Qualification of a distributed optical fiber sensor bonded to the surface of a concrete structure: a methodology to obtain quantitative strain measurements

Abstract

Distributed Optical Fiber Systems (DOFS) are an emerging and innovative technology that allows long-range and continuous strain/temperature monitoring with a high resolution. Sensing cables are either surface mounted or embedded into civil engineering structures to ensure long-term structural monitoring and early crack detection. However, strain profiles measured in the optical fiber (OF) may differ from actual strain in the structure, due to the shear transfer through the intermediate material layers between the OF and the host material (i.e., in the protective coating of the sensing cable and in the adhesive). Therefore, optical fiber sensors (OFS) need to be qualified to provide accurate quantitative strain measurements. This study presents a methodology for the qualification of a DOFS. This qualification is achieved through the calculation of the so-called Mechanical Transfer Function (MTF), which relates the strain profile in the OF to the actual strain profile in the structure. It is proposed to establish a numerical modeling of the system, in which the mechanical parameters are calibrated from experiments. A specific surface-mounted sensing cable connected to an Optical Frequency Reflectometry Domain (OFDR) interrogator is considered as case study. It was found that (i) tensile and pull-out tests can provide full information about materials and interfaces of the numerical modeling; (ii) the calibrated model made it possible to compute strain profiles along the OF and therefore to calculate the MTF of the system, (iii) which proved to be consistent with experimental data collected on a cracked concrete beam during a 4-point bending test. This paper is organized as follows: first, the technical background related to DOFS and interrogators is briefly recalled. Then, the MTF is defined and the abovementioned methodology is presented. In a second part, this methodology is applied to the specific cable. Finally, a confrontation with experimental evidences validates the proposed approach.