The purpose of this paper is to clarify the effect of paint film thickness on the reflection echo height in ultrasonic testing. Using various probe sizes and frequencies, the reflection echo height of test specimens with different paint film thickness was measured. We also calculated the echo height using the finite element method. In addition, the validity of the calculation result was verified by comparison with the experimental results. The change in the echo height was found to be caused by the interference of the reflective wave within the paint film, and the echo height changes nonlinearly by the ratio of the paint film thickness to the ultrasonic wavelength. Furthermore, in order to eliminate the effect of the paint film on the reflection echo height, we propose a new testing technique using a decoupling plate which has the same properties as the paint film.

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A structure of the Mach-Zehnder interferometer type optical fiber sensor was proposed to detect acoustic emission (AE) signals with convenient handling and the characteristics of the sensor were discussed. An optical fiber was adhered to the side of the columnar polycarbonate block to induce elongation, caused by elastic waves propagating in the block, so that AE signals could be detected by simply pushing the block against an object. Since another fiber was settled on the block by means of buffer material made of sponge rubber, the time difference between the sensing and the reference fibers was generated under the condition that both fibers were placed on a single block. It was revealed that the particular directivity of the sensor tends to disappear and the sensitivity is improved by increasing the number of sensing regions around the block. It was also found that the frequency characteristics of the sensor could be changed by altering the length of the block, and that the sensitivity does not depend on the length of each sensing region. Finally, it was demonstrated that the AE signals associated with the crack extension in the PMMA specimen could be successfully detected usingthe proposed sensor.

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An ultrasonic imaging system using a combination of logarithmic and linear amplifiers has been developed. The new system links the two amplifiers effectively to image the crack opening, crack tip, inner configuration of the pipe joint and the weld metal to base-metal interface configuration. The reconstructed image helps an ultrasonic operator to identify crack echoes from geometrical echoes in flaw detection, and to identify the crack tips for crack depth sizing. This paper discusses the use of two amplifiers of different character and reports that there is a suitable combination of amplifiers. The effectiveness of the system is verified through the ultrasonic imaging of stress corrosion cracking in a stainless steel pipe weld.

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