A fiber-optic ultrasonic sensing system was constructed with fiber Bragg gratings based on
the intensity modulation of light reflected from fiber Bragg grating sensors. This fiber-optic
system consists of fiber Bragg gratings for sensing and filtering, a broadband light source as
well as a photo detector. The feasibility of damage monitoring using the fiber-optic system
was investigated. Transient ultrasonic Lamb waves generated with a piezoceramic
pulser were propagated in a cross-ply CFRP with visible impact damage. The
response of the fiber Bragg grating sensor to the Lamb wave propagated through
the damaged area was compared with a reference response in an intact area.
Frequency characteristics of the response signal were analyzed in order to evaluate
Lamb wave interaction with damage. Moreover, Lamb wave detection using a
piezoceramic sensor was performed to compare with the response of the fiber
Bragg grating sensor. The experimental results demonstrated that fiber Bragg
grating sensors are comparable to piezoceramic sensors in ultrasonic detection
and the fiber-optic system proved to be effective in impact damage detection of
CFRP.
An ultrasonic wavefield propagation imaging system is introduced and then applied for ultrasonic wavefield imaging of complex curved surfaces. A Q-switched pulsed laser is utilized as a moving ultrasonic generator, and a PZT ultrasonic sensor is fixed during the laser beam scanning and detects the ultrasonic waves propagated from the points excited by the laser beam. The waveforms are allocated in the spatial domain of the scanned points and then manipulated in the form of a time versus wavefield movie. The visualized wavefields enable easy detection and interpretation of structural defects because anomalies during wavefield propagation can be visualized. Furthermore, this ultrasonic wavefield propagation imaging system enables reference-free inspection, complex curved surface scanning because it does not require control of focal length and incidence angle of the laser beam, and excellent adaptability with built-in structural health monitoring sensors, such as piezoelectric and fiber optic sensors. The system is demonstrated in the applications of wavefield visualization on a drill surface, detection of mass loss parts inside an elbow pipe joint, and detection and characterization of impact damage and stringer disbond in a composite skin–stringer structure.
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