Fiber‐optic interferometric differential strain sensor: “The smart strutt.”

Abstract

Two optical fibers, comprising the legs of a Michelson interferometer, are cast into a long rod (1.33‐cm diameter, 35. 1‐cm length) of epoxy at off‐axis coplanar distances of 5.3 _+ 0. I min. The free‐free bar is electodynamically excited in its gravest flexural, torsional, and longitudinal modes. The absolute displacement of the bar end for each mode was measured using the MTI fotonic vibration sensor and compared with the change in optical path length induced in the interferometer. At the fundamental flexural resonance of the bar (158.5 Hz), the correlation between measured transverse peak displacement (15.4 μm) and the measured number of strain‐induced optical fringes (14.5 at a wavelength of 817 nm) was within 7% of theoretical predictions. No fringes were observed for the torsional and longitudinal modes, as expected, since those modes produce equal strain in each leg of the interferometer. Experimental values and techniques used to obtain data for the complex Young's and shear modulus for the elastomeric casting material (Stycast 1266) will also be presented.