On the use of mechanical and acoustical excitations for selective heat generation in polymer-bonded energetic materials

To address security issues in both military and civilian settings, there is a pressing need for improved explosives detection technologies suitable for trace vapor detection. In light of the strong dependence of vapor pressure on temperature, trace vapor detection capabilities may be enhanced by selectively heating target materials by external excitation. Moreover, polymer-bonded energetic materials may be particularly susceptible to heating by mechanical or acoustical excitation, due to the high levels of damping and low thermal conductivities of most polymers. In this work, the thermomechanical response of polymer-based energetic composites and methods for acoustical excitation are investigated in order to improve the understanding of the temperature rises induced by applied excitation, and to uncover waveforms which may efficiently transmit excitation energy to generate heat and enhance trace vapor detection capabilities. The heat generation in the binder material of energetic and surrogate systems under harmonic excitation was investigated analytically through the application of a viscoelastic material model. Specifically, structural-scale heating was considered under low-frequency direct mechanical excitation as applied to a beam geometry. Experiments were conducted with a mock mechanical material, wherein the mechanical and thermal responses were recorded by scanning laser Doppler vibrometry and infrared thermography, respectively. Direct comparisons between the model and experimental results demonstrated good agreement with the predicted response, with low-order, bulk-scale heating observed along the modal structure in areas of higher strains. In addition, localized heating near individual crystals was investigated analytically by extending the viscoelastic heating model to general three-dimensional stress-strain states. Application of the model to a Sylgard 184 binder system with an embedded HMX (octogen) crystal under ultrasonic excitation revealed predictions of significant heating rates, particularly near the front edge of the crystal, due to the wave scattering and the resulting stress concentrations. In considering methods for such excitation through incident acoustical or ultrasonic waves, the form of the wave profile was tuned in this work for the purpose of maximizing the energy transmission into solid materials. That transmission is generally limited by the large impedance mismatch at typical fluid--solid interfaces, but by varying the spatial distribution of the incident wave pressure, significant transmission increases can be achieved. In particular, tuned incident inhomogeneous plane waves were found to predict much lower values of the reflection coefficient, and hence large increases in the energy transmission in the context of lossless and low-loss dissipative media. Also, material dissipation was found to have a strong effect on the optimal incident waveform, generally causing a shift to lower inhomogeneity values. Similar results were obtained for parameterized forms of bounded incident waves with respect to the local reflection phenomena and surface wave excitation. These results suggest that, depending on the targeted solid material, substantial energy transmission and heat generation increases may be achieved by tailoring the spatial form of the incident wave profile

Overview of Dr. L. L. Beranek\u27s 2006 paper on Analysis of Sabine and Eyring Equations and their Application to Concert Hall Audience and Chair Absorption

Overview of Dr. L. L. Beranek\u27s 2006 paper on Analysis of Sabine and Eyring Equations and their Application to Concert Hall Audience and Chair Absorptio

Development and testing of hermetic, laser-ignited pyrotechnic and explosive components

During the last decade there has been increasing interest in the use of lasers in place of electrical systems to ignite various pyrotechnic and explosive materials. The principal driving force for this work was the requirement for safer energetic components which would be insensitive to electrostatic and electromagnetic radiation. In the last few years this research has accelerated since the basic concepts have proven viable. At the present time it is appropriate to shift the research emphasis in laser initiation from the scientific arena--whether it can be done--to the engineering realm--how it can be put into actual practice in the field. Laser initiation research and development at EG&G Mound was in three principal areas: (1) laser/energetic material interactions; (2) development of novel processing techniques for fabricating hermetic (helium leak rate of less than 1 x 10(exp -8) cu cm/s) laser components; and (3) evaluation and testing of laser-ignited components. Research in these three areas has resulted in the development of high quality, hermetic, laser initiated components. Examples are presented which demonstrate the practicality of fabricating hermetic, laser initiated explosive or pyrotechnic components that can be used in the next generation of ignitors, actuators, and detonators

Formalising attack trees to support economic analysis

Attack trees and attack graphs are both examples of what one might term attack modelling techniques. The primary purpose of such techniques is to help establish and enumerate the ways in which a system could be compromised; as such, they play a key role in the (security) risk analysis process. Given their role and the consequent need to ensure that they are correct, there are good reasons for capturing such artefacts in a formal manner. We describe such a formal approach, which has been motivated by a desire to model attacks from the perspectives of attackers, to support economic analysis. As an illustration, we consider exploitation cost

Formalising attack trees to support economic analysis

Attack trees and attack graphs are both examples of what one might term attack modelling techniques. The primary purpose of such techniques is to help establish and enumerate the ways in which a system could be compromised; as such, they play a key role in the (security) risk analysis process. Given their role and the consequent need to ensure that they are correct, there are good reasons for capturing such artefacts in a formal manner. We describe such a formal approach, which has been motivated by a desire to model attacks from the perspectives of attackers, to support economic analysis. As an illustration, we consider exploitation cost

On the Detection of Supermassive Primordial Stars. II. Blue Supergiants

Supermassive primordial stars in hot, atomically-cooling haloes at $z \sim$ 15 - 20 may have given birth to the first quasars in the universe. Most simulations of these rapidly accreting stars suggest that they are red, cool hypergiants, but more recent models indicate that some may have been bluer and hotter, with surface temperatures of 20,000 - 40,000 K. These stars have spectral features that are quite distinct from those of cooler stars and may have different detection limits in the near infrared (NIR) today. Here, we present spectra and AB magnitudes for hot, blue supermassive primordial stars calculated with the TLUSTY and CLOUDY codes. We find that photometric detections of these stars by the James Webb Space Telescope (JWST) will be limited to $z \lesssim$ 10 - 12, lower redshifts than those at which red stars can be found, because of quenching by their accretion envelopes. With moderate gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope (WFIRST) could detect blue supermassive stars out to similar redshifts in wide-field surveys.Comment: 9 pages, 5 figures, accepted by MNRA

Use of Evanescent Plane Waves for Low-Frequency Energy Transmission Across Material Interfaces

The transmission of sound across high-impedance difference interfaces, such as an air-water interface, is of significant interest for a number of applications. Sonic booms, for instance, may affect marine life, if incident on the ocean surface, or impact the integrity of existing structures, if incident on the ground surface. Reflection and refraction at the material interface, and the critical angle criteria, generally limit energy transmission into higher-impedance materials. However, in contrast with classical propagating waves, spatially decaying incident waves may transmit energy beyond the critical angle. The inclusion of a decaying component in the incident trace wavenumber yields a nonzero propagating component of the transmitted surface normal wavenumber, so energy propagates below the interface for all oblique incident angles. With the goal of investigating energy transmission using incident evanescent waves, a model for transmission across fluid-fluid and fluid-solid interfaces has been developed. Numerical results are shown for the air-water interface and for common air-solid interfaces. The effects of the incident wave parameters and interface material properties are also considered. For the air-solid interfaces, conditions can be found such that no reflected wave is generated, due to impedance matching among the incident and transmitted waves, which yields significant transmission increases over classical incident waves

The Construction of Acoustic Waveforms from Plane Wave Components to Enhance Energy Transmission into Solid Media

The transmission of acoustic energy into solid materials is of interest in a wide range of applications, including ultrasonic imaging and nondestructive testing. However, the large impedance mismatch at the solid interface generally limits the transmission of incident acoustic energy. With the goal of improving the fraction of the energy transmitted into solid materials, the use of various bounded spatial profiles, including commonly-employed forms, such as Gaussian distributions, as well as newly-constructed profiles, has been investigated. The spatial profile is specified as the pressure amplitude distribution of the incident wave. Bounded acoustic beams are represented here as sums of harmonic plane waves, and results obtained for the optimal parameters of incident plane wave components are used to inform the construction of bounded wave profiles. The effect of the form of the spatial profile is investigated, with the total energy carried by the incident wave held constant as the profile is varied, and the relationship with the plane wave components which superimpose to form the bounded wave is discussed. Direct comparisons are made for the efficiency of the energy transmission of different profiles. The results reveal that, by tuning the form of the profile, substantial improvements in the total energy transmission can be achieved as compared to Gaussian and square waveforms

On The Use Of Evanescent Plane Waves For Low-Frequency Energy Transmission Across Material Interfaces

The transmission of airborne sound into high-impedance media is of interest in several applications. For example, sonic booms in the atmosphere may impact marine life when incident on the ocean surface, or affect the integrity of existing structures when incident on the ground. Transmission across high impedance-difference interfaces is generally limited by reflection and refraction at the surface, and by the critical angle criterion. However, spatially decaying incident waves, i.e., inhomogeneous or evanescent plane waves, may transmit energy above the critical angle, unlike homogeneous plane waves. The introduction of a decaying component to the incident trace wavenumber creates a nonzero propagating component of the transmitted normal wavenumber, so energy can be transmitted across the interface. A model of evanescent plane waves and their transmission across fluid-fluid and fluid-solid interfaces is developed here. Results are presented for both air-water and air-solid interfaces. The effects of the incident wave parameters (including the frequency, decay rate, and incidence angle) and the interfacial properties are investigated. Conditions for which there is no reflection at the air-solid interface, due to impedance matching between the incident and transmitted waves, are also considered and are found to yield substantial transmission increases over homogeneous incident waves