It is known among probe manufacturers that sensitivity of an angle probe, which is designed to have angle of refraction above 70 degrees, is considerably low. The reason for this was studied by theoretical calculation based on the theory of Mano. For an angle of refraction above 80 degrees, directivity of an angle probe was calculated on the assumption that it could be replaced by a linear array of point sound sources with different phases. The theoretical calculation predicted considerable decline in sensitivity for the angle of refraction above 70 degrees. Actual directivity of angle probes was measured using side-drilled holes in test blocks, and the validity of the prediction was verified in an experiment. Furthermore, it was found that a side lobe larger than the main lobe appears when the angle of refraction is designed above 82.5 degrees. Thus, an angle probe which has an angle of refraction above 82.5 degrees, cannot be made.

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The authors have developed a unique and non-destructive stress measuring method using a magnetic anisotropy sensor, and have also developed many applications for the method. In this paper, we introduce the principle of the technique, and through experimental results prove its usefulness in evaluating the stress level of pipelines under working conditions for the purpose of maintenance.

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