Direct imaging of traveling Lamb waves in plates using photorefractive dynamic holography

Abstract

Anisotropic stiffness properties of sheet materials can be determined by measuring the propagation of Lamb waves in different directions, but this typically requires multiple positioning of a suitable transducer at several points or scanning over the area of the sample plate. A laser imaging approach is presented that utilizes the adaptive property of photorefractive materials to produce a real-time measurement of the antisymmetric Lamb traveling wave displacement and phase in all planar directions simultaneously without scanning. Continuous excitation and lock-in methodology is employed, enabling the data to be recorded and displayed by a video camera. Analysis of the image produces a direct quantitative determination of the phase velocity in all directions showing plate stiffness anisotropy in the plane. The method is applicable to materials that scatter light diffusely and provides quantitative imaging of the dynamic surface motion exhibited by traveling elastic waves. A description is given of this imaging process and, for the first time, its ability to perform lock-in measurement of elastic wave displacement amplitude and phase.

Video Ultrasonics by Pulsed TV Holography: A New Capability for Non-Destructive Testing of Shell Structures

“…Moreover, it allows extracting the acoustic displacement amplitude and phase and also the instantaneous acoustic displacement field in quasi-real time (several maps per second). This technique was successfully employed to analyze SAW in crystals and metals (Telschow et al 1999;Telschow et al 2003). Another configuration, namely frequency translated holography, combines homodyne phase modulation with spatial filtering to extract the acoustic displacement amplitude, allowing simultaneously to discriminate traveling waves moving in one direction from those moving in the opposite one and from standing wave patterns (Shiokawa et al 1975;Blackshire et al 2002;Blackshire and Duncan 2004).…”

“…In order to allow the comparison, at least one of the wavefronts is frozen and stored in a hologram by some appropriate temporal treatment of the illumination-recording process, and then reconstructed and superimposed to the other wavefronts. Apart from the classical and tedious photographic emulsions (Shiokawa et al 1975;Gagosz 1974, 78-83;Aleksoff 1974, 247-63;Vest 1979, 224-5 and 242-4;Henning and Mewes 1995;Fällströ m et al 1989), real-time recording media like thermoplastic film (Pohl et al 1992;Schroeder and Crostack 1996), bacteriorhodopsin (Blackshire et al 2002;Blackshire and Duncan 2004), or photorefractive crystals (Telschow et al 1999;Telschow et al 2003;Delaye et al 2000;Lemaire et al 2000) have been employed, allowing a much higher refreshing rate (up to tens of Hz for photorefractive media, although at the expense of a large light power demand). , also known as electronic speckle pattern interferometry (ESPI), use the image sensor of the video camera as a recording medium and employ a configuration of image hologram.…”

Laser Acoustic Imaging of Film Bulk Acoustic Resonator (FBAR) Lateral Mode Dispersion

“…This approach performs optical full image detection of the acoustic displacement amplitude and phase motion in a microscopic geometry using photorefractive dynamic holography. The apparatus and operation of this approach have been previously described for measuring UHF acoustical motion at a single point [2] and imaging [3,4]. In this paper we describe results and analysis from imaging measurements of the lateral acoustic mode motion in a nearly square FBAR around its series resonance and compare with model predictions.…”

Laser Measurement of SAM Bulk and Surface Wave Amplitudes for Material Microstructure Analysis

“…Note that the SAM is basically designed to visualize the surface and/or the subsurface of microstructure features of the material, but not directly the acoustic waves propagating within the material. However, observing a wave front traveling within the material would be significant for understanding physical phenomena, such as scattering from microstructure features [4,5,6,7]. In addition, visualization of the acoustic wave front can help in measurement of elastic properties and improve quality evaluation of an acoustic lens.…”