Measuring and testing – Vibration – Sensing apparatus
Reexamination Certificate
1997-06-13
2001-02-06
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Sensing apparatus
C073S602000
Reexamination Certificate
active
06182512
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates generally to non-destructive evaluation, and more particularly to a method and apparatus for laser ultrasound inspection of materials with synthetic aperture focusing.
2. Description of the Related Art
Laser ultrasound involves the generation or detection of ultrasound in materials with lasers. Generally, in laser ultrasound, a source laser irradiates a material with a laser beam along its surface. Ultrasonic waves are generated by the laser beam by non-destructive local heating of the surface to create expansion and a strain wave, or by increasing the amplitude of the beam to vaporize a small amount of material to form a plasma that strikes the surface like a hammer.
The ultrasonic wave propagates throughout the material and is reflected back to the surface of the material. As the reflected ultrasonic wave returns to the surface of the material, typically an interferometric detector is used to detect either displacement or velocity at the surface by irradiation of the surface with another laser beam. The detected displacement or velocity signals are used to generate an image of the material. A more detailed discussion on laser ultrasound is provided in C. B. Scruby et al.,
Laser Ultrasonics
-
Techniques and Applications
(IOP Publishing Ltd. 1990), which is hereby incorporated by reference.
When laser ultrasound is used to inspect thin structures, image resolution generally decreases because Lamb waves are generated. Lamb waves are generated when the object being imaged is less than several wavelengths thick, as surface waves engage the thin structure on its opposing face. Lamb waves are dispersive, which means that different frequency components of the Lamb wave propagate with different velocities. The dispersive characteristic of Lamb waves results in decreased image resolution, since the arrival time of the reflected wave is less well defined.
Although it is possible to compensate for frequency dispersion in Lamb waves through modeling, this technique involves considerable analytical effort. In addition, where multiple modes of propagation exist, it is analytically difficult to model all modes accurately. These problems are compounded in anisotropic materials in which the wave speed is dependent on propagation direction.
It would be desirable, therefore, to have a method and apparatus for accurately and efficiently imaging a thin structure using laser ultrasound.
SUMMARY
A method for forming an image of an article, according to an exemplary embodiment of the invention, comprises the steps of forming a matched filter data set comprising first data points representing a response of the article as a function of a distance to a source; generating waves in the article at a plurality of generation positions on the article, which waves propagate through the article; detecting a motion of the article at a plurality of detection positions to obtain second data points, the second data points forming a scan data set; multiplying the scan data set by the matched filter data set to generate a compensated data set comprising a plurality of third data points; and coherently summing the third data points in the compensated data set to form an image of the article.
An exemplary apparatus comprises a wave generator for generating a wave in an article; a detector for detecting a wave in the article; and a processor which: forms a matched filter based on a response of the article to a wave propagated through the article; directs the wave generator to generate a wave at a plurality of generation positions; directs the detector to detect a motion of the article at a plurality of detection positions; forms a scan data set from the detected motion of the article; multiplies the scan data set by the matched filter to produce a compensated data set; and coherently sums data points in the compensated data set to produce a focused image.
The exemplary method and apparatus are particularly suitable for inspection and imaging aircraft skins which are typically formed of curved sheets of metal, riveted to adjoining panels and supporting structure. As aircraft age, the skins need to be inspected for cracks, disbonds, corrosion, and other defects. The large areas to be inspected, as well as curved or layered structures, can be efficiently and accurately inspected with exemplary embodiments of the invention.
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Fayyaz Nashmiya
General Electric Company
Patnode Patrick K.
Snyder Marvin
Williams Hezron
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