In an attempt to improve the accuracy of ultrasonic thickness measurement of corroded material, we applied soft element probes to corroded steel plates cut out of the hull structure of an aged ship, and examined the properties of the thickness measurement. Using a conventional ultrasonic probe, however, we found that an excess of coupling medium between the probe and the corroded surface resulted in a significant error in the thickness measurement during ship inspection, to the extent that smoothing of the corroded surface was required. We found that using a soft element probe considerably reduced this error in thickness measurement, and clarified the effects of factors such as element size and specimen thickness on this type of measurement. We also carried out beam trace calculations to investigate the measurement properties of the soft element probes. The results of our calculations showed that these properties are closely related to a change in beam divergence caused by surface roughness, as well as by multiple reflections in the excessive coupling medium layer.

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