Shirou Nagano

On Generation of Ultra-High Pressure by Converging of Underwater Shock Waves

The characteristics of a new assembly for the shock consolidation of difficult-to-consolidate powders, such as inter-metallic compounds or ceramic materials, were investigated by both the experimental method and numerical simulation method. The assembly consists of an explosive container, a water chamber, and a powder container. Once the explosive is detonated, a detonation wave occurs and propagates, and then impinges on the water surface of the water chamber. After that, there occurs immediately an underwater shock wave in the water chamber. The underwater shock wave interacts with the wall of the chamber during its propagation so that its strength is increased by the converging effect. We used the usual shadow graph system to photograph the interaction process between detonation wave and water. We also used a Manganin piezoresistance gage to measure the converged pressure of the conical water chamber. Finally, we numerically investigated, in detail, the converging effects of the various conical water chambers on the underwater shock waves. The experimental results and the correspondingly numerical results agree quite well with each other.

Overdriven detonation phenomenon in high explosive

The term “overdriven detonation” refers to detonation process in which the main detonation parameters, such as detonation pressure and propagating velocity, exceed the corresponding Chapman-Jouguet (C-J) values. This kind of detonation state can be realized by the impingement of a high velocity object upon the explosive. This paper presents our initial survey on the occurrence of overdriven detonation in high explosive. The HMX-based PBX is used to accelerate the metal plate being used as the impactor. The target explosive is the so-called SEP, simplified from the term of the ‘safety explosive,’ with the composition of PETN wt.65% and paraffin 35 wt.%. By changing the thickness of the metal plate under an unvaried amount of donor explosive, the different impinging velocities are yielded. The propagation of the detonation wave grown from the impingement of metal plate is recorded by the high-speed streak camera owing to the self-luminosity of detonation. The higher detonation velocity is found from the exp...

High-speed photographic study on overdriven detonation of high explosive

On the common circumstances the detonation of explosives has a steady propagation rate and can be satisfactorily explained by Chapman-Jouguets theory on this phenomenon. Hence, this type of detonation is more frequently called the Chapman- Jouguet (C-J) detonation. The detonation properties such as pressure, density, and temperature, of the detonation products are often characterized as the C-J values of the explosive that represent the corresponding maximums of the variables in the detonation products. However, when an explosive is initiated in some special ways, for instance, high velocity impact of a flyer plate, a strong detonation with properties higher than C-J values will be induced in the explosive. This strong detonation is what we called the overdriven detonation of explosive. The use of overdriven detonation expects to provide much more work to the surrounding matter than does the common C-J detonation. In order to have a basic knowledge of this detonation phenomenon, we designate an experimental set- up for the purpose of acquiring the overdriven detonation in high explosive. The set-up uses a circular metal plate accelerated by a piece of cylinder explosive (donor) to impact another cylinder explosive (acceptor), inducing a detonation wave in the acceptor explosive. The donor explosive used is PBX (85%wt HMX and 15%wt binder) explosive cylinder that has the detonation velocity of 7.84 km/s and the detonation pressure of 25.24 GPa and the acceptor explosive cylinder is SEP (65%wt PETN and 35%wt paraffin) with the detonation velocity of 6.97 km/s and the detonation pressure of 15.9 GPa. The impactor is the copper disc with the same diameter of the donor explosive and 1 mm and 2 mm thicknesses respectively. The detonations occurred in the acceptor explosive from the impact of copper flyer were recorded by the high-speed camera (IMACON 790). The photographs make us possible to estimate the detonation velocities from the distance and time data on them. In addition, we also make a numerical visualization on this phenomenon using a 2-D Lagrangian hydrodynamic code. The calculation, to somewhat extent, reproduces the consequences of the current experimental results.

Converging Underwater Shock Waves for Metal Processing

A new apparatus consisting of an ellipsoidal pressure vessel is proposed for punching holes on pipe walls from the inside. The phenomena taking place in the vessel were investigated experimentally and by a numerical method. When the aspect ratio (i.e. the ratio between the major and minor axes of the ellipsoid) was 2.0, the most suitable pressure distribution was obtained in the apparatus.

Deformation of Metal Pipe due to Underwater Shock Wave

The deformation process of the metal pipes, accelerated by underwater shock wave resulting from the underwater detonation of explosive inside the metal pipe, was investigated by means of both the optical observation experiment and the numerical calculation. The expanding deformation of metal pipes was experimentally viewed by both framing and streak photographic means. A computer code based on the arbitrary Lagrangian and Eulerian (ALE) method was used to perform the numerical simulation on this problem. It has confirmed that the deformations of the metal pipes obtained from the streak photographs agree quite well with those obtained by the numerical calculation. The experimental and numerical results both show that the expanding velocity along the radial direction in aluminum pipe is larger than that in copper pipe, under the same loading conditions: and also, the time needed to reach the maximum radial velocity is shorter in aluminum pipe than in copper pipe. The calculations clearly indicate that the metal pipes are able to acquire a maximum expanding velocity along the radial direction in a very short time after the beginning of the action of underwater shock wave, and also this maximum velocity value only decreases a little in the later time period.

Investigations of fundamental properties of underwater shock waves by high-speed photography

Manufacturing techniques using shock waves generated by underwater explosion have been developed and studied for many years. The major advantages of these techniques are that the pressure acts for a relatively long duration, and there are no thermal effects on the materials. We have been approaching the utilization of the underwater shock wave for various metal processing methods such as in metal forming, explosive welding and shock compacting of difficult-to- consolidate powder. It is necessary to control the underwater shock waves with regard to the processing objectives. In many cases, the underwater shock waves have been used quite close to the explosives. The properties of the underwater shock wave in the region far from the origin of the explosion (more than 1000 mm) have been investigated since long time, but those near the explosive are not yet known. In this paper, we have tried to make clear the properties of shock waves near the explosive. The pressure of the underwater shock wave generated by a detonation cord was measured by a pressure transducer, which was made by a tungsten bar with semiconductor gauges. We found that the pressure of the underwater shock waves decreased almost exponentially. We also investigated the properties of the underwater shock waves by high speed photography. Two kinds of high speed photography were used in experiments. One was the streak photograph and the other was the shadow graph. The propagating of the underwater shock waves was made to clear by framing photographs with an image converter camera. The Profiles of the underwater shock waves were also obtained from high speed photography using a pulses laser as an optical source. Using these profiles of the underwater shock wave, we could obtain the velocity of the shock wave front, and finally we could obtain the pressure across the underwater shock waves by using the Rankine-Hugoniot condition. The pressure obtained by optical measurement agreed with the results obtained by the transducer. As a result of these experiments, the attenuating processes of underwater shock waves, even those near the explosives, were understood.

The irregular reflection from the symmetrical collision of two plane detonation waves in high explosive

The obliquely symmetrical collision of the plane detonation wave in high explosive was observed by means of a high-speed camera in framing mode. The plane detonation wave is generated by two kinds of devices: one using the plane wave generators; the other being the newly devised set-up. The collision angle is set to values greatly larger than the critical angle for irregular reflection of detonation wave from the theoretical prediction. The experimental results show that Mach reflection of detonation wave indeed occurs, but the length of Mach stem is short and the stem shape is smoothly curved. At the same time, the results also illustrate that the collision of detonation wave in high explosive indicates somewhat more complexity than shock reflection in gases and solids.

High-speed photography for pressure generation using the underwater explosion of spiral detonating cord

In recent years we have devoted our efforts to the studies on the various shock processing techniques using explosives for the objectives of gaining materials with the good properties. Those techniques include the punch of pipes, shock consolidation of metallic and ceramic powders, explosive welding of amorphous ribbon on the steel or copper substrate, explosive engraving for the art objects and explosive forming of shells and spheres, and the improvement of the permeability of wood by shock wave. However, to a specific processing technique, it needs to control the shock wave for meeting the demands of that processing purpose. One important control is how to increase the strength of underwater shock wave. Therefore, we propose the following method to converge the underwater shock wave by putting a piece of detonating cord in a spiral way. First, the assignment of the spiral shape of detonation was determined from the geometrical consideration and the basic features of the detonation cord itself. Second, the converging process of the underwater shock wave from the explosion of such designed shape of detonating cord was photographically observed by using the high speed camera in the framing form. The spiral shape with the 100 mm distance from detonating start point to the center of the spiral (indicated by r1) was selected. They were amounted together with the electric detonator and the detonating cord. The photographs confirm that the underwater shock wave moves toward the spiral center in a convergence way. Third, the pressure nearing the spiral center was measured experimentally by means of the pressure transducers. The distance, Dh, between the detonating cord and the transducer was set to be 272 mm. Compared to the case that the detonating cord was placed in straight way, the maximum pressure in the case with the spiral shape is verified to be unchanged, but the impulse, however, is much improved. This reason may be due to over- greatly set Dh. When the distance Dh was set to 50 mm, the pressure measurement was made again and as a result, the large pressure value was record. Compared to the straightly placed detonating cord, it is shown that 3 times higher peak pressure is available in the spiral detonation cord. The results demonstrate that in a small range the pressure of underwater shock wave is indeed converged and higher pressure value is obtained.

Visualizations of underwater shock waves obtained by high efficient explosives

When an explosive detonates in water, it is well known that an underwater shock wave occurs immediately and propagates into the water. Generally speaking, the explosion phenomena near the explosives are too complex for us to understand the shock generating mechanism. Although the characteristics of underwater shock waves far from the explosives have already been investigated by many researchers, the behavior near the explosives has not yet been sufficiently investigated. We have been approaching the metal processing using the underwater shock waves. In this approach, we used the underwater shock waves which were caused in the region close to the explosive. Therefore it is very important to know how the generation and the propagation of the underwater shock waves occurred by the underwater explosion of the explosive. Once the explosives detonate in water, the pressure just behind the detonation wave rises abruptly up to its Chapman-Jougel value. The pressure of the water adjacent to the explosives also immediately rises to its suitable pressure value. The phenomena near the explosives were unsteady, so we used an optical technique to determine their characteristics. We also used a numerical procedure to obtain their characteristics more precisely. Two kinds of explosives were used in experiments. One was a detonating cord and the other a plastic explosive called SEP. The characteristics of underwater shock waves were investigated by shadow graphs and streak photographs. We used a couple Lagrangian-Eulerian method in calculations.

Investigation of a converging underwater shock wave using high-speed photography

We have developed a new assembly for the shock compaction of difficult- to-consolidate powder such as intermetal compounds or ceramic materials. We have successfully obtained crack free bulks of consolidated sample from several kinds of powders for example TiAl alloy or Si3N4 powder etc, using this assembly. This assembly consists of three parts. They are, an explosive container, a water chamber and a powder container. Once an explosive is detonated in the container, the detonation wave occurs and propagates to impinge on the water in the water chamber. When the detonation wave impinges on the water, an underwater shock wave occurs immediately. The underwater shock wave interacts with the solid wall in the chamber, converging its strength as it propagates down stream. Finally, the converged underwater shock wave enters into the powder container to consolidate the powder. The mechanism of this process described above has not yet been fully analyzed. In this paper, the interaction process between the detonating wave and the water are investigated by the streak photograph taken by an image converter camera. The underwater shock wave is accelerated in a short time up to its final velocity, which is slightly larger than that determined by the impedance match method. The converging process of the underwater shock wave was also understood by the high speed photograph obtained by framing photographs using shadowgraph system. The incident shock wave comes up to about 18 GPa after being converged by the wall of the pressure chamber. Finally, we suggest the most suitable design of pressure assembly.