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High-speed non-contact defect imaging for a plate-like structure.pdf

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NDTE International 85 (2017) 53–62 Contents lists available at ScienceDirect NDTE International journal homepage: /locate/ndteint High-speed non-contact defect imaging for a plate-like structure Takahiro Hayashi? Graduate School of Engineering, Kyoto University, Kyoto 615-8540, Japan crossmark ARTICLE INFO Keywords: Elastic wave Defect imaging High speed Scanning laser source Non-contact ABSTRACT Images of defects in a plate-like structure can be obtained from amplitude distributions measured using the scanning laser source technique. This paper discusses high-speed non-contact imaging for defects in a plate. Fundamental experiments conducted using a contact piezoelectric transducer as a receiving device revealed the e?ect of the generated wave shape and measurement speed on the defect images. Generating tone-burst waves consisting of multiple frequency components and averaging the images obtained in multiple frequency bands (frequency image averaging, FIA) gave clearer defect images in which noisy e?ects due to low signal-to-noise ratio and spurious images due to multiple re?ections in high-speed imaging were reduced. Moreover, FIA was shown to be e?ective for high-speed non-contact imaging using a laser Doppler vibrometer. 1. Introduction Periodic inspections are required to maintain large structures as pipes, bridges, and aircraft appropriately. Strain and temperature measurements using optical and infrared cameras are e?ective means for rapidly inspecting the surfaces of such structures. However, X-ray and ultrasonic inspections are required to detect internal defects in pipes and delamination in the bodies and wings of aircraft. Ultrasonic inspection is preferred in practical applications because, unlike X-ray inspection, it does not require large equipment for radiation protection. The most reliable and widely used inspection technique for large structures is the ultrasonic pulse echo technique, which uses longitudinal or transverse elastic waves. However, thi