Application of Scanning Laser Source Technique for Nondestructive Flaw Detection

Abstract

During the last two decades, significant advances in the field of laser generated ultrasounds have been made. In the conventional pulse echo technique, a discontinuity in the specimen can be detected only if an ultrasonic detector is properly oriented to the discontinuity. The 2-D Scanning Laser Source (SLS) technique allows detection of surface-breaking cracks of arbitrary orientation with respect to the direction. This technique also makes detection possible on rough and curved surfaces by scanning changes in ultrasonic amplitude and frequency when the laser source scans over flaws. Conventional ultrasonic scanning technique requires immersion of specimen or water jet for transferring ultrasonic wave between transducer and specimen. However, this SLS technique does not need contacting and couplant to generate surface wave and to get flaw images. Therefore, this SLS technique has several advantages, for complicated production inspection, high speed scanning, non-contact, remote from specimen, and high resolution. In this study, ultrasonic SLS images of fatigue cracks on turbine blade of aircraft and steel plates were obtained to describe shapes of defects by SLS technique.

Complete noncontact laser ultrasonic imaging for automated crack visualization in a plate

“…Next, the CSWE images are compared to cumulative total wave energy (CTWE) images so that the performance of the proposed image processing technique can be examined. CTWE is given by equation (9) and its image is obtained by assembling CTWE at all scanning points: Figures 10 and 11 compare the CTWE and CSWE images obtained from both undamaged and damaged conditions of the plate. For the undamaged plate, the high energy concentration of the incident waves and additional noise components appear in the CTWE image captured in figure 10(a) while they are all eliminated in the CSWE image in figure 10(b).…”

“…To address these shortcomings of contact transducers, there is a strong desire to adopt noncontact laser ultrasonic techniques, which have been extensively studied by the nondestructive testing (NDT) community, to SHM applications. For instance, noncontact laser ultrasonic excitation has been achieved by using pulse [7][8][9][10][11][12][13] and continuous lasers [14,15], and ultrasonic sensing has been performed based on laser interferometry [16][17][18][19][20]. The collected data are processed to detect various types of defects in metal and composite structures [13,[18][19][20].…”

Laser based structural health monitoring for civil, mechanical, and aerospace systems

“…Therefore, there is a strong desire to adopt noncontact laser ultrasonic techniques, which have been extensively studied by the nondestructive testing community, to SHM applications. For example, ultrasonic waves can be generated by using pulse [7][8][9][10][11][12][13] and continuous laser 14,15 , and the corresponding responses can be measured by using laser interferometry [16][17][18][19][20] . Using the measured responses, ultrasonic wave propagation is visualized and various types of defects in metal and composite structures can be detected 13,[18][19][20] .…”