We have developed a generation pulsed-laser scanning method for visualizing the propagation of ultrasonic waves.
While scanning a target object with a pulsed-laser beam to generate thermal-exited ultrasonic waves, we detected the
propagated waves with a fixed PZT transducer. Although the detected waves were generated from different irradiation
points, we were able to produce moving images of the ultrasound generated at the reception-transducer position by
reconstructing the measured waveform data. This method has the following features that make it superior to the
conventional visualization methods such as photo-elasticity method, reception probe scanning method and computer
simulation. (1) it enables us to visualize ultrasonic waves propagating on a complex-shaped object with curved surfaces,
steps, and dents. (2) it provides excellent working efficiency by eliminating the need for adjustments to the laser
incidence angle and the focal distance. For these reasons, we believe that this new method can be effectively applied to
the inspection of defects in the field. In this study, we examined the applicability of this method to CFRP materials, and
the results demonstrate the validity of this method for nondestructive flaw inspection in CFRP-structures.
We introduce a simultaneous multipoint acousto-ultrasonic (AU) sensing system using a tunable laser and fiber wave
Bragg grating (FWBG) sensors. Although the demodulation technique is same as the existing method for a fiber Bragg
grating (FBG), the sensor head is changed to the FWBG sensor for which the FBG is installed in a strain-free
configuration and detects the AU wave not directly but in the form of a fiber-guided wave. Therefore since the strain
cannot make the FBG spectrum move, multiple FBGs with an identical spectrum can be connected with multiple optical
paths realized by equal laser intensity dividers. Temperature difference among the multiple FWBG sensors is passively
resolved by using a short grating, which provides a wider temperature-operating region. Consequently, we can solve the
problem that the FBG spectrum is easily deviated from the lasing wavelength owing to the strain. Also, the simultaneous
multipoint sensing capability based on the single laser improves cost-performance ratio, reduces inspection time, and
enables in-situ monitoring of a real structure exposed to large and dynamic strain. The system feasibility is demonstrated
in the health monitoring examples like acoustic source localization and ultrasonic waves detection burst by a
piezoelectric transducer and a pulsed laser.
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