Detonation Waves In Explosives Research Articles

Simulation of Exploding Foil Initiator Flyers Driving Detonation

Abstract Flyers are the key components that determine the detonation ability of Exploding Foil Initiators (EFI). The incident attitude and velocity of the flyer are the decisive factors affecting the initiation of explosives. This work utilizes a new simulation method to simulate the impact detonation process of flyers. Here we simulate the process of flyers being cut and accelerated, impacting explosives and explosives being detonated. It is possible to obtain the orientation and dynamic velocity changes of the flyer during its motion and the propagation process of detonation waves in explosives. The results provide detailed flyer morphology during the operation of the EFI and pressure changes in explosives. The results indicate that during the formation process of 10 μm and 20 μm flyers, the flip will occur. Additionally, 20 μm and 25 μm flyers can detonate HNS-IV, while 10 μm and 30 μm flyers can not detonate HNS-IV.

Design and characterisation of the high‐current DC breaker driven by explosive

AbstractThe development of a high‐current DC breaker is very important for the protection of the expensive superconducting magnet. A kind of an economic switch called pyrobreaker, which is driven by explosive detonation waves, has been used as the backup quench protection due to its inherent reliability and fast response. A prototype pyrobreaker with a special structure that can withstand and break 100 kA is discussed. The numerical material model is taken to calculate the fracture demand of contact and the mass demand of explosive. Then, the detonation wave and contact breaking process are simulated by the ANSYS software. The performance of the design and characterisation during dynamic breaking are compared with simulation and no‐load experiment test. At the end, the temperature rise test and the breaking test under 100 kA are completed successfully. These results demonstrate that the design of 100 kA pyrobreaker is reliable.

Characteristics of Fragment Impacting Steel Cover at High Speed to Initiate Heated Explosives

AbstractThe temperature of the explosive has a great influence on its shock initiation sensitivity. The response characteristics of the heated explosive under fragment impact is a concern for the safety of the explosive. In order to study the initiation characteristics of heated explosives subjected to fragment impact, an experimental device that uses the shaped charge to generate the high‐velocity fragment (2599 m/s) to impact the cover to initiate the heated explosive was designed in this paper. This method achieves uniform temperature control of explosives via heating the upper and lower ends and preserves heat on the side of the explosive charge. We experimentally tested the method on hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX)‐based aluminized explosives (61 wt.% RDX, 30 wt.% Al, 9 wt.% binder) at different temperatures. The fragment impact behavior and explosive detonation wave growth were observed via X‐ray photography. A numerical model of fragment initiating explosive, which considered the temperature change of the explosive, was set up, a numerical simulation of the explosive charge initiated by fragment impacting the cover was carried out, and the reaction degree of explosive impacted by fragment and its change with temperature were analyzed. The study found that the precursor shock wave formed by the fragment impacting the cover initiated the explosive, and the explosive temperature greatly influences the fragment initiation process. Through the numerical simulation calculation of the explosive charge initiated by fragment impact under different cover thicknesses and temperatures, the relationship among the critical cover thickness, the run‐to‐detonation distance and explosive temperature was given. In the temperature range of 25 °C–111 °C, the shock sensitivity of RDX‐based aluminized explosives under fragment impact decreases with the increase of explosive temperature, while in the temperature range of 111 °C–170 °C, the sensitivity increases with the increase of explosive temperature.

Acting law of in-hole initiation position on distribution of blast vibration field

In rock drilling and blasting, the in-hole initiation position determines the propagation direction of the explosive detonation wave, and thereby affects the distribution of blast vibration field (BVF). In this study, the acting mechanism of the initiation position was investigated via the comprehensive analysis of the distribution of the detonation products, the explosion energy as well as BVF of the cylindrical charge. Then, the distribution law of BVF under different initiation positions was analyzed using the Heelan’s short-column-solution based superposition model of an extended charge. At last, the acting effect of the initiation position on the distribution of BVF was demonstrated by the onsite blasting experiment. Results indicate that the acting mechanism of the initiation position lies in the axial non-uniform distribution of the explosive energy and the phase delay effect of the superposition of BVF. The in-hole initiation position has the adjustment effect on the distribution of BVF, due to which, the blast vibration amplitude is strengthened at the forward direction of the detonation wave. It needs to be pointed that the non-uniformity of the distribution of BVF is under some control of the explosive length and the explosive velocity of detonation. For the common initiation modes, the field test results indicate that the ground peak particle velocities under the bottom are larger than those under the top and mid-point initiations, and the top initiation is the smallest. Besides, the blast vibration differences becomes more obvious as the blast-hole depth increases, but the vibration difference gradually vanishes with distance.

Evolution of stress field in cylindrical blasting with bottom initiation

The dynamic caustic method and numerical simulation method were used to study explosive stress field of cylindrical blasting with bottom initiation. The explosive stress distribution of cylindrical blast holes with bottom initiation was not uniform, and can be divided into side areas with stronger stress and end areas with weaker stress with notable end effects. In the side areas, the explosive stress field at the initiating end increased, and the stress was high at the initial stage after initiation. However, the stress field at the non-initiating end was the strongest with the explosive detonation wave propagating from initiating end to non- initiating end. Differences in the principal stress difference near the blast hole were significant. The explosive stress field in the initiating end was weaker than that in the non-initiating end. Caustic spots increased and maximum principal stress directions is consistent with crack propagation direction when cracks propagated near circular holes. According to the simulation results, the Mises stress wave has a “conical” distribution. The Mises stress attenuation coefficient in the vertical direction, the axial direction, and the oblique direction increased at initiating end. The Mises stress attenuation coefficient in vertical direction, oblique direction, the axial direction increased at non-initiating end.

Shock interactions with heterogeneous energetic materials

The complex physical phenomenon of shock wave interaction with material heterogeneities has significant importance and nevertheless remains little understood. In many materials, the observed macroscale response to shock loading is governed by characteristics of the microstructure. Yet, the majority of computational studies aimed at predicting phenomena affected by these processes, such as the initiation and propagation of detonation waves in explosives or shock propagation in geological materials, employ continuum material and reactive burn model treatment. In an effort to highlight the grain-scale processes that underlie the observable effects in an energetic system, a grain-scale model for hexanitrostilbene (HNS) has been developed. The measured microstructures were used to produce synthetic computational representations of the pore structure, and a density functional theory molecular dynamics derived equation of state (EOS) was used for the fully dense HNS matrix. The explicit inclusion of the microstructure along with a fully dense EOS resulted in close agreement with historical shock compression experiments. More recent experiments on the dynamic reaction threshold were also reproduced by inclusion of a global kinetics model. The complete model was shown to reproduce accurately the expected response of this heterogeneous material to shock loading. Mesoscale simulations were shown to provide a clear insight into the nature of threshold behavior and are a way to understand complex physical phenomena.

The Dynamics of Detonation in Explosive Systems

This article reviews advances in modeling condensed-phase explosive detonation waves and their interaction with inerts for precision applications. We describe how constitutive data are obtained for a basic, predictive hydrodynamic model for explosives that subsequently can be studied numerically and analytically. Theory for multidimensional, time-dependent detonation dynamics is reviewed with a focus on freely propagating detonation and the asymptotic theory for quasi-one-dimensional, quasi-steady, detonation shock evolution (detonation shock dynamics). We discuss verification of these theories by direct numerical simulation (DNS) and validation by experiment. We describe a subscale model of detonation that uses an evolution equation to predict detonation dynamics and front states in complex engineering geometries that otherwise could not be computed by DNS. Four areas for future research are identified.

Shaped Charge Jet Heating

AbstractA one‐dimensional theoretical model of shaped charge jet heating is developed. The model includes heating due to the shock wave transferred to the liner by the explosive detonation wave and due to the plastic work performed during liner collapse and jet elongation. Results calculated by a one‐dimensional shaped charge computer code using this model fall within the lower bound of the experimental range.

Reaction zone measurements in high explosive detonation waves by means of shock-induced polarization

Information on the reaction zone of a detonation wave in condensed high explosive charges can be obtained by measuring shock-induced polarization (SIP) signals in Plexiglas. The shape of the early part of the SIP signal appears to be determined mainly by the pressure history at the high explosive/Plexiglas interface. This enables the measurement of the detonation reaction time and hence the reaction zone length. Detonation waves in a number of high explosives have been investigated and the results obtained are presented in this paper. Using the same experimental technique, shock transit times can be measured in Plexiglas transducers of various thicknesses behind the high explosive charge. From these measurements not only a value for the reaction time, but also values for the Chapman-Jouguet pressure and the von Neumann spike parameters can be derived.

Zig-Zag Oscilloscope Presentation of Millimicrosecond Accuracy for Microsecond Time Intervals

In the study of metal-high explosive detonation wave interaction, by the pin technique, it is desirable to obtain time measurements to an accuracy of less than 1 mμsec over a total time interval of 5 μsec. Commercial instruments capable of a high degree of time resolution were not available and a special instrument was developed for this purpose. A cathode-ray oscilloscope, displaying deflection type event pulses, is converted into a precise time-measuring instrument by a calibrated zig-zag sweep. The record is obtained by single-shot photography of the cathode-ray tube sweep which is triggered by the event to be measured. The calibrated zig-zag sweep is crystal controlled at 2 Mc and has superimposed deflection type time marks, 2 mμsec wide, at 50-mμsec interval, synchronized with the zig-zag frequency. The apparatus can drive six oscilloscopes simultaneously or in sequence from delayed triggers.

The sensitiveness of explosives - The effect of compression on the sensitiveness of initiators

The sensitiveness of the following initiators has been compared when used loose, and when compressed up to 2300 kg./cm. Crystalline Service azide (crystals 75 X 10-4 cm.). Powdered Service azide (fragments 1 to 25 x 10~4 cm.). Dextrinated lead azide. Mercury fulminate. Sensitiveness to heat was measured by determining the induction period at various temperatures, and also by evaluating threshold temperatures below which no detonation was observed for various masses of initiators. Tests were also made to see how far the action of flame, and of percussion, could be correlated with the action of heat on these initiators. Compression reduces the induction period in all cases, but the values of E and B (see text) are differently affected in the case of different initiators. Compression also lowers the threshold temperature below which a given mass of initiator will not detonate. Thus in all cases sensitiveness to heat is increased by compression. On the other hand, compression lessens the sensitiveness of mercury fulminate to flash and percussion, corresponding with the well-known phenomenon of ‘dead-pressing’. If anything, compression increases the sensitiveness of lead azide to flash and percussion. A new detonation mechanism has been observed for both service and dextrinated lead azides. The experimental results throw further light on the build-up of the detonation wave in explosives. When the volume of explosive ‘primarily’ involved in the sensitiveness phenomenon is small, as is usually the case for initiators, the mechanism of build-up differs from the ‘self-heating’ mechanism which may overtake it with larger volumes of explosive. A simple explanation is suggested for the ‘dead-pressing’ of explosives, which is to be expected when ‘self-heating’ is the mechanism controlling the build-up of detonation.

Spread Around the Initiating Point of the Detonation Wave in High Explosives

THERE is an old belief that the detonation wave in an explosive must travel a certain distance from the initiating point before attaining its final speed. In order to elucidate this problem, cylindrical charges of pressed T.N.T. have been fired with the detonator in different orientations and at various distances from the end surface of the charge. During experiments the flame was photographed on a film running with a constant speed of about 100 m./sec. Between the charge and the camera a screen with a vertical slit, 5 mm. wide, was fitted 1 m. from the charge. In all cases the axis of the charge was horizontal and pointed at the camera.

MECHANISM OF DETONATION IN EXPLOSIVES

Results of photographic work on the detonation of high explosives are presented and discussed in their relationship to the hydrodynamic theory of detonation. The two most important properties of an explosive are its strength and its detonation velocity. Methods for determining these quantities are described. A moving film camera of the rotating drum type is especially useful in determining detonation velocity and in studying the nature of detonation. The photographic results, which are illustrated by a number of pictures, are helpful in demonstrating various elements of the theory. These photographs illustrate the nature of detonation waves in explosives and of shock waves in air. The duration of the detonation waves in nitroglycerin and blasting gelatin is less than one millionth of a second and the duration of the shock waves produced by these explosives in air is of the same order. The high temperature of shock waves which is predicted by theory is confirmed by the intense luminosity shown in photographs. The relatively low temperature predicted for shock waves in liquids is similarly confirmed by the absence of luminous shock waves in water. Photographs are included showing the propagation of detonation from one cartridge of blasting gelatin to another across air gaps and water gaps. In the latter case no visible shock wave is produced in the water and the highly luminous after‐burning is eliminated. Calculated values of the detonation velocity for nitroglycerin, blasting gelatin and 60% gelatin dynamite are in approximate agreement with the experimentally determined values. Calculations indicate that pressures in the detonation wave may run as high as 140,000 atmospheres and temperatures to 4300°C. The shape of the detonation wave front in a high velocity explosive like blasting gelatin is apparently planar whereas in low velocity explosives it is convex. The actual mechanism of energy propagation in detonation is not clearly understood out probably involves activation of the explosive at the detonation wave front by high velocity products of the detonation which are projected forward at speeds even greater than the detonation velocity.