IMPROVED ULTRASONIC TESTING OF NEARSURFACE REGIONS BY MEANS OF A MOBILE HOLOGRAPHIC SOUNDFIELD CAMERA

Abstract

Publisher Summary Ultrasonic method has less importance in the detection of flaws in sub-surface zones. The reasons for this are the inadequate axial resolution and the influencing zone of the transmitter pulse. In order to reduce the problems of conventional ultrasonic testing methods, this chapter discusses a new concept based on holographic sound field visualization. By this technique, the object to be tested is illuminated twice during insonication by strong laser pulses, their time separation being matched to the applied ultrasonic frequency. Thus, two object surface states are holographically recorded. On reconstruction of the developed hologram, these different surface states are superimposed and give rise to the existence of interference fringes representing the insonicated ultrasonic wave. By proper modulation of the optical phase of the object lighting beam, the sensitivity of the system could be raised enabling the visualization of ultrasonic waves with amplitudes in the range of less than 1 nm. In practical applications, flaws in the near surface region of the component to be tested are seen in the holographic soundfield image as disturbances of the otherwise homogeneous (flawless) interference pattern directly at the flaw location. Using a strong ruby double pulse laser, large areas (about 1m2) can be illuminated and tested simultaneously. On account of the extremely short exposure (measuring) time, this technique is independent of relative motions between the measuring system and the test object as concerns, for example, vibrations, tremors, or refractive index variations.