Simulating increased Lamb wave detection sensitivity of surface bonded fiber Bragg grating

Abstract

Fiber Bragg grating (FBG) sensors are excellent transducers for collecting ultrasonic wave signals for structural health monitoring (SHM). Typically, FBG sensors are directly bonded to the surface of a structure to detect signals. Unfortunately, demodulating relevant information from the collected signal demands a high signal-to-noise ratio because the structural ultrasonic waves have low amplitudes. Our previous experimental work demonstrated that the optical fiber could be bonded at a distance away from the FBG location, referred to here as remote bonding. This remote bonding technique increased the output signal amplitude compared to the direct bonding case, however the mechanism causing the increase was not explored. In this work, we simulate the previous experimental work through transient analysis based on the finite element method, and the output FBG response is calculated through the transfer matrix method. The model is first constructed without an adhesive to assume an ideal bonding condition, investigating the difference in excitation signal coherence along the FBG length between the two bonding configurations. A second model is constructed with an adhesive to investigate the effect of the presence of the adhesive around the FBG. The results demonstrate that the amplitude increase is originated not from the excitation signal coherence, but from the shear lag effect which causes immature signal amplitude development in the direct bonding case compared to the remote bonding case. The results also indicate that depending on the adhesive properties the surface-bonded optical fiber manifests varying resonant frequency, therefore resulting in a peak amplitude response when the input excitation frequency is matched. This work provides beneficial reference for selecting adhesive and calibrating sensing system for maximum ultrasonic detection sensitivity using the FBG sensor.

Optical Fiber Sensors for Ultrasonic Structural Health Monitoring: A Review

“…Several effects contribute to the increase in sensitivity for the remote bonding configuration, recently demonstrated through numerical simulations. Wee et al [101] considered the effect of the shear lag through the adhesive bond. The shear lag effect is a function of the shear modulus and thickness of the adhesive.…”

“…As the directly bonded FBG is in the adhesive region, the shear lag effect reduces the average amplitude of the wave transfer. Additionally, the remotely bonded FBG is measuring the L 01 mode, which can be enhanced through local resonance of the optical fiber-adhesive system [101]. Huang and Balusu [102] investigated this phenomena by treating the optical fiber-adhesive-structure system as an ultrasound coupler and calculating the scattering parameters for broadband time-frequency analysis.…”

Ultrasonic Lamb wave measurement sensitivity of aligned carbon nanotube coated fiber Bragg grating

“…fiber through an adhesive bond cannot be simply improved through the impedance matching of the adhesive bond, but rather should be optimized by tuning the adhesive mechanical properties, such as adhesive elastic modulus and adhesive thickness [24].…”

Ultrasonic Structural Health Monitoring Using Fiber Bragg Grating

“…Thus, a common fiber installation approach in ultrasonic SHM is to mount the sensor onto the structural surface by using epoxy, cyanoacrylate, or some other adhesives, as shown in Figure 8 a. The ultrasonic sensitivity of the FBG can be improved by carefully choosing the adhesive materials and fiber coating materials, as both of them can influence the strain transfer from the structural plate to the optical fiber [ 80 , 81 , 82 , 83 ].…”