An imaging method to detect flaws in composite pressure vessels used in the aerospace industry has been developed. Solid-rocket-motor casings and fuel or oxidizer tanks for liquid rocket motors can now be evaluated with the endoscopic shearography inspection device.

The original concept for the endoscopic shearography inspection apparatus was to replace the telephoto zoom lens of a shearography camera with a commercially available borescope. The shearography camera would then be placed outside the test article with the objective end of the borescope inserted through the end boss of the pressure vessel for internal inspection. Either the camera, borescope, or test article would be rotated between inspections to provide full radial or azimuth flaw detection. The camera and borescope or the test article would be translated between inspections to provide full axial detection.

Endoscopic Shearography can detect flaws in composite pressure vessels such as solid-rocket-motor casings.

In the final design of the endoscopic shearography inspection device, a pair of borescopes (one for imaging and one for illumination) is positioned parallel to the other. The telephoto lens of a shearography camera was replaced with a side-view rigid borescope. This borescope uses relay lenses and a mirror to image the test article from the objective lens on the borescope tip to the viewing lens of the eyepiece.

In the second borescope, an integrated fiber-optic bundle provides the illumination path, with light entering through the pistol-grip hand and exiting adjacent to the objective lens on the borescope tip. A C-mount adapter was used to provide mechanical stability between the adapter and interferometer. as well as optical coupling of the imaging beam. A gel light guide is used to couple the unexpanded shearography laser beam to the fiber-optic-bundle light guide, which is integrated internal to the borescope.

The unexpanded laser beam enters the eyepiece of the illumination borescope, passes through a series of relay lenses, and is imaged to the borescope objective. The unexpanded laser beam exits the borescope objective and passes through a lens pair, causing the beam to diverge. The distance between the lens-pair elements may be adjusted to increase or decrease the beam divergence to fit the appropriate field of view. The expanding beam illuminates the surface of the test article and is then collected by the objective lens of the imaging borescope. The coherent image passes through a series of relay lenses and is imaged to the borescope eyepiece. The C-mount adapter relays this image to the interferometer for image processing.

Testing of this modified design demonstrated that the endoscopic shearography inspection apparatus with the dual borescopes is able to detect flaws in laminar composite structures.

The demonstrated feasibility of endoscopic shearography suggests that a similar technique can also be used for endoscopic inspections with other nondestructive methods. Thermography, in particular, seems a likely candidate method since it is also an imaging technique often used for the same type of application as shearography.

This work was done by Samuel S. Russell of Marshall Space Flight Centerand Matthew D. Lansing of the University of Alabama in Huntsville Research Institute. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 107 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). MFS-26494


Photonics Tech Briefs Magazine

This article first appeared in the April, 1998 issue of Photonics Tech Briefs Magazine.

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