Michael Mello

Heterodyne transverse velocimetry for pressure-shear plate impact experiments

Pressure-shear plate impact experiments have traditionally relied on free space beam interferometers to measure transverse and normal particle velocities at the rear surface of the target plate. Here, we present two different interferometry schemes that leverage heterodyne techniques, which enable the simultaneous measurement of normal and transverse velocities using short-time Fourier transforms. Both techniques rely on diffracted 1st order beams that are generated by a specular, metallic grating deposited on the rear surface of the target plate. The diffracted beam photonic Doppler velocimetry technique interferes each 1st order beam with a reference of slightly higher wavelength to create a constant carrier frequency at zero particle velocity. The second technique interferes the 1st order beams with each other and employs an acousto-optic frequency shifter on the +1st order beam to create a heterodyne transverse velocimeter. For both interferometer techniques, the 0th order beam is interfered in a heterodyne photonic Doppler velocimetry arrangement to obtain a measurement of the normal particle velocity. An overview of both configurations is presented along with a derivation of the interferometer sensitivities to transverse and normal particle velocities as well as design guidelines for the optical system. Results from normal impact experiments conducted on Y-cut quartz are presented as the experimental validation of the two proposed techniques.

Pressure-shear plate impact experiment on soda-lime glass at a pressure of 30 GPa and strain rate of 4·10^7 s^(-1)

Recent modifications of a powder gun facility at Caltech have enabled pressure-shear plate impact (PSPI) experiments in a regime of pressures and strain rates that were previously unaccessible. A novel heterodyne diffracted beam photonic Doppler velocimeter (DPDV) has also been developed for simultaneous measurement of the normal and transverse particle velocity histories using the ±1st order diffracted beams produced by a 400 lines/mm diffraction grating deposited onto the polished rear surface of the impacted target plate. We present and interpret the results of a PSPI experiment conducted on a 5 µm thick soda-lime glass sample subjected to a normal stress of 30 GPa and a shear strain rate of 4 · 10^7 s^(–1). Transverse particle velocity measurements reveal a peak shear stress level of 1.25 GPa up to a shear strain value of 2.2, followed by a precipitous drop in stress and complete loss of shear strength.

Heterodyne diffracted beam photonic Doppler velocimeter (DPDV) for measurement of transverse and normal particle velocities in pressure-shear plate impact experiments

Pressure-shear plate impact (PSPI) experiments have traditionally relied on free space beam interferometers such as the transverse displacement interferometer (TDI) and normal displacement interferometer (NDI) or normal velocity interferometer (NVI), to measure transverse and normal velocities at the rear surface of the target plate [1]. Alternative interferometer schemes feature a dual beam VISAR arrangement [2] and a recently developed all fiber-optic TDI-NDI/PDV configuration [3]. Here, we present a heterodyne diffracted beam PDV (DPDV) which interferes a pair of symmetrically diffracted 1st order beams produced by a thin, specular, metallic grating deposited on the rear surface of the target plate. Each beam is collected by a fiber-optic probe and directed to interfere with a reference beam of a slightly increased wavelength to create an upshifted carrier signal frequency at zero particle velocity. Signal frequencies are extracted from the two fringe records using a moving-window DFT algorithm and then linearly combined in a post processing step to decouple the normal and transverse velocities. The 0th order beam can also be interfered in a heterodyne PDV to obtain an additional independent measurement of the normal particle velocity [4]. An overview of the DPDV configuration is presented along with a derivation of the interferometer sensitivities to transverse and normal particle velocities. Results from a normal impact experiment conducted on y-cut α-quartz are presented as experimental validation.