Non-Contact Tube Inspection Technique Using Laser Generation of Guided Wave and Its Reception by Air-Coupled Transducer

Abstract

Ultrasonic guided wave has been widely used for the tube inspection. The conventional method is to use piezoelectric transducers that should be contacted to the target surface. In recent years, however, the non-contact method is strongly required in the automation of inspection process for the manufacturers as well as in dangerous environmental in-service inspection. In this paper, we have proposed a non-contact inspection method generating the ultrasonic guided wave by laser and receiving it by an air-coupled transducer. This method can generate and receive the guided wave of a specific mode with selectivity, which makes the interpretation of received signal clearer and resultantly improves the accuracy of inspection. Also the detected signal was analyzed by using the wavelet transform and it was shown that the wavelet analysis is useful for the mode identification. The proposed method was applied to the tube of 1mm thickness and 20mm outer diameter with three different types of artificial notch defects; through-wall, inside and outside of tube. The size of defects were 2∼8mm long in the circumferential direction, with 100μm width. It was proven that all kinds of defects were detectable. Finally we have developed a practical automatic inspection system, in which the inspection result is displayed in a 2-D image.