Akio Yoshinaka

Attempts to initiate detonations in metal-sulphur mixtures

The possibility of self-sustained solid-solid detonations (SSD) in Mn+S mixtures has been investigated. Charges 50 mm in diameter under different degrees of confinement were used. The initiation charges used include nitromethane, tetryl, and C-4. No self-sustained SSD were observed in the present study. Neither did the degree of confinement nor the type or weight of the initiation charge appear to make a significant difference in the results obtained. The shock and reaction front are initially coupled but decay rapidly. Decoupling occurs after about 25 mm of propagation when the shock has decayed to about 2.5 km/sec which corresponds to the estimated sound speed of the reactant. The lack of significant feedback of the chemical energy released to the shock via work done by the expansion of the solid products is probably the cause for failure to obtain SSD in Mn+S.

Effect of Shock Precompression on the Critical Diameter of Liquid Explosives

The critical diameter of both ambient and shock‐precompressed liquid nitromethane confined in PVC tubing are measured experimentally. The experiment was conducted for both amine sensitized and neat NM. In the precompression experiments, the explosive is compressed by a strong shock wave generated by a donor explosive and reflected from a high impedance anvil prior to being detonated by a secondary event. The pressures reached in the test sections prior to detonation propagation was approximately 7 and 8 GPa for amine sensitized and neat NM respectively. The results demonstrated a 30% – 65% decrease in the critical diameter for the shock‐compressed explosives. This critical diameter decrease is observed despite a significant decrease in the predicted Von Neumann temperature of the detonation in the precompressed explosive. The results are discussed in the context of theoretical predictions based on thermal ignition theory and previous critical diameter measurements.

A magnetic braking and sensing technique for deceleration and recovery of moving non-magnetic metallic particles

A magnetic braking and sensing technique developed as a potential alternative to assist with the non-contact deceleration and detection of explosively dispersed non-magnetic metallic particles is discussed. In order to verify the feasibility of such a technique and gain an understanding of how the underlying forces scale with particle size and velocity, a study was conducted whereby an aluminum particle moving along a spatially varying but time-invariant magnetic field was modeled and the corresponding experiment performed.

PHASE VELOCITY GENERATOR FOR A TWO‐STAGE IMPLOSION DRIVEN HYPERVELOCITY LAUNCHER

Operation of a two‐stage implosion‐driven hypervelocity launcher in which the launch tube is imploded behind the projectile is examined. In particular, techniques to generate a high phase velocity of implosion are explored, and the technique of using a variable thickness flyer plate with a constant thickness layer of explosive is selected for further investigation. An analytic model, based on the Gurney analysis for flyer plate acceleration, is developed to determine the thickness profile necessary to deliver the desired phase velocity. Finite element simulations and experimental implementation of this approach results in the correct increase in phase velocity along the launch tube, however, the velocity profile does not generate the desired constant acceleration profile.

HIGH‐SPEED PHOTOGRAPHY OF DETONATION PROPAGATION IN DYNAMICALLY PRECOMPRESSED LIQUID EXPLOSIVES

The propagation of detonation in shock‐compressed nitromethane was observed with a high‐speed framing camera. The test explosive, nitromethane, was compressed by a reverberating shock wave to pressures as high as 10 GPa prior to being detonated by a secondary detonation event. The pressure and density in the test explosive prior to detonation were determined using two methods: manganin stress gauge measurements and LS‐DYNA simulations. The velocity of the detonation front was determined from consecutive frames and correlated to the density of the reverberating shock‐compressed explosive prior to detonation. Observing detonation propagation under these non‐ambient conditions provides data which can be useful in the validation of equation of state models.

Initiation of Detonation in Multiple Shock‐Compressed Liquid Explosives

Initiation and resulting propagation of detonation via multiple shock reverberations between two high impedance plates has been investigated in amine‐sensitized nitromethane. Experiments were designed so that the first reflected shock strength was below the critical value for initiation found previously. Luminosity combined with a distinct pressure hump indicated onset of reaction and successful initiation after double or triple shock reflection off the bottom plate. Final temperature estimates for double or triple shock reflection immediately before initiation lie between 700–720 K, consistent with those found previously for both incident and singly reflected shock initiation.

Detonation Propagation in Shock‐Compressed Liquid Explosives

The propagation of detonation in a liquid explosive (nitromethane) that has been precompressed by a shock wave is investigated experimentally. An explosive donor charge is used to transmit a shock wave into a capsule containing a tube of test explosive (sensitized nitromethane) perpendicular to the direction of the compressing shock wave. The shock wave compresses the liquid in the test capsule, then reflects from a steel anvil and further compresses the liquid. A detonation then propagates through the tube of shock‐compressed liquid at velocities as large at 7.5 km/s. Propagation of the detonation is recorded with an electronic streak camera, and the degree of compression is measured using a manganin gauge. The observed velocity as a function of the degree of compression is compared to predictions of detonation velocity as a function of density.

Detonation Initiation in Preshocked Liquid Explosives

The initiation of detonation in a homogenous liquid explosive by the reflection of a strong shock from a high impedance anvil is investigated. By transmitting a sub‐critical shock through a test sample of sensitized nitromethane and then reflecting it normally off a steel plate bounding the explosive, detonation can be initiated in the pre‐shocked medium. The initiation of detonation is observed via fiber optics monitored by photodiodes and by manganin pressure gauges mounted on the steel plate. The initiation of detonation by the reflected shock is inferred from the appearance of intense luminosity and an increase in pressure at the explosive/steel interface, both appearing about 1 μs after shock reflection. The manganin gauge measurements indicate that the critical pressure for incident initiation by a 100 mm diameter shock is 4–5 GPa, while the critical pressure for reflected shock initiation is 7 GPa.