Title: AMPLIFICATION OF RAYLEIGH WAVES IN A SURFACE LAYER OF VARIABLE THICKNESS.

The mechanical effects of a thin surface layer in reducing the growth of the Rayleigh-Taylor instability of a plastic solid plate is investigated by representing the surface layer as an elastic plate of high yield strength and high bonding strength to the plastic. This plate may be either the surface layer itself or a composite material formed by interactions of the surface layer with the solid surface layers. In this approximation it is found under wide ranges of conditions that short-wavelength disturbances are stabilized in linear approximation. In this limit the ''gravitational'' energy increment of the displaced plastic solid ismore » overcome by the elastic shear energy of the stretched plate. Long-wavelength disturbances are very weakly affected by this mechanism. For a plastic plate of approximately 1-cm thickness subjected to a pressure difference of approximately 100 kbar we expect wavelengths shorter than lambda/sub c/ approximately 10/sup -1/ cm to be effectively stabilized by this mechanism.« less

The measurement of the thickness of the boundary layer with a capacity probe is described. This probe flush inserted into the wall of the shock tube and thus avoiding any disturbances in the flow is sensitive to the thickness of the boundary layer, when the gas in the undisturbed flow is ionized. The experimental results for argon at Mach numbers between 5 and 10 show the usefulness of the probe for measuring the thickness of the boundary layer, the relaxation time of ionization and the coefficient of diffusion. (auth)

The successful deployment of an MX missile system requires a careful consideration of ground shock effects which were of secondary importance in previous applications. For example, with the system sited in a valley and under a multiple attack scenario, untargeted points can be expected to experience a significant ground motion environment as a result of the superposition of motions originating from attacks on a variety of surrounding aimpoints. Furthermore, reflections of outgoing energy from the valley boundaries can be expected to complicate the ground motion environment within the valley, particularly at late times. The objective of the work described inmore » this report has been to develop a better quantitative understanding of these late-time, long-period ground motions in order to provide a firmer basis for scaling to new geologic conditions. Particular emphasis has been placed on the identification of the characteristic mode of propagation associated with these arrivals. A theoretical model is described which has been used to compute the surface waves produced by a propagating airblast load acting on the surface of a multilayered, elastic half-space. This model was then applied to the analyses of both the observed data and finite difference simulations of the Pre-Mine Throw and Pre-Dice Throw 100 tone HE surface blasts. In addition, a preliminary assessment has been made of the influence of valley boundaries on the late-time ground motion environment within the valley.« less

The acoustoelastic effect refers to the fact that elastic wave velocities vary with stress. Measurements of the change in stress induced velocity yield information which leads to the determination of stresses. Some work has been done to explore the possibility of using ultrasonic waves, including bulk longitudinal and shear waves as well as surface waves, for the nondestructive evaluation of stresses. This paper focuses on Rayleigh surface waves, which have the advantage of detecting both surface stresses and stress gradients. Piezoelectric or electromagnetic acoustic transducers have been used for the application of Rayleigh waves in the acoustoelastic measurement of stress.more » The size of these transducers and their fixtures limit the distance between transducers. Sharp edge wedges are usually bound to the pick-up piezoelectric transducers, known as the Rayleigh or surface wave device (SWD), to provide a better spatial resolution when the stress field is not uniform. The smallest distance reported is 11 mm. On the other hand, acoustic microscopes (AM) can measure localized stress for a very small area, from 30 {micro}m to 2 mm. Both SWD and AM method may lack the agility required for general applications. Laser ultrasonics (LU) is a method for optical generation and detection of ultrasound. The generation and detection areas can be focused to very small spot sizes, less than 1 mm, which allow velocity measurements to be made over a path length of a few millimeters. The LU technique is non-contact and can be applied remotely; it has many potential applications. The authors used the LU technique to measure the acoustoelastic behavior of an aluminum alloy, A16061-T6.« less