beam. Two fiber Bragg grating (FBG)–based displacement sensors were also used for direct measurement of crack opening displacements at the locations of the flaws. The process involved defining and computing the bond length, which is the segment of the optical fiber affected by the presence of the crack. The distributed strain was integrated over the bond length for describing the crack opening displacements. In developing the method, it was necessary to also perform direct tension tests of the optical fiber for the acquisition of the mechanical properties. The proposed method takes into account the elastic and elastoplastic stages of deformations in the optical-fiber coating and the influence of plastic phenomena on the bond length.

Deflection monitoring plays a key role in bridge Structural Health Monitoring, as deflection data may offer valuable insights into potential structural issues, such as cracks and corrosion. However, monitoring bridge deflections under operative loads presents challenges, particularly in scenarios where establishing a fixed reference point for displacement sensors is impractical, such as when a bridge spans a waterway or the sensors may be vulnerable to vandalism. In recent years, the inverse Finite Element Method (iFEM) has emerged as a possible solution. This method provides a load- and material-independent means of computing structural displacements from strain measurements, in real time at a minimal computational cost. Additionally, the iFEM can serve as a virtual sensing approach to monitor strains at locations lacking sensors, enabling the assessment of the fatigue life consumption at virtually any point in the structure. Despite its potential, applications of the iFEM in the civil engineering field are very limited, with no validation on full scale bridges has been performed to date. This study addresses this gap by investingating the application of the iFEM for bridge deflection monitoring through distributed strain measurements. The proposed approach is tested on experimental case studies, involving cross-validatiion of reference-free deflections inferred by the iFEM with those obtained from displacement sensors. This work contributes to the advancements of structural health monitoring methodologies in civil engineering, demonstrating the practicality and reliability of iFEM in bridge applications.