Ultrahigh-sensitivity moire interferometry

Abstract

An ultra-high sensitivity whole-field in-plane displacement measurement method was developed for micromechanics. It is called the optical/digital fringe multiplication method , or O/D multiplication method. The specific objective was displacement sensitivity of 50 nm per fringe contour or better, which corresponds to that of moire with 20,000 lines per mm or more. The objective was achieved by a two-step process. Microscopic moire interferometry was used for step 1 to map an original displacement field, which provides a sensitivity of 208 nm per fringe order. For step 2, a fringe multiplication scheme was implemented. Here, an automatic fringe shifting and fringe sharpening scheme was developed, wherein βN fringe contours are 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 optical, mechanical and electronic systems implemented here are remarkably robust and quick. The method was applied to three practical applications: fiber/matrix deformation of a metal/matrix composite, interface strains in a thick 0°/90° graphite/epoxy composite, and thermal deformation around a solder joint in a microelectronic subassembly.