R. Mendes

Differences between the detonation behavior of emulsion explosives sensitized with glass or with polymeric micro-balloons

The differences between the detonation behaviour of ammonium nitrate based emulsion explosives sensitized with polymeric and those sensitized with glass micro-balloons is presented and discussed. Expancel® are hollow polymeric micro-balloons that contain a hydrocarbon gas. The mean particle size of these particles is 30 μm with a wall thickness of about 0.1 μm. The detonation velocity and the failure diameter of the emulsion explosive sensitized with different amounts of these particles have been measured in cylindrical charges by optical fibers. The detonation velocity demonstrates non-linear behaviour in relation to density and reaches the maximum value for a density lower than that of the matrix. The detonation fails when the density approaches that of the matrix. The detonation in the emulsion explosives extinguishes itself at a porosity value that seems to be independent from the nature of the sensitizing agent. For low densities, the detonation velocity is almost independent of the charge diameter, and is close to the values predicted by BKW equation of state.

Extinction and initiation of detonation of NM-PMMA-GMB mixtures

The influence of the addition of a small amount of glass microballoons (GMB) on the extinction and initiation of NM-PMMA mixtures is investigated. Numerical simulation using DYNA 3D code shows the collapse of small GMB, under NM detonation. GMB of a mean particle diameter of 45 μm were added into NM-PMMA explosive mixture. Polymethylmetacrylate (PMMA) was added to nitromethane (NM) to avoid buoyancy movements of GMB, increasing the viscosity of the original NM. Different failure tests were done initially proving the low values of critical extinction diameter and thickness. Accurate failure thickness experiments were performed using a prismatic configuration with a small opening angle of 2.9°. A double resistive wire technique and an aluminium witness bar were used to determine its critical thickness, as a function of GMB mass concentration. This double resistive wire technique shows, in real time, the induction time and distance of initiation. The failure thickness was correlated with those previously obt...

Privacy-Preserving Data Mining: Methods, Metrics, and Applications

The collection and analysis of data are continuously growing due to the pervasiveness of computing devices. The analysis of such information is fostering businesses and contributing beneficially to the society in many different fields. However, this storage and flow of possibly sensitive data poses serious privacy concerns. Methods that allow the knowledge extraction from data, while preserving privacy, are known as privacy-preserving data mining (PPDM) techniques. This paper surveys the most relevant PPDM techniques from the literature and the metrics used to evaluate such techniques and presents typical applications of PPDM methods in relevant fields. Furthermore, the current challenges and open issues in PPDM are discussed.

Review of the weldability window concept and equations for explosive welding

Explosive cladding/welding is usually considered a solid state process in which the detonation of a certain amount of an explosive composition is used to accelerate one of the materials to be weld against the other in order to promote a high velocity oblique collision that will be responsible for bonding the materials. The conditions that should be met to achieve good welds define what is called as a weldability window or criteria. A weldability criteria based on the collision point velocity (Vc) and on the collision angle (β) is the most used today. In the β-Vc space the weldability window is defined by four lines or limits. Despite of its wide used in explosive welding works, neither the concepts behind those limits neither the equations used to define them in the β-Vc space are particularly clear. Contradictory concepts, and equations with undefined variables or parameters, are commonly found in the literature. This paper aims to clarify those concepts and equations through an integrated description of the weldability limits and a reviewed presentation of the associated equations with the variables and parameters, including their units, clearly defined. The reviewed concepts and equations are then used for the description of the explosive weld of stainless steel to carbon steel in cylindrical configuration.

Influence of base material properties on copper and aluminium–copper explosive welds

ABSTRACT The influence of base material properties on the interfacial phenomena in copper and aluminium–copper explosive welds was studied. Two explosive mixtures with different detonation velocities were tested. Sound aluminium–copper joints with effective bonding were achieved by using an explosive mixture with a lower detonation velocity. High energy explosives led to extensive interfacial melting, preventing the production of consistent dissimilar welds. Unlike to the similar copper joints, the aluminium–copper welds presented very asymmetrical interfacial waves, rich in intermetallic phases and displaying a curled morphology. The interaction of the materials in dissimilar welding was found to be completely different depending on the positioning of each alloy in the joint, i.e. positioned as the flyer or as the baseplate.

Effect of Shear Stress in Shock Initiation of PBX

Initiation of hot spots‐reaction sites in PBX explosives by a linear shock generator has been studied under wide variation of the input shock pressure, from 3 GPa up to 20 GPa. A multi‐channel high‐resolution optical method based on application of a 96 optical fiber strips has been used for a comprehensive registration of the reaction spot initiation and following the processes of reaction growth or extinction. We have obtained strong evidence that reaction originates in the shear regions established by the induced shock. This suggests that shear has a major role in shock initiation. The pressure field in the front of reacted flow and the minimum size of reaction sites, which results in reaction growth, are defined as a function of the input shock pressure. Effect of shear stress is discussed concerning the grain sizes and mass concentration of HMX in the PBX’s.

Detonation Meso‐Scale Tests for Energetic Materials

The objective of the present study is to characterize, on the meso‐scale level, the detonation behaviour of PBX based on HMX , based in the minimisation of the test samples of energetic materials up to 10 mg. The development of a non‐intrusive, high resolution, optical metrology procedures, using multi‐fibber strip, allows the testing of PBX micro‐samples, formed by few crystals surrounded by binder, with the simultaneous registration of parameters as local detonation velocity and pressure, geometrical shape of detonation front and the structure of the shock‐to‐detonation transition zone. The enhanced information allows a better understanding of the processes of formation and propagation of detonation wave. This procedure can be applied to the study of new advanced energetic materials.

Shock Wave Propagation Process in Epoxy Syntactic Foams

The Shock Wave Propagation Process [SWPP] in epoxy syntactic foams [SF] (Hollow Glass Micro Spheres [HGMS] within an epoxy binder) with a nano‐second temporal and micrometer spatial resolution is presented and discussed. Samples with three different characteristic HGMS sizes were studied (42, 92 and 135 μm). For the samples with characteristic HGMS size of 92 μm, the effects of the density (0.64, 0.81 and 0.92 g/cm3) and of the loading pressure (20.0 and 9.6 GPa) were also analyzed. The obtained results show the effects of self‐organization (synergetic/cooperative) in the initial phase of the SWPP, associated to a layer‐by‐layer collapse of HGMS, and in the following phase of propagation, associated to the cellularization of the SW front. Specific experimental results, showing the SWPP in one or two layers of HGMS, and numerical simulations (LS‐DYNA 2D), of the pore collapse process in one layer of HGMS, were also performed in order to clarify the mechanisms of SW propagation.

Thermistor based, low velocity isothermal, air flow sensor

The semiconductor thermistor technology is applied as a flow sensor to measure low isothermal air velocities (<2 ms−1). The sensor is subjected to heating and cooling cycles controlled by a multifunctional timer. In the heating stage, the alternating current of a main AC power supply source guarantees a uniform thermistor temperature distribution. The conditioning circuit assures an adequate increase of the sensors temperature and avoids the thermal disturbance of the flow. The power supply interruption reduces the consumption from the source and extends the sensors life time. In the cooling stage, the resistance variation of the flow sensor is recorded by the measuring chain. The resistive sensor parameters proposed vary significantly and feature a high sensitivity to the flow velocity. With the aid of a computer, the data transfer, storage and analysis provides a great advantage over the traditional local anemometer readings. The data acquisition chain has a good repeatability and low standard uncertainties. The proposed method measures isothermal air mean velocities from 0.1 ms−1 to 2 ms−1 with a standard uncertainty error less than 4%.

Non ideal detonation of emulsion explosives mixed with metal particles

The detonation of ammonium nitrate based compositions like emulsion explosives mixed with metal particles was experimentally investigated. Aluminum powder with a mean particle size of 6 μm was used, and the mass concentration of aluminum on the explosive charge ranged from 0 to 30% wt. The values of the detonation velocity, the pressure attenuation – P(x) – of the shock front amplitude in a standard PMMA monitor and manganin gauges pressure-time histories are shown as a function of the explosive charge porosity and specific mass. All these parameters except the pressuretimes histories have been evaluated using the multi-fiber optical probe (MFOP) method which is based on the use of an optical fiber strip, with 64 independent optical fibers. The MFOP allows a quasicontinuous evaluation of the detonation wave run propagation and the assessment of spatial resolved measurements of the shock wave induced in the PMMA barrier. Results of that characterization process are presented and discussed for aluminized an...