레이저 유도 초음파를 이용한 비접촉 NDT 기법과 초음파 신호 영상화에 의한 금속 소재 용접 결함 평가

Abstract

The longitudinal, shear and surface waves have been used to an Ultrasonic wave exploration method to identify internal defects but it has technical difficulties to detect defects in a limited space with having several millimeters board thickness. It is applicable to use Lamb wave, a kind of induction ultrasonic wave that has a relatively high inspection efficiency and defect detection sensitiveness compare to perpendicular or square explorations in terms of the internal defect detection in these kinds of thin board. The Lamb wave is a special type of induction ultrasonic wave to propagate to a board and it is effective to inspect such as board structure, cell structure, etc. Recently a laser is being used to generate Lamb wave but it not being widely utilized because of difficulties in receiving and selective generation in various modes Lamb wave generating and a low S/N ratio, wide range of generating frequency band when a laser is used. This paper aims to study that the defect exploration method to specimen's thickness, using an characteristic of an ultrasonic wave generating in ablation area and thermoelasticity area, has been applied to using a way of receiving it by Air-Coupled Transducer and transmitting it by an ultrasonic wave and a way of noncontact receiving and transmitting ultrasonic wave using a laser. The longitudinal, shear waves have been used in 60mm thickness specimen and Lamb wave in 3mm to run artificial defect detection test. In this way, the usefulness of non-contact laser ultrasonic wave has been verified. To confirm the possibility of implementation on natural defect existing at real welding part, thickness in 3mm SM45C and STS304 artificial defect specimen has been made and defect detection test using Lamb wave has been executed. Detected signal by signal treatment has been expressed in visual not RF signal. By doing so the convenience of defect detection has been attempted and the effectiveness of defect signal visualization using Lamb wave has been confirmed compare with defect image through SAT. The understanding of generating theory of a laser ultrasonic wave and the usefulness of laser ultrasonic wave exploring Bulk Wave with long distance non-contact has been verified by using an interferometer. At this time ultrasonic wave propagation path in ablation area and thermo elasticity area has been identified and the internal circular defect and line defect have been detected by making artificial defect. But the laser ultrasonic wave incidence by dot type using existing laser has a high S/N ratio and it has a difficulty on a thin board to make a distinguish between longitudinal, shear and surface waves in exact defect detection. And a laser has been used for non-contact receiving of ultrasonic wave signal. It is confirmed that it is difficult to form nondestructive inspection automation and system since?this method also getting a high impact according to the surface conditions of a specimen. To overcome the limitation of generating method of the Lamb wave by laser because of this fact, the wave length has been consolidated using configuration slit and possible to generate the Lamb wave and it has been applied to nondestructive inspection. Lamb wave has a feature in generating several modes and Lamb wave's feature to propagate flat board using Rayleigh-Lamb frequency related equation, analyzed theoretically and the predict has been made to generate the mode and frequency of the Lamb wave. The conformity of the ultrasonic wave mode for generating has been identified through the testing. An artificial defect specimen has been made to include natural defect by using process parameter and Nd:YAG laser and the artificial defects with 1, 2 and 3mm in diameter on a welding part have been made. Manufactured defect specimen used as a reference in a way of defect measuring test using Lamb wave to identify the location and type of defects using SAT. The result of measuring artificial and natural defects has been able to be possible to be visualized through signal treatment. In this way, the defect detection has been easy identifying the defectby visualization not signal. Through this study, the Lamb wave which is an induction ultrasonic wave using laser has been showing high defect detection capability at welding part on a board of forming multi-Layer due to the deteriorated material by welding heat. In this way, it shows it is possible to do automation of nondestructive inspection.