Crack detection in riveted lap joints using fiber laser acoustic emission sensors

Abstract

Fiber laser ultrasonic sensors are demonstrated to be capable of measuring acoustic emission generated by cracks in aluminum panels. A single laser sensor is integrated into a riveted lap joint, which is subject to accelerated fatigue. Acoustic emission generated by crack formation in the panel in addition to other acoustic events due to fretting within the joint are clearly resolved by the laser sensor. Localization of an acoustic emission event is demonstrated with a multiplexed array of three laser sensors. This manuscript also calculates the fundamental limit to displacement resolution of the fiber laser sensor and presents measurements of the directional response to Lamb waves. The high measurement resolution and multiplexing capability of the fiber laser ultrasonic sensor makes it an ideal candidate for structural health monitoring applications.

Prospects on ultrasound measurement techniques with optical fibers

“…Takeda et al [53] developed a small diameter optical fiber with 40 µm cladding and 52 µm polyimide coating to improve the stress detection by the FBG from ultrasound wave. For the sensing of signals in the ultrasonic range, viscous coupling of the optical fiber to the structure provides an extremely high SNR, such as vacuum grease [54]. In the regime where Λ a /L FBG ≫ 1, the FBG grating is subjected to uniform deformation, which leads to a shift of the Bragg wavelength proportional to the amplitude of the GW, which is the ultrasound pressure wave induced displacement.…”

Optical Fiber Sensors for Ultrasonic Structural Health Monitoring: A Review

“…Takeda et al [94] developed a small diameter optical fiber with 40 µm cladding and 52 µm polyimide coating to allow embedding in a carbon fiber composite and improving the amplitude of strain detected by the FBG. For the sensing of signals in the ultrasonic range, viscous coupling of the optical fiber to the structure provides an extremely high SNR, for example by using vacuum grease [95]. However, viscous coupling is not practical for many SHM applications where the sensor needs to remain connected to the structure.…”

Coherent Fiber-Optic Sensor for Ultra-Acoustic Crack Emissions

“…The localization of AE events, in addition to other acoustic events due to fretting within joints, was also demonstrated in [ 18 ] with a multiplexed array of three sensors. Even though these results are promising and, for the first time, a fiber-optic spatial resolution was investigated in AE sensing, the layout presented comprises a distributed-fiber laser sensor, which contributes to creating a complex and expensive solution, which suffers from directional sensitivity of the measurand.…”