Papers by Author: Yuji Wada

Abstract: Nitrocellulose (NC) is known to undergo spontaneous ignition, and its thermal stability has been previously reported to decrease in the presence of sulfuric acid. In this study, we evaluated the decomposition and ignition behavior of NC in the presence of sulfuric acid using an accelerating rate calorimeter (ARC) and hand-built ignition testing equipment. The results of the ARC experiment indicated that the thermal stability of NC decreased when the amount of sulfuric acid was increased above 10^-5 mol/g (NC). In addition, when NC (4 g) mixed with sulfuric acid (6.6 mol/L, 1 g) was isothermally stored at 75°C in a glass flask, the color of the reaction mixture changed from white to brown. After 83–127 min, NC spontaneously ignited and produced a loud explosion.

Abstract: In order to know accurate information on the non-ideal detonation pressure, steel tube test was carried out on ammonium nitrate (AN) and activated carbon (AC) mixtures. In this test, detonation velocity and pressure were measured simultaneously by varying thickness of PMMA placed between AN/AC and pressure gauge. The length and the diameter of the steel tube were 350 mm and 35.5 mm. The results showed that shock pressure attenuation in PMMA was not observed for this experimental condition (PMMA gap; 3-5 mm). The averaged measured peak pressure and detonation velocity were 3.4 GPa and 3.2 km/s.

Abstract: The crater on the reinforced concrete wall generated by the explosion of a few grams of Composition C4 explosive was investigated. A series of the single-shot blast experiment were conducted. The structural specimens for the reinforced concrete wall (750 mm × 750 mm × 250 mm) were used for estimation of the crater size. The diameter of the borehole was fixed 16 mm, and the length of borehole was varied to generate the various size of crater. The amount of explosive was also varied from 3.0 to 8.0 g. The effect of the length of the borehole to the crater depth was examined. High-speed photography was used to observe the fracture process of the crater. As the results, in the difference of the side where the crater was generated, the situation of crushing was different. The strain measurements were also conducted.

Abstract: A multidimensional analysis code for reactive shocks (MARS), which is developed to solve various problems in the physical hazard analysis of high energetic materials, has been applied to such complex problems as multi-material problem and sympathetic problem because it can employ various types of equations of state and a materials database. However, it was difficult to meet a growing demand for large-scale analysis and fluid-structure interaction (FSI) analysis. To address these issues, this study reports a parallelization of the code and an implementation of the functional capability of FSI analysis, and performance results for sample problems were also shown.

Abstract: The crack on the reinforced concrete wall generated by the explosion of several grams of Composition C4 explosive was investigated. A series of the single-shot blast experiment were conducted. The structural specimens for the reinforced concrete wall () were used for estimation of the crater size. The diameter of the borehole was fixed 16 mm, and the depth was varied to generate the various size of crater. The amount of explosive was also varied from 2.0 to 3.5 g. The effect of the depth of the borehole to the crater depth was examined, and the optimum charge condition for making the large crater at borehole side was clarified. For the case in which the fragmentation at the back side was allowed, the optimum charge condition was also found. High-speed photography was used to observe the growth process of the surface cracks. As the results, it was predicted that the crater was generated at 2 ms after explosion at least. The strain measurements were also conducted.

Abstract: In this study, a high strain rate test method of a steel plate under blast loading from high explosive was designed and was conducted by a combined experimental/numerical approach to facilitate the estimation process for the dynamic stress-strain curve under practical strain rate conditions. The steel plate was subjected to a blast load, which was generated by Composition C4 explosive and the dynamic deformation of the plate was observed with a high-speed video camera. Time-deformation relations were acquired by image analysis. A numerical simulation for the dynamic behaviors of the plate identical to the experimental condition was conducted using a coupling analysis of finite element method (FEM) and discrete particle method (DPM). Explosives were modeled by discrete particles and the steel plate and other materials were modeled by finite element. The blast load on the plate was described fluid-structure interaction (FSI) between DPM and FEM. As inverse analysis scheme to estimate dynamic stress-strain curve, an evaluation using a quasistatic data was conducted. In addition, two types of approximations for stress-strain curve were assumed and optimized by least square method. One is a 2-piece approximation, and was optimized by least squares method using a yield stress and a tangent modulus as parameters. The other is a continuous piecewise linear approximation, in which a stress-strain curve was divided into some segments based on experimental time-deformation relation, and was sequentially optimized using youngs modulus or yield stress as parameter. The results showed that the piecewise approximation can gives reasonably agreement with SS curve obtained from the experiment.

Abstract: To obtain a better understanding of detonation properties of ammonium nitrate (AN) and activated carbon (AC) mixtures, steel tube test was carried out for stoichiometric composition of powdered AN and AC mixtures and the detonation velocity and the pressure profile were measured. Based on the results obtained the relation between the detonation velocity and the peak pressure was discussed with the theoretically predicted values which were obtained by the thermohydrodynamic CHEETAH code with the BKWC equation of state. The measured detonation velocity and peak pressure were far below the theoretically predicted values and the non-ideal detonation behaviour was confirmed.

Abstract: To obtain a better understanding of detonation properties of ammonium nitrate (AN) and activated carbon (AC) mixtures, steel tube test with several diameters was carried out for various compositions of powdered AN and AC mixtures and the influence of the charge diameter on the detonation velocity was investigated. The results of test indicated that the detonation velocity increased with the increase of the charge diameter. The experimentally observed values were far below the theoretically predicted values made by the thermohydrodynamic CHEETAH code and they showed so-called non-ideal detonation. The extrapolated detonation velocity of stoichiometric composition to the infinite diameter showed a good agreement with the theoretical value.

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: 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.