Patent Number: 054835712
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the field of x-ray inspection, more particularly to x-ray inspection of macroscopic phenomena such as stresses in a material. 2. Description of the Related Art Moire inspection is widely known in use for inspection of areas accessible to visible light. X-rays are widely known for use in observation of areas that cannot be penetrated by visible light. Numerous methods for examining strain are available including electronic and optical strain gages and optical methods such as holography, shearography and speckle interferometry. The present invention allows for the inspection of areas that cannot be observed with optical methods. The interior of materials that are not transparent to visible light cannot be observed with the optical methods enumerated above. Strain gages have two large limitations, the first is that they monitor only a single point, the second is the need for cables running from the gage to its accompanying electronics. It is sometimes impractical when examining the interior of structures to have cables leading from the interior to the exterior. It would be difficult to prevent leakage from a high pressure tire, for example, if there were a cable running from its interior. The first limitation can be overcome by constructing the pattern of this method such that it covers a large area of the inspected object. The second is overcome as the remote nature of this method allows for the elimination of any such cables, There are some systems that make use of both x-rays and the Moire effect in diffraction measurements. These make use of reflection patterns to make measurements at the surface of a material. They are useful mostly on a very small scale and again fail to allow measurements at remote locations. The claimed method allows for remote measurements and large scale investigations. SUMMARY OF THE INVENTION The present invention is a method for using x-rays and the Moire effect to characterize materials. Its advantage lies in its use with materials that are substantially opaque to visible light, or interior regions that are not accessible making visible light Moire techniques unusable. The first step involves attaching a first pattern to the material being inspected. By attaching is meant the process of attaching to the exterior, embedding within the material, or using a pattern already extant in the material. This could include the belting of a radial tire or a pattern of metal dots fastened to or embedded in a specimen. A pattern could also be attached first to an intermediate object that would in turn be attached to a specimen to be tested. The pattern used as the first pattern should be chosen to provide a radio opacity contrast of at least 2:1 with respect to the material being inspected. Alternatively the pattern could provide a radio opacity contrast of less than 2:1 and the resulting image could be enhanced using signal processing techniques. To produce a Moire pattern a second pattern is necessary. This second pattern could be produced in a number of ways. It may be imposed over the detector so that the detected image already contains the Moire pattern. It could alternatively be imposed after the detecting step, for example by digitally scanning a film image into a computer, the line density of the digital scanner providing the second pattern. The nature of the two patterns need not be important, any two patterns are capable of producing a Moire pattern, however for them to be useful in making quantitative measurements it is preferable that they be regular, for example, sets of substantially parallel lines with substantially constant separation distances. In addition, it is well known in the art that the geometry of an x-ray set up limits the resolution of that system. It is beneficial to detect an image as close as possible to the object being imaged. The size of the pattern elements and the sizes of their images will in turn govern the final resolution of the data produced through the present method. That is, a pattern of thin wires will be able to produce an image that will provide more detailed information than one of very thick wires. Both of the patterns could be attached to the material being inspected. If the patterns were attached at different depths, for example the first at the surface and the second within the material, this would allow observation of differential motions within the material. The images produced are created by x-radiation. X-radiation's penetrative character allows for characterization of areas that are inaccessible. The x-radiation should be beamed through the area of the material containing the first pattern and may be detected by any means. Film is an inexpensive detecting means and thus it lends itself to this method. It would be advantageous to make use of real time x-ray detecting methods to save the time necessary to develop film as well as reduce sample irradiation time. Digital real time radiography systems are capable of directly transferring detected images to a computer, by controlling the density of the imaging the Moire image can be produced and stored directly. Dynamic or flash x-ray systems are capable of even faster imaging that would allow dynamic measurements to be made and very short x-ray exposure times.