Reproducible and predictable electrical pulses with peak powers of a few hundred kilowatts lasting for a few microseconds can be obtained from shock−wave compressed ferroelectrics. In this work, impact−loading techniques are used to investigate the electromechanical response of poled specimens of a ferroelectric ceramic, PZT 95/5, to long−duration shock pulses. The experiments are conducted in the normal mode in which the shock propagation vector is perpendicular to the remanent polarization. Current histories are obtained as a function of load resistance for a fixed shock amplitude of 1.4 GPa, and few additional experiments investigate the stress dependence of the electrical response. A simple, though specific, model is developed that gives good agreement with observed results. The extension of this model to other materials and shock−loading conditions is discussed.
Release adiabats and Hugoniot curves centered at shock states can be readily determined by impacting a projectile disk onto a stationary reverberation disk made of a linear elastic material of known shock properties. The reverberation disk may have a free back surface or may be backed by a buffer disk made of the specimen or some other material. The reverberation disk is very thin compared to the thicknesses of the other disks so that many wave reverberations occur in it during the experiment. Depending on the impedance of the reverberation disk relative to the other disks, each reverberation successively unloads, or loads, the projectile disk, thus establishing points on a release adiabat or on recentered Hugoniot curves of the specimen material. The technique is particularly valuable for measurements on compressible nonlinear materials, and it generates a large amount of information in a single experiment. Experiments have been performed with X-cut quartz, Lucalox, and 60° orientation sapphire reverberation disks which illustrate the technique, and results are presented for an epoxy resin and a porous tuff.
The propagation of the higher modes of longitudinal wave propagation in a dispersive elastic rod was investigated, both analytically and experimentally. The Pochhammer-Chree equation was solved for the mode shapes and group velocities as a function of wavenumber for the axisymmetric modes of longitudinal wave propagation. By integration of the axial displacement across a cross section, an indication of the planeness of the waves was obtained. In the experimental investigation, elastic waves generated in a coloredglass pressure bar by absorption of luminous energy from a Q-switched laser were measured using a thin piezoelectric crystal sandwiched in the pressure bar. The gross experimental results showed good agreement with the response as predicted by Love's modified wave equation. Higher modes of wave propagation not predicted by the one-dimensional theory were evident in the experimental record. These effects were related to the results obtained from the Pochhammer-Chree equation. Dimensionless quantities: &=w/w0 circular frequency •= r/a radial coordinate •= z/a axial coordinate •= u/ae •' radial displacement •= w/ae •' axial displacement •r = •rr/t•e i'ø' ao= •ro/ue i•* normal stresses •= r•/ue •t shearing stress /•=fia wavenumbers •=•a a bar radius
Elastic stress waves generated in a pressure bar by in-depth absorption of laser energy were measured using a thin piezoelectric quartz crystal. The experimental results showed very good quantitative agreement with theoretical predictions based on Love's modified wave equation. Higher modes of wave propagation not predicted by the one-dimensional theory were evident in the experimental results.
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