| 摘要 |
PROBLEM TO BE SOLVED: To determine by simple work the propriety of a crack occurring in a welded portion in a bottom plate of a petroleum tank. SOLUTION: In this method for determining the propriety whether a size of the crack C occurring in the welded portion B, in which a large number of metal plates are joined together by welding and of which the surface is applied with a coating film, in the bottom plate P of the petroleum tank T is within an allowable range or not, a crack of a size indicating a limit of the allowable range is formed in the welded portion, a reflectance when an ultrasonic wave is transmitted to the crack is used as a reference reflectance, the reference reflectance is stored preliminarily in every film thickness of the coating film L applied on the welded portion, the ultrasonic wave is transmitted to a prescribed position of the welded portion to measure the reflectance and to measure the film thickness of the coating film applied on the prescribed position in the welded portion, the measured reflectance in the prescribed position is compared with the reference reflectance corresponding the film thickness in the position, an acceptance is determined when the measured reflectance is the reference reflectance for less, and a rejection is determined when it is larger than the reference reflectance.
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