Automatic Inspection of Pipe Using Non-Contact Guided-Wave Technique with Enhanced Mode-Selectivity

Ultrasonic guided wave has been widely used for the pipe inspection. The conventional method is to use piezoelectric transducers that should be contacted to the target surface. However, in order to automatically inspect pipes in the manufacturing stage, non-contact inspections are strongly required. In this paper, we propose a new non-contact method for the automation of pipe inspection, which generates the ultrasonic guided wave by laser and receives by air-coupled transducer. Also we have developed a rotational canning system to show the inspection result as a 2-D image, which makes the operator find out defects easily. Then, in practical applications of guided wave techniques, it is preferred to generate and detect a single or less dispersive mode. So this paper considers the enhancement of mode-selectivity, where a laser beam illuminated through arrayed line slits is used as the transmitter and the air-coupled transducer is used as the receiver. The line arrayed laser illumination is a wavelength matching technique that can generate only a few modes. The air-coupled transducer detects the leaky wave of the propagated guided wave, and by tuning its detection angle we can detect the selected single mode. This method can generate and receive the guided wave of a specific mode, which makes the interpretation of received signal clearer and resultantly improves the accuracy of inspection. The proposed method was applied to the stainless pipe specimen with artificial defects and its inspection performance was verified. The experimental results showed that the fundamental axial-longitudinal mode was dominantly generated and received, and the location and the size of pipe defects could be displayed well in the 2 dimensional scanning images in real time.