Extension of Moire interferometry into the ultra-high sensitivity domain
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The objective was achieved by the following developments. First, the basic sensitivity of moire interferometry was increased beyond the previously conceived theoretical limit. This was accomplished by creating the virtual reference grating inside a refractive medium instead of air, thus shortening the wavelength of light. A very compact four beam moire interferometer in a refractive medium was developed for microscopic viewing, which produced a basic sensitivity of 208 nm per fringe order. Its configuration made it inherently stable and relatively insensitive to environmental disturbances. An optical microscope was employed as the image recording system to obtain the desired spatial resolution.
Secondly, a fringe multiplication scheme was implemented. Here, an automatic fringe shifting and fringe sharpening scheme was developed, wherein very thin fringe contours of order N* = Î²N were produced, where N is the fringe order in the basic moire pattern and Î² is a fringe multiplication factor. A factor of 12 was achieved, providing a sensitivity of 17 nm per fringe contour. This corresponds to moire with 57,600 lines per mm (1,463,000 lines per in.), which exceeds the sensitivity objective. The mechanical and electronic systems implemented here are remarkably robust and quick.
The method was demonstrated with three practical applications: interface strains in a thick 0Â°/90Â° graphite/epoxy composite, fiber/matrix deformations of metal/matrix composites, and thermal deformation around a solder joint in a microelectronic subassembly.
- Doctoral Dissertations