A VARIABLE SENSITIVITY DISPLACEMENT INTERFEROMETER WITH APPLICATION TO WAVE-PROPAGATION EXPERIMENTS

The present paper introduces: a variable sensitivity displacement inte rferometer (VSDI) used to monitor both normal and in-plane particle di splacements in wave propagation experiments. The general system consis ts of two interferometers working in tandem. Normally reflected light is interfered with each of two symmetrically diffracted light beams ge nerated by the specimen rear surface. In cases where the surface motio n simultaneously exhibits both in-plane and normal displacements, the fringes represent a linear combination of the longitudinal and transve rse components of motion. Decoupling of the normal and in-plane displa cement histories may be achieved through a linear combination of the t wo VSDI records. Alternatively, it is always possible to decouple the components of motion motion by combining a VSDI record with art indepe ndent measurement of either component. Moreover, it is shown that in t he case of pure normal motion, the VSDI system functions as a desensit ized normal displacement interferometer (DNDI). Similarly, in situatio ns involving purely in-plane motion, the VSDI is shown to function as a desensitized transverse displacement interferometer (DTDI). The DNDI and DTDI fringe sensitivities are in general shown to depend on the a ngle a or equivalently, the frequency sigma of a grating manufactured at the observation point and the order n of the diffracted beams. The variable sensitivity feature of the VSDI greatly desensitizes normal d isplacement measurements and is particularly well suited for wave prop agation studies in which normal particle velocities in excess of 100 m /s are generated. Experimental results are presented which demonstrate the application of this technique to monitoring particle motion histo ries in plate impact recovery experiments.