The INL Laser Ultrasonic Camera directly images (without the need for scanning) the surface distribution of subnanometer ultrasonic motion at frequencies from Hz to GHz. Ultrasonic waves form a useful nondestructive evaluation (NDE) probe for determining physical and mechanical properties of materials and parts. The reason for this is that ultrasonic waves or "sound" can be generated in all forms of matter: liquids, solids and gases and exhibit information about the material in which they travel. Measurement of the characteristics of ultrasonic wave motion, such as wave speed, attenuation and the presence of scattered waves from microstructural features or flaws are used to perform NDE for quality control. Laser ultrasonics refers to the process whereby lasers are used for both generation and detection of ultrasonic waves in materials, thereby providing a noncontacting method for performing ultrasonic NDE. The current state of the art utilizes a pulsed laser for ultrasonic generation through the process of thermoelastic expansion or weak ablation. The method of detection involves interferometry of the Michelson, Fabry-Perot, and Photorefractive (adaptive) types. Commercially available systems utilize these interferometric methods and provide a "point and shoot" single point measurement capability. In order to perform measurements over a large surface, the laser generation and detection spots must be scanned in a raster fashion over the area recording ultrasonic signals at each location. (See examples of ultrasonic camera data movies.)
Camera Schematic
In contrast, the INL Laser Ultrasonic Camera employs a photorefractive (adaptive) approach to interferometry to provide full-field real-time images of ultrasonic motion over large areas. The basic information to be measured, the ultrasonic motion of the surface, is impressed onto the phase of the detection laser beam just as with the other passive methods. The entire optical image of the vibrating surface is formed inside the photorefractive material where it undergoes real time processing due to the dynamics of the photorefractive process. Nonlinear optical mechanisms within the photorefractive recording material are utilized to produce an output image that is a "picture" of the vibrating surface. The net effect is that interferometric detection is accomplished over the entire vibrating surface all at once without scanning, producing an output that can be viewed directly with the eye or with a television camera. No additional electronic or computational processing is required! By eliminating the need for scanning over large areas or complex parts, the inspection process is greatly speeded up. Laser ultrasonic methods provide noncontacting approaches that are desired for field applications, such as for remote measurements and in-situ manufacturing process monitoring. An example concerns the anisotropic properties of sheet materials that can be determined by measuring the propagation of elastic waves, known as Lamb waves, in different directions. The INL Laser Ultrasonic Camera produces a real-time image of propagating Lamb wave modes in all directions along the sheet simultaneously. The resultant image provides a direct quantitative determination of the phase velocity (which depends on the material microstructure, density, and elastic properties) in all directions immediately, showing plate anisotropy in the plane. Ultrasonic motion of all types in most materials can be imaged and measured with this new approach.
Diagram showing elastic wave anisotropy
Elastic wave anisotropy in a plate can be directly imaged and quantitatively measured through recording a single image of a Lamb wave traveling outward from a central excitation point (typically produced by a contact or noncontact transducer). The resulting image shows the material elastic anisotropy through the direction dependence of the wave speed. The figure shows traveling A0 waves in a sheet of ordinary copy paper, produced by a continuously oscillating contact source at the center. One could estimate the wave speed anisotropy fromt his figure, but a better way is to Fourier Transform the entire image into an image of essentially the wave slowness in all directions.
Wave slowness figure
The wave slowness figure exhibits a single oblong closed curve that gives the inverse wave speed in all directions simultaneously. Recently, the wave speed anisotropy has been predicted from first principles by collaborators Professor Subhendu K. Datta and Osami Mukdadi of the Mechanical Engineering Department, University of Colorado using othrotropic elastic coefficients measured by collaborators P. Brodeur, C. Habeger, J. Gerhardstein, B. Prufahl, and E. Lafond at the The Institute of Paper Science and Technology in Atlanta, GA.
- Contacts:
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Ken Telschow, Ph.D., (208) 526-1264, Send E-mail
Vance A. Deason, (208) 526-2501, Send E-mail