Experimental and Numerical Investigation on the Strain Response of Distributed Optical Fiber Sensors Bonded to Concrete: Influence of the Adhesive Stiffness on Crack Monitoring Performance

Abstract

The present study investigated the strain response of a distributed optical fiber sensor (DOFS) sealed in a groove at the surface of a concrete structure using a polymer adhesive and aimed to identify optimal conditions for crack monitoring. A finite element model (FEM) was first proposed to describe the strain transfer process between the host structure and the DOFS core, highlighting the influence of the adhesive stiffness. In a second part, mechanical tests were conducted on concrete specimens instrumented with DOFS bonded/sealed using several adhesives exhibiting a broad stiffness range. Distributed strain profiles were then collected with an interrogation unit based on Rayleigh backscattering. These experiments showed that strain measurements provided by DOFS were consistent with those from conventional sensors and confirmed that bonding DOFS to the concrete structure using soft adhesives allowed to mitigate the amplitude of local strain peaks induced by crack openings, which may prevent the sensor from early breakage. Finally, the FEM was generalized to describe the strain response of bonded DOFS in the presence of crack and an analytical expression relating DOFS peak strain to the crack opening was proposed, which is valid in the domain of elastic behavior of materials and interfaces.

A Review of Recent Distributed Optical Fiber Sensors Applications for Civil Engineering Structural Health Monitoring

“…to the fiber core is required [19,41]. Alj et al [42], for example, indicated that the strain transfer process depended on mechanical and geometrical properties of the different intermediate layers, i.e., their elastic modulus, their height and the initial crack opening. Alj et al continued reporting that several analytical models had been proposed in the literature among which the one of Feng et al [41] who developed a theoretical approach to describe the strain transfer process in zones containing strain singularities (cracks) and the one of Henault et al [43] who defined a Mechanical Transfer Function (MTF) that relates the strain profile along the DOFS core to the strain profile of the host cracked concrete.…”

“…to the fiber core is required [ 19 , 41 ]. Alj et al [ 42 ], for example, indicated that the strain transfer process depended on mechanical and geometrical properties of the different intermediate layers, i.e., their elastic modulus, their height and the initial crack opening.…”

“…Alj et al [ 42 ] investigated the strain response of thick-coated DOFS sealed in a groove on the surface of a concrete structure using a polymer adhesive and aimed to identify optimal conditions for crack monitoring. Initially, a Finite Element Model was developed (see Figure 10 ) to understand the strain transfer process between the DOFS core and the host structure which showed that the softer the adhesive is, the lower the strain transferred to the DOFS glass core due to redistribution over a broader length of the sensor (similarly to what was observed in [ 48 , 49 , 50 ]).…”

“…This was evident in the strain profiles provided by the DOFS (see Figure 11) which was bonded to the test specimen with three polymer adhesives used in this experiment (Bi-component epoxy, Mastic silicone and Silicone rubber) and subjected to an actuator displacement reaching 0.6 mm. Finite element mesh of (a) the 3D geometry and (b) the cross-sectional view of the system [42].…”

“…Feng et al [41] and Henault et al [43] developed theoretical approaches to describe the strain transfer mechanism (also defined Mechanical Transfer Function) that relates the strain profile of the host cracked concrete with the DOFS core; • Bassil et al [44] proposed a new analytical model formulating a general expression of the crack-induced strain transfer from fractured concrete material to optical fibers; • Alj et al [42] Several DOFS-powered experimental campaigns detected a behavioral feature of silicone whenever it was used for the protection of DOFS. That is a certain redistribution of the strains present in a specific point/section of the DOFS along a larger segment, consequently sampling lower strains than the ones originally present; • This is in line with the results of a performed FEM analysis stating that the softer the adhesive is (in this case silicone can be characterized as a "soft" adhesive), the lower the strain transferred to the DOFS glass core due to redistribution over a broader length of the sensor; •…”