Small Explosive Charge Research Articles

Limiting currents in the compression of magnetic flux between flat and coaxial conductors

Experiments are described on magnetic flux compression by flat and coaxial conductors. As the initial current I0 is increased the final current I1 obtained as a result of flux compression at first increases proportionally to I0 and then reaches a maximum and remains constant for further increases in I0. Analysis of the experiments shows that in coaxial structures when a small explosive charge accelerates the conductors the limiting current is determined by the maximum work which a conductor can perform in compressing the magnetic field. In experiments with flat busbars and large explosive charges the limiting currents appear to be determined by the flux losses in short-circuited voids formed in the linking of irregular surfaces of the busbars. This assumption is shown to be in qualitative agreement with experiment.

Vertical Coherence of Shallow-Water Reverberation

During a field experiment in 200 ft of water off the west coast of Florida, the vertical coherence of the reverberation following a small explosive charge was measured by recording, and later correlating, the outputs of a string of vertically separated hydrophones. The reverberation coherence, in terms of the cross-correlation coefficient between hydrophone pairs, was found to increase with time after detonation, so as to indicate that the reverberation reaches the hydrophone string, within an ever-narrowing vertical angle, as time or range increases. The equivalent vertical angle of the reverberation was found to decrease with time or range and to lie in the range 5°–30° at frequencies between 200 and 8000 Hz. It is so small that only an extremely long vertical array can yield an appreciable array gain for active sonars operating in a background of shallow-water reverberation.

Vertical Coherence of Shallow-Water Reverberation

During a field experiment in 200 ft of water off the west coast of Florida, the vertical coherence of the reverberation following a small explosive charge was measured by recording, and later correlating, the outputs of a string of vertically separated hydrophones. The reverberation coherence, in terms of the cross-correlation coefficient between the sound pressures on hydrophone pairs, was found to increase with time after detonation, so as to indicate that the reverberation reaches the hydrophone string within an ever-narrowing vertical angle as time or range increases. The equivalent vertical angle of the reververation—here called the coherence angle—was found to lie in the range 5°–30° at frequencies between 200–8000 Hz. It is so small that only an extremely long vertical array can yield an appreciable array gain for active sonars operating in a background of shallow-water reverberation.

An underwater explosive shock gun

A theoretical study is made of a simple design for an underwater shock gun, which consists of a chamber in the form of a hollow circular cone, with a spherical sector of explosive charge fitted into the apex. When the explosive is initiated at the apex, the resulting sector of a spherical blast wave will be diffracted by expansion waves moving inwards after the leading shock has emerged from the rim of the cone.The progress of the expansion wave-fronts is calculated, and the results show a surprising inability of the diffraction process to ‘eat into’ the full-strength sector of spherical blast. It is found to be possible to design such a gun so that it is capable of projecting a high intensity shock-pressure beam over a considerable range, using only a very small explosive charge.

The Measurement of Dynamic Strains Using Birefringent Strips

An experimental study is presented of the bonded birefringent strip as a means of observing transient deformations on the surfaces of opaque bodies. The particular situation investigated consists of a birefringent sheet bonded to a metal rod in which a strain pulse is initiated by a small explosive charge. Major emphasis is placed upon the study of two problems associated with the use of the technique. The first is the development of a reliable method of interpreting the observed photoelastic fringe patterns. The second is an evaluation of the influence the sheet has on the strain pulse in the bar. Satisfactory agreement is obtained between the strain profile at the rod-sheet interface determined by using a birefringent strip and the profile given by resistance-wire strain gages.

Energy Spectra of Small Explosions at Great Depths

The total energy spectrum of a small explosive charge fired at great depths differs markedly from the smooth shock-wave spectra that have been demonstrated by Weston and others. The oscillations due to the interaction of shock-wave and bubble pulses are pronounced even at kilocycle-per-second frequencies, and the bubble contributes significant additional spectral energy at several harmonics of the bubble frequency (inverse of the bubble period). In a study of the effect of charge weight and depth on the acoustic signal from an explosive source, spectra were digitally computed from broadband pressure recordings made at the surface above small charges fired in the ocean. Charge weights varied by a factor of 150, depths by a factor of 25, and bubble frequencies ranged from about 50 to 1500 cps. Methods of estimating spectrum level using semi-empirical expressions for the pressure-pulse parameters are discussed.

An Experimental Study of Pulse Propagation in Elastic Cylinders

The distortion of longitudinal pulses in elastic cylinders resulting from the effects of lateral inertia is discussed in terms of the Pochhammer treatment The experimentally observed displacement-time curve produced at one end of a steel cylinder when a small explosive charge is detonated at the opposite end is compared with that obtained theoretically using Fourier synthesis. It is found that only the fundamental mode of propagation need be considered in predicting the pulse shape under such conditions.