Direct Imaging of Anisotropic Material Properties using Photorefractive Laser Ultrasound

Anisotropic properties of materials can be determined by measuring the propagation of elastic waves in different directions. A laser imaging approach is presented that utilizes the adaptive property of photorefractive materials to produce a real-time measurement of the antisymmetric Lamb or flexural traveling wave mode displacement and phase. Continuous excitation is employed and the data is recorded and displayed in all directions simultaneously at video camera frame rates. Fourier transform of the data produces an image of the wave slowness in all planar directions. The results demonstrate imaging of microstructural isotropy and anisotropy and stress induced ansiotropy in plates. INTRODUCTION A powerful method for imaging ultrasonic motion has been developed at the INEEL that utilizes the photorefractive effect in optically nonlinear materials to perform adaptive interferometry. Utilizing this approach, no postprocessing of the data recorded by a video camera is required to produce images of the surface vibration amplitude over large areas. This paper describes application of this approach, referred to as the INEEL Laser Ultrasonic Camera, to imaging of traveling antisymmetric Lamb or flexural waves in plates for determining anisotropic elastic properties of materials produced by microstructural texture and by the external application of stress. Resonant motion in plates has been previously described. Optical interference is developed within a photorefractive material with this technique and the output is an optical image whose intensity distribution is directly proportional to the surface vibration amplitude, for small ultrasonic displacements. In certain nonlinear optical materials, the photorefractive process can be established, whereby, optical excitation and transport of charge carriers produces a diffraction grating or hologram from an optical interference pattern. A charge distribution results spatially and temporally in the material that is determined by the phase information