Laser vibrometer measures surface acoustic waves for nondestructive testing

Abstract

In assessing the safety of a structure, a nondestructive testing (NDT) technique should be able to detect the position of any defects and calculate their size. Most NDT techniques are thus necessarily complex, require qualified staff, and are unsuited to automation. In this article, we describe an experimental NDT methodology for crack detection using surface acoustic waves (SAWs) and optical laser Doppler vibrometer measurements. We evaluate whether ultrasonic laser measurements appropriately detect damage and imperfections in structural components. SAWs are elastic waves that are confined to the surface of a material. They travel along, penetrating about one wavelength into the surface (where the depth of penetration is defined by the depth at which 95% of the wave energy is transmitted). 4 We chose an ultrasonic optical technique instead of other currentlyavailable methods (e.g., ultrasonic scanning, magnetic particle inspection, x-ray imaging, and imaging techniques in general5) for two reasons. First, using the laser as a receiving ultrasound transducer enables remote access to the structure being tested. Second, laser scanning a region of interest makes it possible to obtain many measurements regarding the incident, transmitted, and reflected waves. These measurements, in turn, elucidate the behavior of the SAWs and of the crack itself. In the presence of surface discontinuities such as surface roughness and cracks, part of an incident wave is reflected. The rest is transmitted. Hence, gauging SAWs in the vicinity of a discontinuity allows one to infer information about the state of the surface. The material we tested was the slat track of an Airbus A320. This part of the plane is the movable support structure connecting the wing with the leading-edge slats (see Figure 1). A hydraulic test rig was used to induce in the structure the kind Figure 1. Shown is an Airbus A320 and the slat track used for testing.