The objective of this research was to provide means for the experimental
analysis of deformations encountered in micromechanics. Whole field contour
maps of U and V displacements in a microscopic field of view were desired.
Since displacements within a small field can be very small even when strains
are large, ultra-high sensitivity is required. The specific objective was
displacement sensitivity of 50 nm per fringe contour, which corresponds to that
of moire with 20,000 lines per mm, in combination with spatial resolution of the
optical microscope (2-5 μm).
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* = βN were produced, where N is
the fringe order in the basic moire pattern and β 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 0°/90° graphite/epoxy composite, fiber/matrix deformations of
metal/matrix composites, and thermal deformation around a solder joint in a
microelectronic subassembly.
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