配管エルボ部の欠陥検出におけるガイド波試験の欠陥検出感度の周波数依存性

Guided wave testing offers an efficient screening method for thinning of pipe walls because of its long inspection range and its ability to inspect pipes with limited access. However, the existence of an elbow in pipes makes it difficult to interpret echo signals in guided wave testing. In the present study, to investigate the sensitivity of defect detection at a pipe elbow by guided wave testing, guided wave testing was performed on several 50A Sch 40 aluminum alloy piping specimens of the same configuration that includes an elbow. An artificial defect was produced at one of 12 different locations on the outer surface of the elbow of each piping specimen. The defect signals were observed as the defect depth was gradually increased at each defect location to obtain the defect sensitivity. The transmitted guided wave frequency was in turn set to 30 kHz, 40 kHz, and 50 kHz. At 30 kHz, high sensitivity values were obtained at the intrados of the elbows, whereas at 40 kHz and 50 kHz, high sensitivity values were obtained at their extrados. This paper also shows the results of computer simulations that used the same configuration as that used in the experiments to analyze the propagation behavior of guided waves passing through the elbow. In addition to the experimental results, the simulation results indicate that the defect-sensitive locations are controlled by the guided wave frequency. Thus, proper selection of the excitation frequency for guided wave testing enables defect detection in an intended area of an elbow with high sensitivity.