Bridgewire Detonators Research Articles

Investigation of thermal damage in explosive bridgewire detonators via discrete element method simulations

AbstractExploding bridgewire (EBW) detonators are used to rapidly and reliably initiate energetic reactions by exploding a bridgewire via Joule heating. While the mechanisms of EBW detonators have been studied extensively in nominal conditions, comparatively few studies have addressed thermally damaged detonator operability. We present a mesoscale simulation study of thermal damage in a representative EBW detonator, using discrete element method (DEM) simulations that explicitly account for individual particles in the pressed explosive powder. We use a simplified model of melting, where solid spherical particles undergo uniform shrinking, and fluid dynamics are ignored. The subsequent settling of particles results in the formation of a gap between the solid powder and the bridgewire, which we study under different conditions. In particular, particle cohesion has a significant effect on gap formation and settling behavior, where sufficiently high cohesion leads to coalescence of particles into a free‐standing pellet. This behavior is qualitatively compared to experimental visualization data, and simulations are shown to capture several key changes in pellet shape. We derive a minimum and maximum limit on gap formation during melting using simple geometric arguments. In the absence of cohesion, results agree with the maximum gap size. With increasing cohesion, the gap size decreases, eventually saturating at the minimum limit. We present results for different combinations of interparticle cohesion and detonator orientations with respect to gravity, demonstrating the complex behavior of these systems and the potential for DEM simulations to capture a range of scenarios.

Analyzing the effectiveness of the Gurney method for small scale fragmentation propulsion using Exploding Bridgewire detonators

AbstractExplosives are common in military, mining, and construction applications where the explosive properties are understood, but mechanics of how the explosive's energy fragments and throws materials are less known. Considering the type of confining material around an explosive, creates variability in fragmentation behavior due to the individual material characteristics. The most common method for assessing fragmentation behavior is the Gurney method, which eliminates any consideration of fragmenting material properties. The Gurney method assumes that, on a large scale, the inconsistencies in material are irrelevant and only the mass of the confiner need be considered. However, it is known in many fields that energy is consumed in the breaking of a material. In this paper, the detonation and resultant fragmentation propulsion of Exploding Bridgewire (EBW) detonators with the same explosive material, but different casing characteristics, is observed. The Gurney method was used to predict fragmentation velocities of the casing following detonation, which were compared to the behavior observed through high‐speed video of the actual event. The EBWs were selected to provide variability in casing material, casing thickness, charge length, and charge diameter. It was found that when the amount of explosive is small, the material properties of the casing play a significant role, with 70 % of the total explosive mass lost in fragmenting PMMA EBWs and 30±3 % lost in Aluminum EBWs. There is significant energy loss to breaking the casing material that cannot be ignored on the small scale and could impact large explosives with high casing to explosive ratios.

A Liquid Stratification Model to Predict Failure in Thermally Damaged EBW Detonators

AbstractIn previous work, commercially available downward facing exploding bridgewire detonators (EBWs) were exposed to elevated temperatures. These detonators were then initiated using a firing set which discharged a high amplitude short duration electrical pulse into a thin gold bridgewire. Responses of the detonators were measured using photonic doppler velocimetry (PDV) and high‐speed photography. A time delay of 2 μs between EBW initiation and first movement of an output flyer separated operable detonators from inoperable detonators or duds. In the current work, we propose a simple method to determine detonator operability from the calculated state of the detonator at the time the firing set is initiated. The failure criterion is based on the gap distance between the exploding bridgewire (EBW) and the adjacent initiating explosive within the detonator which is low‐density pentaerythritol tetranitrate (PETN) that melts between 413–415 K (140–142 °C). The gap forms as PETN melts and flows to the bottom of the input pellet. Melting of PETN is modeled thermodynamically as an energy sink using a normal distribution spread over a temperature range between the onset temperature of 413 K and the ending temperature of 415 K. The extent of the melt is determined from the average temperature of the PETN. The PETN liquid is assumed to occupy the interstitial gas volume in the lower part of the input pellet. The vacated volume from the relocated liquid forms the gap between the EBW and the PETN. The remaining sandwiched layer consists of solid PETN particles and gas filling interstitial volume. We predict that a threshold gap between 17–27 μm separates properly functioning detonators from duds.

The modeling of weak shock waves in highly porous powder beds and comments on its relevance to exploding bridgewire (EBW) detonators

A computer modeling study is reported about the constant-piston-velocity compaction response of a 50% porous fine particle and air mixture. It is established that for the 10- $$\upmu \hbox {m}$$ average particle diameter powder chosen (sugar), over the piston velocity range 100–1000 $$\hbox {m}\,\hbox {s}^{-1}$$ , a self-similar shock is formed in 12–14 particle diameters of motion ( $${\approx }\,140~\upmu \hbox {m}$$ ). The shock is found to be statistically bounded, but temporally and spatially variable. At the resulting shock velocities identified for this powder, this results in self-similar behavior if the shock is supported for approximately 100 ns. Therefore, the use of the shock jump equations to estimate the post-shock state is valid after this duration. Further, it is found that a linear shock- and particle-velocity relation works well to model the low-pressure compaction of the powder. The intercept of this relation (commonly called the bulk sound speed in solid materials) is very low (50–100 $$\hbox {m}\,\hbox {s}^{-1}$$ ), but positive. The implications of this powder response on the functioning of exploding bridgewire detonators and the porous pentaerythritol-tetranitrate fill commonly used in them are discussed and compared with other literature on the topic.

The action-integral and energy to explode short gold wires in ambient air

This paper is about electrically induced explosions of short (1 mm) and thin (38 μm diameter) gold bridgewires in ambient air and contrasts this behavior with that observed for longer wires in a vacuum used in applications such as Z-pinch systems. The action-integral (the time integral of the wire current squared) and energy-to-burst are measured using high current discharge systems. It is found that in contrast to some reports, the action-integral to burst is not a constant for a specific wire geometry and material but, instead, is a function of the rate of current change in the wire around the time of burst and the related metric of the current flux at burst. Furthermore, it is found that the accuracy of the constant action-integral to burst approximation is dependent on the discharge system used. Short gold wires such as these have relevancy to exploding bridgewire detonators.

Experimental observations of exploding bridgewire detonator function

Exploding bridgewire detonators are an industry standard technology used for over 75 years and valued for their precise timing and safety characteristics. Despite widespread use, their functional mechanism remains controversial with both shock and non-shock mechanisms attributed. In this work, we reexamine the bridgewire detonator function with a suite of modern diagnostics and compare these observations with the existing literature. Traditional detonator observations consisted of voltage applied to the bridgewire and time dependent current, integral response measurements such as case motion, and more recently Schlieren imaging of the detonator surface. In this work, we add visible light emission, x-ray transmission, proton radiography, and temperature measurements during detonator function in addition to voltage, current, and function times. The addition of in situ observations of light emission, temperature, and density gives us new insight into the mechanisms of explosive bridgewire detonator function. We see a distinct separation in time, location, symmetry, and velocity of bridgewire output and detonation onset. During the time between bridgewire burst and the initiation of detonation, we observe a temperature ramp in the input pellet. In this paper, we present the suite of measurements and comparisons with the literature on integral response measurements.

The effects of air gaps and inert layers on exploding bridgewire detonator function

ABSTRACT Exploding bridgewire (EBW) detonators work by electrically exploding a thin conducting wire as a result of a high voltage discharge from a capacitor. The process creates a strong UV (ultraviolet) emission and a shock compaction wave into the surrounding porous explosive bed. To investigate the significance of each, the research presented here utilized air gaps, an optical window, or inert porous powders between the bridge and explosive powder bed. The thickness of the air gap was varied to attenuate the air shock but allow the transmission of UV light. The optical properties of the inert porous bed were varied to allow a similar shock compaction to occur, but with different intensities of UV light. Finally, a thin optical window was used that blocked the shock, but allowed transmission of the UV light. It is clearly shown that both UV light and shock compaction are important to the functioning of EBW detonators at practical energy levels although each is individually capable of resulting in detonation if sufficient pure energy of either form is applied.

Optical absorption in polycrystalline PETN, RDX, HMX, CL-20 and HNS and its possible effect on exploding bridgewire detonator function

ABSTRACT Exploding bridgewire (EBW) detonators work by bursting a thin conducting wire as a result of a high voltage discharge. This process is known to produce a strong UV emission. The roles of optical absorption, photochemistry, shock sensitivity, and impact sensitivity are assessed with respect to the relative roles each plays in the operation of an EBW detonator. The optical absorptivity and penetration depth have been measured using dilute polycrystalline samples of PETN, RDX, HMX, CL-20 and HNS in KBr between 190 and 1100 nm. The results show strong absorption at UV wavelengths with a gradual decline at longer ones. It is found that while strong UV absorption appears to be necessary, it is not sufficient to make an effective EBW detonator.

A review of the mechanism by which exploding bridge-wire detonators function.

An introduction to exploding bridge-wire (EBW) detonators is given followed by an extensive critical review of open source literature pertaining to these devices. The aim is to better establish the mechanism of operation. Some authors state that the key mechanism is shock-to-detonation while others maintain it is more thermal in nature, or a complex combination of both. In addition to EBW detonators, arc detonators and direct optical initiation detonators are also reviewed, and it is demonstrated that in this manner the usually coupled effects from both shock and deflagration can be somewhat decoupled. As a result, it is hypothesized that the mechanism of operation in all three detonators is in fact the same: the formation of a hot plasma with a power of ≈1 MW and emission in the ultraviolet drives a thermal explosion process.

Finding the “lost-time” in detonator function

Exploding bridgewire (EBW) detonators were invented during the Manhattan Project over 75 years ago. Initially developed for precise timing and reproducibility, they continue to be used in many applications. Despite widespread use and reliability, their mechanism for function remains controversial. They provide precision timing, yet their function is described in terms of a “lost time” accounting for nearly half of the function time. Buried in understanding the EBW function is the mystery of how an incoherent impulse such as powering a bridgewire yields the coherent energy output of a detonation. Even the general phenomena by which release of chemical energy in a crystalline organic explosive becomes associated with the sonic plane of a steady detonation wave remain uncertain. Here, we investigate the EBW function with a suite of diagnostics and show that stationary heating occurs during the “lost-time.” We use x-ray radiography to observe the propagation of a shock wave from bridgewire vaporization and establish that the origin of the radially emanating detonation wave is spatially separated from the initial shock. Utilizing the observed temperature as a boundary condition in our explosive response models yields a thermal ignition consistent with the “lost-time” and detonation location consistent with previous work. With these results, we define a direct thermal initiation mechanism for the EBW function consistent with previous integral observations and explain the displacement of initiation from the bridgewire burst in time and space.

A view on the functioning mechanism of EBW detonators -part 1: electrical characterisation

Exploding Bridgewire (EBW) Detonators are in widespread use and have proven reliability and performance characteristics. Since their invention there have been numerous studies to identify the mechanism by which the exploding bridgewire initiates the explosive. However, there is still not a universally accepted mechanism. This paper is the first of three characterising the initiation of PETN in an exploding bridgewire detonator to understand the underlying mechanism. The approach taken was to understand the transfer of energy through the system, beginning with the fireset / bridgewire interactions. The measurement of current, time to bridgewire burst and the transient voltage across the bridgewire at burst have enabled the determination of the energy used in bursting the bridgewire. This in turn has led to the calculation of the energy efficiency of the fireset bridgewire system and an estimate of the energy delivered post bridgewire burst. The results of the experimental work will be presented, together with the implications for the initiation mechanism of PETN in an exploding bridgewire detonator.

A view on the functioning mechanism of EBW detonators -part 2: bridgewire output

This is the second paper of three papers describing the studies to identify the initiating mechanisms in Exploding Bridgewire (EBW) detonators. In this paper the results of experiments to quantify the effect of the bridgewire explosion are described. Experiments have been performed to characterise the output from the bridgewire in terms of the stimulus it would apply to the surrounding explosive in an EBW detonator. The expansion speed of the bridgewire at burst as a function of input energy has been measured using Photonic Doppler Velocimety (PDV). To complement the bridgewire expansion velocity determinations aquarium experiments were carried out in which the shock wave velocity in water was measured, as a function of energy, by high speed photography. The shock pressures were calculated and compared to initiation criteria for PETN.

Effects of electrostatic discharge on the performance of exploding bridgewire detonators

The performance of exploding bridgewire detonators affected by electrostatic discharge (ESD) were studied by microfocus X-ray industrial computed tomography (μCT), and high voltage stimulus to detonators was provided by an ESD simulator. Two stimulation modes were investigated, and the performance indexes of detonators after ESD were tested. The results show that there was no obvious change in bridge resistance when detonators were simulated by the mode of Pin-Pin, while the primary explosive of detonators was partly damaged by the mode of pin-shell, and the performance indexes of 2 detonators were affected by ESD. The reasons for this phenomenon were analyzed. This work contributes to understanding the performance of exploding bridgewire detonators affected by ESD.

On the laser ignition and initiation of explosives

Energetic materials are conventionally ignited by the application of heat to some part of an explosive target. This is most often provided by a flame or by electrical heating using a resistive wire. The material responds by thermally heating and starting a burning zone, which spreads out from the ignition point generating gas. In some cases this zone can accelerate (due to the effect of this gas) into a reactive shock wave, termed detonation. Lasers are used in a variety of applications to thermal heat a range of materials, and thus seem an obvious candidate to trigger chemical reaction in energetic ones. A light pulse offers several advantages over an electrical one, since it may be delivered down a path both immune to electrical effects and chemically stable. Thus, the triggering of safety apparatus (such as the firing of bolts on aircraft exits) represents a major thrust in the development of laser–triggered explosive devices. The energy of the pulse may be used in one of two ways to achieve these effects. In the first, the laser is shone directly upon the chemical medium, which absorbs the light at discrete wavelengths. In the second, it is used to vaporize a metallic flyer that is launched to impact on a target creating a high–pressure zone. Either mechanism starts the required reaction. However, when the pulse is delivered directly to the material, detonation is found to proceed immediately with no transition via burning. This makes the process inexplicable using present concepts. This review will address a range of the experiments conducted and theories developed for this novel field. However, it should be emphasized that the fundamental mechanisms remain to be fully explained making the application both academically stimulating as well as industrially important.

Evaluation of a new organic azide: Hexakis(azidomethyl)benzene (HAB)

AbstractHAB is a thermally‐ and hydrolytically‐stable solid, easily prepared from available raw materials. It is not highly sensitive to shock, friction, or electrostatic charge, but is sensitive to some types of impact, although not to others. It shows preliminary promise for possible use as a substitute for normal lead styphnate in less‐sensitive bridgewire detonators, and as a substitute for tetracene in percussion detonators, e.g. the M‐42 primer. Easy ignition, and a high burning rate without detonation, suggest application as an igniter. HAB is not sufficiently powerful to dent an aluminum witness block; therefore, it would not be suitable as a replacement for lead azide as an intermediate detonator charge.