Energy localization and heat generation in composite energetic systems under high-frequency mechanical excitation

In this work, the ability to use high frequency mechanical excitation to generate significant heating within plastic bonded explosives, as well as single energetic particles embedded within a viscoelastic binder, is studied. In this work, the fundamental mechanisms associated with the conversion of high-frequency mechanical excitation to heat as applied to these composite energetic systems are thoroughly investigated. High-frequency contact excitation has been used to generate a significant amount of heat within samples of PBX 9501 and representative inert mock materials. Surface temperature rises on the order of 10 °C were observed at certain frequencies over a range from 50 kHz to 40 MHz at thermal steady state conditions. The mechanical responses of these samples were also measured to explore the connection between the thermal and bulk motion of the samples. It was found that significant heating of the samples near the transducer resonance was driven by the bulk motion of the material while heating observed at higher frequencies were attributed to particle-scale interactions. To further investigate the interactions occurring at the particle scale, similar excitation was applied to samples of an elastic binder embedded with individual inert or energetic particles. Samples were excited over a range of 100 kHz to 20 MHz, and two distinct frequency regions were observed with separate characteristic heating trends. Through the comparison of the measured surface motion of the sample to the spatial temperature maps of the surface, it was determined that for heating observed in the samples at excitation frequencies above 1 MHz, the heat generation was due viscoelastic effects of the binder near the sample surface. However, at excitation frequencies near the transducer resonance of 215 kHz, it was determined that significant heat was generated at the inclusion and was associated with particle-binder interactions. For these cases of particle associated heating, an analytical heat conduction model was fit to the collected surface temperature data to estimate the heating rates and temperatures associated with the embedded particles. To investigate the potential of stress concentrations to generate localized heating near an inclusion due to viscoelastic losses, an analytical solution of the stress and temperature fields caused by wave scattering effects due to a spherical inclusion within a lossy binder was developed. Results indicate that under certain excitation and sample configurations, significant heating can occur due to stress concentrations caused by constructive interference of the waves near the inclusion and temperatures are predicted to approach or exceed realistic decomposition temperatures of various energetic materials. This analysis indicates that significant heating of the embedded particles can be induced without the presence of delamination or voids; however, this phenomenon it thought to mainly be a precursor or driver to more dynamic events associated with debonding between the particle and binder. Finally, high speed X-ray phase contrast imaging and high speed visible microscopy were used to demonstrate the individual heating mechanisms associated with the heating and subsequent decomposition of an HMX particle within a viscoelastic binder under ultrasonic excitation. Additional analysis of the transient surface temperature of the sample was used to characterize and quantify the heat generation produced from each observed heating mechanism. The results and developed methods presented in this work should prove useful in the understanding of the conversion of mechanical to thermal energy via various mechanisms within composite energetic systems. (Abstract shortened by ProQuest.

Using time-frequency analysis to determine time-resolved detonation velocity with microwave interferometry

Two time-frequency analysis methods based on the short-time Fourier transform (STFT) and continuous wavelet transform (CWT) were used to determine time-resolved detonation velocities with microwave interferometry (MI). The results were directly compared to well-established analysis techniques consisting of a peak-picking routine as well as a phase unwrapping method (i.e., quadrature analysis). The comparison is conducted on experimental data consisting of transient detonation phenomena observed in triaminotrinitrobenzene and ammonium nitrate-urea explosives, representing high and low quality MI signals, respectively. Time-frequency analysis proved much more capable of extracting useful and highly resolved velocity information from low quality signals than the phase unwrapping and peak-picking methods. Additionally, control of the time-frequency methods is mainly constrained to a single parameter which allows for a highly unbiased analysis method to extract velocity information. In contrast, the phase unwrapping technique introduces user based variability while the peak-picking technique does not achieve a highly resolved velocity result. Both STFT and CWT methods are proposed as improved additions to the analysis methods applied to MI detonation experiments, and may be useful in similar applications

Heat generation in an elastic binder system with embedded discrete energetic particles due to high-frequency, periodic mechanical excitation

High-frequency mechanical excitation can induce heating within energetic materials and may lead to advances in explosives detection and defeat. In order to examine the nature of this mechanically induced heating, samples of an elastic binder (Sylgard 184) were embedded with inert and energetic particles placed in a fixed spatial pattern and were subsequently excited with an ultrasonic transducer at discrete frequencies from 100 kHz to 20 MHz. The temperature and velocity responses of the sample surfaces suggest that heating due to frictional effects occurred near the particles at excitation frequencies near the transducer resonance of 215 kHz. An analytical solution involving a heat point source was used to estimate heating rates and temperatures at the particle locations in this frequency region. Heating located near the sample surface at frequencies near and above 1 MHz was attributed to viscoelastic effects related to the surface motion of the samples. At elevated excitation parameters near the transducer resonance frequency, embedded particles of ammonium perchlorate and cyclotetramethylene-tetranitramine were driven to chemical decomposition

Localized Heating Near a Rigid Spherical Inclusion in a Viscoelastic Binder Material Under Compressional Plane Wave Excitation

High-frequency mechanical excitation has been shown to generate heat within composite energetic materials and even induce reactions in single energetic crystals embedded within an elastic binder. To further the understanding of how wave scattering effects attributable to the presence of an energetic crystal can result in concentrated heating near the inclusion, an analytical model is developed. The stress and displacement solutions associated with the scattering of compressional plane waves by a spherical obstacle (Pao and Mow, 1963, “Scattering of Plane Compressional Waves by a Spherical Obstacle,” J. Appl. Phys., 34(3), pp. 493–499) are modified to account for the viscoelastic effects of the lossy media surrounding the inclusion (Gaunaurd and Uberall, 1978, “Theory of Resonant Scattering From Spherical Cavities in Elastic and Viscoelastic Media,” J. Acoust. Soc. Am., 63(6), pp. 1699–1712). The results from this solution are then utilized to estimate the spatial heat generation due to the harmonic straining of the material, and the temperature field of the system is predicted for a given duration of time. It is shown that for certain excitation and sample configurations, the elicited thermal response near the inclusion may approach, or even exceed, the decomposition temperatures of various energetic materials. Although this prediction indicates that viscoelastic heating of the binder may initiate decomposition of the crystal even in the absence of defects such as initial voids or debonding between the crystal and binder, the thermal response resulting from this bulk heating phenomenon may be a precursor to dynamic events associated with such crystal-scale effects

High Speed X-ray Phase Contrast Imaging of Energetic Composites under Dynamic Compression

Fracture of crystals and frictional heating are associated with the formation of “hot spots” (localized heating) in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and the fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and the fracture mechanisms in PBXs composed of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene binder doped with iron (III) oxide. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binder, even at ultrafast recording rates. Under dynamic compression, most of the cracking in the crystals was observed to be due to the tensile stress generated by the diametral compression applied from the contacts between the crystals. Tensile stress driven cracking was also observed for some of the crystals due to the transverse deformation of the binder and superior bonding between the crystal and the binder. The obtained results are vital to develop improved understanding and to validate the macroscopic and mesoscopic numerical models for energetic composites so that eventually hot spot formation can be predicted

Cristina Peri Rossi. Erotismo, transgresión y exilio: Las voces de Cristina Peri Rossi, Jesús Gómez-de-Tejada (coord.). Sevilla, Editorial Universidad de Sevilla, 2017

Obra ressenyada: Jesús GÓMEZ-DE-TEJADA (coord.), Cristina Peri Rossi. Erotismo, transgresión y exilio: Las voces de Cristina Peri Rossi. Sevilla: Editorial Universidad de Sevilla, 2017