Douglas Venable

Technique for the Determination of Dynamic‐Tensile‐Strength Characteristics

A technique for the examination of the dynamic‐tensile‐strength characteristics of materials is presented. The dependence of tensile strength upon stress gradient, hence stress rate, is demonstrated for Al, Cu, Ni, and Pb. The results were tested and found to be self‐consistent in that once the dynamic‐tensile‐strength characteristics have been measured they can be incorporated into the numerical calculations, which then can be used to predict complex multilayer spall behavior even in layers of dissimilar materials.

SHOCK-FRONT-THICKNESS MEASUREMENTS BY AN ELECTRON BEAM TECHNIQUE

An electron beam densitometer is described for investigating the structure of a shock front in low‐density gases. The parameters measured by this technique are directly related to those parameters used in the accepted definition of shock front thickness. Hence the interpretation of the data is direct; no model of the shock front structure must be assumed and no theoretical treatment of nonequilibrium flow conditions is needed. An approximate method of data reduction is used for treating preliminary results. The measured value of the thickness of a Mach 4 shock in argon is about three mean free paths, measured in terms of the undisturbed gas. This technique provides a new tool for investigating relaxation phenomena behind shock waves.

Apparatus for Precision Flash Radiography of Shock and Detonation Waves in Gases

An apparatus is described which is based on the technique introduced by Kistiakowsky utilizing the absorption of soft x‐rays to measure densities behind gaseous shock and detonation waves. Experimental conditions leading to the smallest absorption statistical uncertainty consistent with maximum sensitivity are defined. These may be approximated at reasonable pressures and tube diameters by adding a strongly absorbing rare gas diluent to the experimental gas mixture. Under such conditions measured densities are accurate to ±1%, as demonstrated by comparison of experimental and calculated densities for shock waves in krypton. At some sacrifice in accuracy, space resolution of 1 mm and time resolution of 1 μsec may be achieved.A continuously pumped, laboratory‐built, demountable, pulsed x‐ray tube with an L‐cathode and tungsten target is used at an accelerating voltage of 20–‐30 kv to obtain currents up to 0.2 amp for durations of the order of 1 msec. Copper and chromium targets were also investigated.For ca...

PHERMEX—A High‐Current Electron Accelerator for Use in Dynamic Radiography

A high‐current, three‐cavity, standing‐wave linear electron accelerator (PHERMEX) has been designed and built to produce a pulsed bremsstrahlung beam. This beam was desired for use in flash radiography of explosively driven metal configurations, as an aid in the study of shock‐induced hydrodynamic phenomena. An unfocused beam exceeding 70 A for 0.10 or 0.20 μsec is available, at a peak energy of about 20 MeV. A 20 A beam for radiographic use, for the same time intervals and peak energy, is focused directly onto a tungsten target to form a 3 mm diam bremsstrahlung source.

Dynamic Observations of the Course of a Shock‐Induced Polymorphic Phase Transition in Antimony

Flash radiographs have been taken which clearly illustrate the formation of both the first and second plastic waves in shocked Sb. The first plastic wave, whose initial pressure and speed are 115 kbar and 3.25 mm/μsec, respectively, is formed in a time much less than 0.2 μsec, the time exposure of the radiograph. However, the second wave builds up gradually to a steady state where the pressure and speed are 135 kbar and 2.9 mm/μsec. At an initial specimen temperature of about 25°C, the time required to complete the phase transformation behind the first plastic wave is determined to be 2–3 μsec.

Single Cavity High Current Electron Accelerator

A single 50‐Mc cavity electron accelerator has been built. When operating in the TM010 mode, it is capable of delivering a 29‐amp, 0.2‐μsec, 8‐Mev, 2‐cm radius beam to a target downstream from the cavity. When focused to a 3‐mm‐diam spot, the beam current is reduced to about 9 amp; the resulting radiation flux is about 1.2 r at one meter for a 0.2‐μsec pulse length.

An Aluminum Splash Generated by Impact of a Detonation Wave

Experimental observations are presented of two‐dimensional flow attending the impact of a detonation wave with an aluminum target plate. A time sequence of flash radiographs of the events provides a means for measuring the formation and growth of the resulting splash. Space‐time histories of various features of the splash are presented. The radiographs also display the course of the reflected shock in the explosive products, the shock transmitted into the target, the rarefaction wave which arises when this latter shock strikes the free surface of the target, the spalling resulting from this rarefaction wave, and the flow of the gases expanding from the burned explosive. Adequate equation of state data exist for both the gaseous products of the high explosive and the aluminum whereby the gross features that are observed can be replicated by todays computer techniques. However, viscosity and dynamic shear and tensile strengths, as functions of local conditions, are needed before a complete computer represe...

Beam Loading in a High‐Current Standing‐Wave Electron Accelerator

Electron‐beam loading of a single‐cavity standing‐wave accelerator has been investigated experimentally and theoretically for the case of continuous injection over ten cycles. The particular cavity used was a 2.4‐m long right‐circular cylinder excited at 50 MHz in its TM010 mode with an unloaded Q of 1.2×105. Beam currents were varied from 40 to 210 A, field strengths from 0.2 to 6 MV/m, and the energy of the ejected beam from 1 to 10 MeV. During beam injection the energy supplied per cycle to the accelerator by its power amplifiers was negligibly small compared to the energy absorbed per cycle by the injected electron beam. The energy extracted by the electrons was determined from cycle‐to‐cycle oscilloscopic observations of the decay of the accelerating field. Energy extraction per cycle was as great as 8% of that residing in the electromagnetic field at the time of beam injection. The results agree with predictions over most of the range of the experimental parameters when assuming paraxial trajectorie...