Flaw Imaging Technique for Plate-Like Structures Using Scanning Laser Source Actuation

Recently, the longitudinal, shear, and surface waves have been very widely used as ultrasonic wave-based exploration methods to identify internal defects of host structures. In this context, a noncontact nondestructive testing (NDT) method is proposed to detect the damage of plate-like structures and to identify the location of the damage. To achieve this goal, a scanning laser source actuation technique is utilized to generate a guided wave and scans a specific area to find damage location more precisely. The ND:YAG pulsed laser is used to generate Lamb wave and a piezoelectric sensor is installed to measure the structural responses. The measured responses are analyzed using 3-dimensional Fourier transformation (3D FT). The damage-sensitive features are extracted by wavenumber filtering based on the 3D FT. Then, flaw imaging techniques of a plate-like structure are conducted using the damage-sensitive features. Finally, the plates with notches are investigated to verify the effectiveness and the robustness of the proposed NDT approach. 1. Introduction Recently, there have been increasing demands on structural 2 health monitoring (SHM) and nondestructive testing (NDT) in the fields of civil, mechanical, and aerospace engineering and so on to prevent losses of life and property by continuously monitoring the systems. Especially, local monitoring methodologies have been studied to overcome the limitation of global monitoring techniques [1–4]. Effective SHM/NDT methods must be intuitive so that inspection results can be easily understandable and must have high throughput [5]. To tackle this issue, a lot of researches based on acoustic and ultrasonic technologies have been proposed using laser interferometry, laser vibrometer, pulsed laser, and so on because the ultrasonic waves are sensitive to the mechanical properties of structures, while the wave responses are hardly affected by radiation [5]. Holography based imaging technique, one of the full-field ultrasonic wave imaging techniques, requires highly diffusive target surface. Also, holographic images can be obtained clearly in dark environments, and hence this method is not suitable to remote automatic inspection although this has noncontact inspection capability [6]. Dynamic responses can be collected by laser Doppler vibrometry which measures vibrational velocity of target structures. Although the performance of the laser Doppler vibrometry is improved in terms of scan angles and automatic focusing [7], it has still disadvantage that retroreflective film should be attached on target surfaces to