Abstract
Surface breaking cracks in a structure can be ultrasonically detected by using Rayleigh or Lamb waves. Either reflected echoes or transmitted signals may be monitored in the pulse-echo or pitch-catch modes of operation. Pulse-echo and pitch-catch techniques can also be used with laser ultrasonics where a high-power pulsed laser is used to generate ultrasound thermoelastically. Laser-based ultrasonic (LBU) techniques provide a number of advantages over conventional ultrasonic methods. However, small flaws give rise to weak reflections or small changes in the amplitude of transmitted signals. These small variations are often too weak to be detected with existing laser detectors. In this paper we provide the theoretical background for an alternate approach for ultrasonicetection of small surface-breaking cracks using laser-based techniques - the Scanning Laser Source (SLS) technique. This approach does not monitor the interaction of a well-established ultrasonic surface wave with a flaw, but rather monitors the changes in the generated ultrasonic signal as the laser source passes over a defect. Changes in amplitude and frequency of the generated ultrasound are observed which result from the changed constraints under which the ultrasound is generated over uniform versus defective surface areas. These changes are quite readily detectable using existing laser detectors even for very small flaws. To analyze the ultrasound generated when the laser beam is right above the defect, the locally important effects of heat absorption and diffusion must be carefully considered.
