Laser Doppler velocimetry systems for measuring wave profiles in shock-loaded solids are critically reviewed with a comprehensive listing of all systems and a thorough examination of optical heterodyne principles.
The diaphragm separating nitrogen gas at high pressure from one at low pressure is bursted and a shock wave travelling forward to the low pressure chamber and the rarefaction wave travelling back to the high pressure chamber are generated. In this experiment, the velocities of the shock waves are measured by two different methods; that of using pressure transducer and that of employing laser beam shear displacement. When He-Ne laser beam traverses the windows across the travelling shock wave, it is transversely displaced by the shock pressure induced tilt of the window plate, so that the laser beam coverage on the detector aperture changes. This change produces change in the output of the photodiode. In the present experiment, two pairs of windows separated by 10 cm give two separate photodiode outputs, and from the time intervals of two voltage signals, the velocity of shock wave is determined. The results obtained by this laser optical method accurately agree with those obtained from the method of using a pair of pressure transducers.
A mild steel projectile (65.6 gm) with an elivated impact area (1.0cm in dia.) is driven to 25.6 m/sec by shock wave toward the fixed target plate with thickness 0.5-0.8 cm and the bulging deformation at the polished free surface is measured by using laser Doppler velocity interferometer. Initially thick target (1.2-2.4cm) is used for the determination of the velocity of the elastic wave in the solid. In the thinner targets, the free surface gets bulging plastic deformation. Its maximum bulging velocity, which is thickness dependent, are determined for the thickness 0.5-0.8 cm. It is found the maximum bulging velocity decreases approximately inversely with respect to the thickness of the target. and target thickness dependent behaviours of bulging plastic deformation of the mild steel target are obtained for the constant impact velocity and they are discussed on the basis of data obtained by using the laser Doppler velocity interferometer.
