Imaging of Lamb Waves in Plates for Quantitative Determination of Anisotropy Using Photorefractive Dynamic Holography

Abstract

INTRODUCTIONAnisotropic properties of sheet materials can be determined by measuring the propagation of Lamb waves in different directions. Electromagnetic acoustic transduction and laser ultrasonic methods provide noncontacting approaches that are often desired for application to industrial and processing environments. This paper describes a laser imaging approach utilizing the adaptive property of photorefractive materials to produce a real-time measurement of the antisymmetric Lamb wave mode in all directions simultaneously. Continuous excitation is employed enabling the data to be recorded and displayed by a CCD camera. Analysis of the image produces a direct quantitative determination of the phase velocity in all directions showing plate anisotropy in the plane.Many optical techniques for measuring ultrasonic motion at surfaces have been developed for use in applications such as vibration measurement and laser ultrasonics. Most of these methods have similar sensitivities and are based on time domain processing using homodyne, Fabry-Perot [1], and, more recently, photorefractive interferometry [2]. Generally, the methods described above do not allow measurement at more than one surface point simultaneously, requiring multiple beam movements and scanning in order to produce images of surface ultrasonic motion over a large area. Electronic speckle interferometry, including shearography, does provide images directly of vibrations over large surface areas. This method has proven very durable in the field for large displacement amplitudes of several wavelengths. In addition, a sensitivity of /3000 has been demonstrated under laboratory conditions [3]. Full-field imaging of traveling ultrasonic waves using digital shearography has been recently reported with sensitivity in the nanometer range [4]. With this method, optical interference occurs at the photodetector

Novel trends in optical non-destructive testing methods

“…Pulsed excitation of surface or full body waves is done two-dimensionally by either using a piezoelectric transducer or by a short pulsed laser. The detection can be done by holography [35], shearography [19], vibrometry or one-dimensionally by a pump-probe technique [36]. In BIAS a setup was developed for the observation of Lamb waves on carbon-fibre reinforced plastics (CFRP).…”

<title>Advanced electronic phase stepped interferometry (EPSI) for detection and characterization of early-stage damage in aerospace materials</title>

“…The object field is the result of the source reflecting off the specimen under observation, and the reference field usually comes directly form the source. The reference field at one specific point may be expressed in complex amplitude form as Ur (1) and the object field may be expressed as U0 = U0 e (2) where IU! represents the real amplitude, and represents the phase of each wave.…”

Direct imaging of anisotropic material properties using photorefractive laser ultrasound

“…The mechanism for image recording is that of optical lock-in operation, whereby the only image recorded is that which is coherent with the reference modulation. 6 The first time varying or AC term in the intensity of the diffracted beam is given by …”