Materials Science Forum Vol. 566

Abstract: Numerous experimental investigations on the reflection of plane shock waves over straight wedges indicated that there is a domain, frequently referred to as the weak shock wave domain, inside which the resulted wave configurations resemble the wave configuration of a Mach reflection although the classical three-shock theory does not provide an analytical solution. This paradox is known in the literature as the von Neumann paradox. While numerically investigating this paradox Colella & Henderson [1] suggested that the observed reflections were not Mach reflections but another reflection, in which the reflected wave at the triple point was not a shock wave but a compression wave. They termed them it von Neumann reflection. Consequently, based on their study there was no paradox since the three-shock theory never aimed at predicting this wave configuration. Vasilev & Kraiko [2] who numerically investigated the same phenomenon a decade later concluded that the wave configuration, inside the questionable domain, includes in addition to the three shock waves a very tiny Prandtl-Meyer expansion fan centered at the triple point. This wave configuration, which was first predicted by Guderley [3], was recently observed experimentally by Skews & Ashworth [4] who named it Guderley reflection. The entire phenomenon was re-investigated by us analytically. It has been found that there are in fact three different reflection configurations inside the weak reflection domain: • A von Neumann reflection – vNR, • A yet not named reflection – R, • A Guderley reflection – GR. The transition boundaries between MR, vNR, R and GR and their domains have been determined analytically. The reported study presents for the first time a full solution of the weak shock wave domain, which has been puzzling the scientific community for a few decades. Although the present study has been conducted in a perfect gas, it is believed that the reported various wave configurations, namely, vNR, R and GR, exist also in the reflection of shock waves in condensed matter.

Abstract: This paper reports on the design and performance of a large diameter diaphragmless shock tube that has been recently developed in order to experimentally study various basic characteristics of the gas-dynamic laser (GDL). The main engineering element of the shock tube is a diaphragm-like sliding piston (in place of a rupturing diaphragm) by which normal shock waves are formed. The role of such a structure in generating repeatable shock waves is discussed. The shock tube performs in good accordance with the simple shock tube theory, as has been verified so far by experiments with some conventional lasing gases (gaseous mixtures of CO2 and N2 and those diluted with an excess of He) at shock wave Mach numbers ranging from 1 to 5. Recent results of the stagnation conditions achieved in the shock tube with application to GDL experiments are included as well.

Abstract: The behaviors of the high explosive near the critical conditions for shock initiation of detonation are investigated by high speed photography and pressure measurements in gap tests. The sample is RDX base explosive, and the inner diameter of donor and acceptor charges is 26 mm. Gap material is PMMA. Near the critical condition, the results under the following conditions have been discussed. 1) Shock to detonation transition (SDT) take place in acceptor, 2) The SDT does not occur, but the reaction wave affects the leading shock front in acceptor, and 3) The gap length in which the effect of the reaction wave to shock front almost disappears. These results are very useful to construct the initiation model for solid explosive.

Abstract: Shock waves are indispensable tools for medical applications, and hence their interactions with human tissue become one of the most important basic research topics. In this paper, the determination of shock Hugoniot curves for liquids that can model human tissue, namely water, castor oil, and aqueous solutions of sodium chloride, sucrose and gelatin, at 10 and 20 weight percent are presented. Underwater shock waves were generated by ignition of 10 mg silver azide pellets and time variations of over-pressures were measured and simultaneously the shock speed was measured by the time of flight technique. Then shock Hugoniot curves were obtained, by assuming the Tait type equation of state, to relate the estimated density and measured pressure values. Results show in the cases of aqueous solutions that increasing amount of additives into water causes only a very minute decrease in the compressibility of the solution. This difference was more pronounced in the case of sodium chloride, less for gelatin, and almost none for sucrose aqueous solution.

Abstract: To design a cylindrically-shape explosion container, the experiment of a high explosive charge detonating in a steel pipe has been performed. The charges, composition C4, were positioned at the geometrical centre of the steel pipe. Two kinds of measurements were performed on the steel pipe: circumferential strain and outside diameter. The strain-time history shows that the pipe structure vibrates and the vibration is decaying. It has been reported that this type of response is explained as the mechanism of strain growth, and this problem is taken up to verify computer simulation in this study. This simulation code could be strong tool to estimate the geometries of the explosion container. The relationship among the pipe parameter, explosive charge and pipe’s final deformation is proposed as practical guidance for predicting radius and thickness of the pipe correspond to the level of internal blast loading.

Abstract: This paper presents the design of a compact size projectile accelerator, and its application. To meet the various needs such as a compact body size to use under various experimental conditions, an easy maintenance for repetitive experiments during a certain period, and a capability of the velocity control, the compact accelerators were newly designed with a direct explosive drive method. Two different types of accelerator were designed: a PMMA accelerator and a metal accelerator. The pictures of the projectile shoot using the designed accelerators were recorded by SHIMADZU HyperVision HPV-1 high-speed video camera. As a result, it was recognized that the PMMA accelerator was failed to accelerate the projectile, while the metal accelerator succeeded to accelerate it effectively. The accelerating performance of the metal accelerator was further investigated. The explosives for projectile acceleration were Emulsion explosive and Composition C4 explosive weighing 5 to 35g. It was found that the metal accelerator has the capability to control the projectile velocity adjusting the weight of the explosives, and there is an approximate linear correlation between them in our experimental range. A series of impact tests on 5052S aluminum alloy targets was examined using the accelerator.

Abstract: Some liquid explosives have two different detonation behaviors: high velocity detonation (HVD) or low velocity detonation (LVD). The detonation behavior depends on the level of the initiating shock pressure. The detailed structure of LVD in liquid explosives has not yet been clarified. A physical model was proposed that LVD is not a self-reactive detonation, but rather a supported-reactive detonation from the cavitation field generated by precursor shock waves. In this study, high-speed photography was used to investigate the detonation behavior of nitromethane (NM) with the various initiating shock pressures. Stable LVD was not observed, only transient LVD was observed. A very complicated structure of LVD was observed: the interaction of multiple precursor shock waves, multiple oblique shock waves, and a cavitation field. Multiple shock waves propagating in non-detonating NM were observed for shock pressures below the range required for LVD, while above the LVD range HVD was observed.

Abstract: New multiphysical computational models for simulation of regular open and closed-cell cellular structures behaviour under compressive impact loading are presented. The behaviour of cellular structures with fluid fillers under uniaxial impact loading and large deformations has been analyzed with the explicit nonlinear finite element code LS-DYNA. The behaviour of closed-cell cellular structure has been evaluated with the use of the representative volume element, where the influence of residual gas inside the closed pores has been studied. Open-cell cellular structure was modelled as a whole to properly account for considered fluid flow through the cells, which significantly influences macroscopic behaviour of cellular structure. The fluid has been modelled by applying a Smoothed Particle Hydrodynamics (SPH) method. Computational simulations showed that the base material has the highest influence on the behaviour of cellular structures under impact conditions. The increase of the relative density and strain rate results in increase of the cellular structure stiffness. Parametrical numerical simulations have also confirmed that filler influences the macroscopic behaviour of the cellular structures which depends on the loading type and the size of the cellular structure. In open-cell cellular structures with higher filler viscosity and higher relative density, increased impact energy absorption has been observed.