Kazuhito Fujiwara

Explosive-driven cylindrical imploding shocks

Cylindrical imploding shocks are analyzed numerically. The shocks are supposed to be generated by detonating cylindrical explosive shells. Detonations are assumed to be initiated either from the inner or the outer surface of the shells. Explosive shells of different thicknesses are compared. The conversion mechanism of the chemical energy of explosives into gas energy is investigated. In the implosion stage general features of the flow near the shock front are much the same between two different initiations. The same is also true among different explosive thicknesses. However, in the reflection stage after the shock is reflected from the axis, thicker explosives produce higher pressure and temperature near the axis. The difference in the initiation method does not make a very noticeable difference, even in the reflection stage. Shock trajectories, temperatures at the shock front, and evolutions of the pressure at a fixed point are compared between numerical and experimental results. Agreements between the two results are close.

Numerical simulation of cylindrical converging shocks in solids

Cylindrically converging shock waves in solids have been analyzed by the random choice method (RCM). A Riemann solver for fluidlike solids with the Gruneisen‐type equation of state is constructed and incorporated into the RCM. It is then applied to the cylindrical shock tube problems for solid copper with driving pressures of 20 and 200 GPa. The numerical results are compared with those of the finite difference method (FDM). The shock speed is smaller and the discontinuity at the shock front is smeared out due to the artificial viscosity in the FDM calculation. Spatial distributions of pressure, density, and particle velocity calculated by the RCM show that the steepness at the shock front is maintained both in the converging and reflecting stages. It is shown that the pressure on the shock front and the total energy contained in the central circular area are much larger in the reflecting stage than in the focusing stage. The dimensional analysis has shown that the similarity solution exists; however the ...

The sterilization of dry powdered foods by successive impacts

The thermal sterilization is limited to use for fungi and bacteria in some kinds of dry powdered foods because of the heat sensitivity of their flavor. The sterilization method using impulsive loads or shock waves has the characteristic to sterilize foods without heating. Higher shock pressure can shows higher disinfecting action, but has some problems such as the degradation of foods and the hardness of handling. The proper choice of energy sources is one of points to make the method push to the industrial level. Safety and facility are also important factors to develop the practical system. In this paper as the case of the lowest limit of the impact load, the powder was impacted by mechanical hammer. It was found that the iterative impacts showed the disinfecting effect although the efficiency was low at an impact. Several processing cycles were required to show the significant decreasing of bacteria, because the apparent colony number (not actual bacteria number) increased in the impact process. The numerical model to estimate the mechanical condition in sterilizing process is also presented in this paper.

Explosively Driven Expansion and Fragmentation Behavior for Cylinders, Spheres and Rings of 304 Stainless Steel

Explosive loading techniques are applied to expand tubular cylinders, spherical shells and rings of 304 stainless steel to fragmentation, and the effects of wall thicknesses, explosive driver diameters and the constant proportionality of the in-plane biaxial stretching rates are investigated on the deformation and fracture behavior of three basic structures experimentally and numerically. In the cylinder tests, the driver is a column of high explosive PETN, inserted coaxially into the bore of a cylinder and initiated by exploding a fine wire bundle at the column axis using a discharge current from a high-voltage capacitor bank. In case of the ring tests, ring specimens are placed onto a single cylinder filled with the PETN as a expansion driver, and for sphere tests, specimens filled with the PETN are also initiated by exploding a fine copper wire line with small length located at the central point. Two types of experiments are conducted for every specimen and test condition. The first type uses high speed cameras to observe the deformation and crack generation of expanding specimens showing the final maximum in-plane stretching rate of above , and the second uses soft capturing system recovering typically most fragments successfully. The fragments are measured and investigated using a fragmentation model. The effects of test parameters on the deformation and fracture behavior for three types of structures are discussed in need of modified fragmentation model for shell structural elements.

Spall fracture of metallic circular plates, vessel endplates and conical frustums driven by direct explosive loads

Dynamic fracture experiments are conducted for circular plates, vessel endplates and conical frustums of A2017‐7075 aluminum alloys and 304 stainless steel, using a testing apparatus developed applying wire‐row explosion technique to initiation, where tensile stress waves are generated producing spall in the specimens by the direct incidence of plane detonation waves of the explosive PETN. A VISAR system is adopted to observe the free‐surface velocity histories of the specimens. The signals for basic circular plate specimens indicate the characteristics of the failure for tested materials, effects of explosive thickness variations and the configuration of specimens. Hydro codes are satisfactorily applied to simulate the experimental signal data and observed damage phenomena of recovered specimens. Next, an explosive‐filled cylindrical vessel with an endplate at the one end is initiated at the other end surface and expanded by axially propagating explosive detonation to fracture. Both the VISAR signals and...

Analysis of Explosion Combustion Phenomenon with Ammonium Nitrate

In this study, an explosion combustion phenomenon of ammonium nitrate (ρ=1.7kg/cm3) was used instead of the explosive PETN and the ultra-high-speed destruction phenomenon of aluminum cylinder was analyzed. The mix powder obtained by mixing aluminum powder (ρ=2.7kg/cm3) and ammonium nitrate was used instead of the explosive PETN, and an explosive combustion phenomenon was generated using copper wire explosion by high-voltage capacitor bank (40kV, 12.5 /F). Ammonium nitrate and aluminum powder are kneaded in a combination ratio of 5 : 1 in mass. An aluminum cylinder was destroyed by the phenomenon. The experiments were conducted using various diameters of ammonium nitrate particle and the photographs of the phenomenon were taken by the high-speed camera (IMACON468) and the high-speed video camera (HPV-1). The fragments of aluminum cylinder were collected and their dimensions were measured. The explosion phenomenon and fragments were compared with the result by explosive PETN. This paper presents these experiments and analysis result. And, hydro codes have been applied to simulate the deformation behavior of the aluminum cylinder.

Mechanical changes in materials caused by explosive precompression shock waves and the effects on fragmentation of exploding cylinders

Explosive driven rapid fracture in a structural body will be preceded by a compression process, and the compression effects on mechanical properties of the materials are clearly important to understand shock-induced failure such as spall or fragmentation phenomena. In this study, incident shock waves in plate specimens of aluminum A2017-T4 and 304 stainless steel are generated by plane detonation waves in the high explosive PETN initiated using wire-row explosion techniques, and the compressed specimens are successfully recovered without severe damages due to the reflected expansion waves with use of momentum trap method. A hydro code, Autodyn-2D is applied to determine test conditions: thicknesses of explosives, attenuators, specimens and momentum traps and to evaluate experimental results, simulating time-histories of stress waves in the layers of the test assembly. Microhardness distributions in cross-sections, tensile strength, fracture ductility and yield stress are measured for the recovered specimens, using miniature tensile and compression test pieces machined from them. They are compared with those of virgin specimens, showing significant increase of hardness, tensile and yield strength and remarkable reduction of elongation and ductility for shocked specimens. The results are taken into consideration for evaluation of experimental fragmentation energy in cylinder explosion tests.

Rapid Expansion and Fracture of Metallic Cylinders Driven by Explosive Loads

Smooth walled tubular specimens of stainless steel and low‐carbon steels were explosively expanded to fragmentation. The driver was a column of the high explosive PETN inserted into the central bore and initiated by exploding a fine copper wire using a discharge current from a high‐voltage capacitor bank. The variation of wall thickness and the effect of different explosive driver diameters are reported. A fully charged casing model was also exploded with initiation at the end surface for comparison. Streak and framing photos show both radially and axially symmetric expansion of cylinders at average strain rates of above 104 s−1 and a wall velocity of 417–1550 m/s. Some framing photos indicate the initiation and spacing of fractures during the bursting of the cylinders. Hydro codes have been applied to simulate the experimental behavior of the cylinders, examining numerical stresses, deformation and fracture criteria. Most of the fragments were successfully recovered inside a cushion‐filled chamber, and t...

The Effect of Wall Configuration on Deformation and Fragmentation for Explosively Expanded Cylinders of 304 Stainless Steel

Tubular specimens of stainless steel with smooth and notched walls were explosively expanded to fragmentation. The driver was a column of the high explosive PETN inserted into the central bore and initiated by exploding a bundle of fine copper wires using a discharge current from a high-voltage capacitor bank. The effect of wall thicknesses and different explosive driver diameters are reported for smooth walled cylinders. Fully charged notched cylinders with single axial groove or slit in the walls were also exploded for comparison. Streak and framing photos show both radially and axially symmetric expansion of cylinders at average strain rates of above 10 4 s -1 and a wall velocity of 667-1550 m/s for smooth cylinders. Some framing photos indicate the initiation and spacing of fractures during the bursting of the cylinders. A hydro code based on FDM and SPH has been applied to simulate the experimental behavior of the cylinders, examining numerical stresses, deformation and fracture criteria. The circumferential fracture spacing of recovered fragments is investigated using a fragmentation model. There were some differences in fragmentation between smooth and notched cylinders, but overall deformation behavior was almost same. Introduction An accidental explosion of high energy storage vessels or cases generally causes serious hazard and the understanding of high speed deformation and fracture behavior of basic cylindrical structures has been of great concern in safety evaluation and fracture control design. Wire explosion techniques had been applied by authors and Forrestal et al. to uniform expansion [1, 2] and fragmentation [3] of smooth-wall metallic cylinders, producing cylindrically diverging detonation waves in the installed powder high explosive pentaerithritoltetranitrate (PETN). In other studies, Grady [4] proposed a fragmentation model for explosive-filled steel cylinders, detonated at one end. In this study, uniformly expanding behavior and fragmentation for a smooth cylinder of 304 stainless steel are compared with those for cylinders with different wall configuration regarding thickness, surface groove or slit. The observed behavior is reproduced by the numerical simulation using Autodyn 2D, and a modified Grady’s model is proposed for fragmentation. Experimental and Numerical Method The developed test assembly for uniform expansion of cylinders is illustrated in Fig.1. Tubular specimens of 18Cr-8Ni stainless steel (JIS SUS 304, dynamic proof stress: 340 MPa) are explosively expanded to fragmentation. A column of the high explosive PETN (charged density: 0.90-0.95 g/cc) is inserted into the central bore and initiated by exploding a bundle of three copper wires (diameter 175μm) set along the central axis of the column using a discharge current from a high-voltage capacitor bank (40kV, 12.5μF). The generated cylindrical diverging detonation wave becomes the driver for the specimens. The cylinder of 100 mm length, outer diameter (Do) 34 mm and wall Materials Science Forum Online: 2004-09-15 ISSN: 1662-9752, Vols. 465-466, pp 225-230 doi:10.4028/www.scientific.net/MSF.465-466.225

Shock Sterilization of Dry Powder Foods

The sterilization for fungi and bacteria in some kinds of dry powder foods is limited to keep their grade. Especially in spices the heat sterilization is restricted to minimum use, because the heat reduces the hot-taste that is a principal component in spices. The operation for the sterilization has to act only on fungi and bacteria, not to degrade the volatile components contained in flavor elements. In this paper, the instance of shock sterilization is shown, and the development of the idea to the sterilization equipment and its performance are presented. Experimental results showed the potential of the shock for the sterilization and the feasibility for the industrial use.Copyright