An ultrasonic sensing system using fiber Bragg gratings (FBGs) was constructed. The system utilizes wavelength-optical intensity modulation technique and includes a broadband light source, FBGs for sensing and filtering, as well as a photodetector. Feasibility of active sensing by the system was examined. Ultrasonic Lamb wave generated by an ultrasonic transducer was propagated in a cross-ply CFRP with impact damage. Response of FBG sensor to Lamb wave propagated through a damaged area was compared with the reference response in an intact area. In this experiment, the ultrasonic transducer was driven by four kinds of signals to determine the optimal transducer drive signal for impact damage detection. Response to Lamb wave propagated through the damaged area had narrower frequency bandwidth and lower frequency characteristics than response in the intact area. A spike pulse proved to be optimal for the transducer drive signal in active sensing. The experiments demonstrated that the FBG ultrasonic sensing system was effective to detect impact damage of CFRP.

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Impulse hammer measurements on steel plates were carried out to clarify the potential of a new technique of thickness estimation using a hammering test during a hull inspection. Load signals of an impulse hammer with a impact speed control jig were recorded with a digital oscilloscope and a personal computer. We investigated relationships between characteristic values (impact, peak load, half value width, min-max ratio - the absolute ratio of minimum value to maximum value of differential load signal) of the load signals and thickness. We obtained 4th order polynomial expressions which transform characteristic values to thickness, and examined their effectiveness using 35 load signals picked up on 1m x 1m steel plates of different thickness. The thickness estimation with the impact value was proved to have the smallest estimation error among 4 characteristic values.
We also examined the effect of slit, fillet weld, impact speed, hammer weight, paint layer and surface roughness caused by corrosion to our thickness estimation technique and found the extent of the effective estimation.

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