L. A. Merzhievskii

Experience of using synchrotron radiation for studying detonation processes

Results of studying detonation processes in condensed high explosives, which are obtained by methods based on using synchrotron radiation, are summarized. Beam parameters are given, and elements of the station and measurement system are described. Data on the density distribution in the detonation front for several high explosives are presented, and values of parameters in the Neumann spike and at the Jouguet point are determined. A method used to reconstruct a complete set of gasdynamic characteristics (density fields, particle velocity vector, and pressure) from the experimentally measured dynamics of the x-ray shadow of the examined flow is described. Results of using this method for studying detonation of a charge of plastic-bonded TATB are presented. A method of measuring small-angle x-ray scattering in the course of detonation conversion is described. Based on results obtained by this method for a number of high explosives with an excess content of carbon, kinetics of condensation of free carbon and dynamics of the mean size of nanoparticles being formed thereby are analyzed.

Implementation of the capability of synchrotron radiation in a study of detonation processes

24 Modern detonation theory is based on the gas dynamic model developed by Ya.B. Zeldovich, J. von Neumann, and W. Doring. In addition to it, the reaction zone sizes and Neumann peak have been experimentally studied and simulated, the phenome nological kinetics of the detonation transformation has been constructed, attempts to describe the reac tion zone by molecular dynamics methods have been undertaken, the data on unloading adiabats have been obtained, numerous equations of state of detonation products have been constructed, and the actual curva ture of the detonation front and a number of other properties of the process have been considered. At the same time, there are new facts not complying with the commonly accepted notions and requiring experi mental studies and subsequent explanations. Among the actively studied problems are the features of the fine structure of the reaction zone, the Chapman– Jouguet surface shape, the carbon condensation kinet ics during the detonation transition, the possibility of transition without a chemical peak, and a number of other problems. Their solution is complicated by the absence of adequate experimental techniques, since the available ones are often perturbative or do not pro vide spatial and temporal resolution sufficient for unambiguous interpretation. Partial answers to some of these questions can be obtained using the technique developed and implemented by the authors, based on the use of the soft X ray component of synchrotron radiation (SR).

Deformation models under intense dynamic loading (Review)

This paper considers currently available models of irreversible deformation processes of materials under dynamic, in particular shock-wave, loading. The models can be divided into three groups: (1) macroscopic (continuum) models—traditional models of continuum mechanics, primarily classical models of elastic-plastic deformation, their various generalizations to the case of dynamic processes and models of viscoelastic relaxation media; (2) microstructural models based on the description of microstructural mechanisms of irreversible deformation (usually, the concept of the kinetics of a dislocation ensemble); (3) atomistic molecular dynamics models and calculations. A special category includes the most promising (from the point of view of the author) multilevel models which combine the advantages of each of these approaches and consider deformation mechanisms of various levels. Examples of calculations using such models are presented.

Modeling of shock-wave deformation of polymethyl metacrylate

A model of a Maxwellian elastoplastic body is constructed to describe the behavior of polymethyl metacrylate (C5O2H8)n under loading. A principal feature of this model is supplementing the governing equations with the relaxation time of shear stresses in the form of a continuous dependence on parameters characterizing the state of the medium. The analytical form of the dependence is chosen with allowance for microstructural and mesostructural mechanisms of irreversible deformation. Another specific feature of the model is the equation of state of the medium, which includes the dependence of the internal energy on the first and second invariants of the strain tensor. Such an approach allows obtaining a unified mathematical description of all physical states of polymers. Particular attention is paid at the stage of model verification to comparisons of model predictions with experimental data for the temperature of the shock-compressed material and decay of the shock wave due to its interaction with overtaking and side rarefaction waves. This comparison shows that the model provides an adequate description of shock-wave processes in polymethyl metacrylate.

Simulation of the dynamic compression of polycrystalline Al2O3

A model for the behavior of Al2O3 ceramics under dynamic and shock-wave loading is developed on the basis of the model of a Maxwell-type viscoelastic body, which is used here for ceramics for the first time. An equation for the variation in the internal energy for a nonspherical strain tensor and the relation between the time of relaxation of tangential stresses and the state parameters of the medium are derived. The time of relaxation serves to describe the microstructural mechanisms of irreversible deformation. The applicability of the model is validated by comparing the results of solving a number of problems of dynamic and shock-wave deformation with experimental data.

Observation of Compression and Failure Waves in PMMA by Means of Synchrotron Radiation

The possibility of using synchrotron radiation for density measurements in shock‐compressed polymethylmethacrylate destroyed in a failure wave is demonstrated for the first time. Parameters of the compression and failure processes are presented.

Shock-wave structure in a unidirectional composite with differently oriented fibers

We have studied experimentally stress profiles upon propagation of a shock wave in a unidirectional composite in the case where the normal to the surface of the wave front is directed at angle θ to a reinforcing fiber. For θ=5 and 15°, the elastic precursor behind which the shock wave propagates was registered. For the case θ=45°, the elastic precursor becomes a plastic wave with a smeared front, and, for θ=90°, a single shock wave was recorded. Measurement results show that the stress at the yield point depends on the orientation of the fiber and on the direction of the shock-wave motion.

Temperature measurements for shocked polymethylmethacrylate, epoxy resin, and polytetrafluoroethylene and their equations of state

This paper presents the results of computational and experimental studies of the temperature along the shock adiabat for three polymers. Measurements of the brightness temperatures of shock-compressed epoxy resin and polymethylmethacrylate and the brightness and color temperatures of shock-compressed polytetrafluoroethylene were carried out. The temperatures of the shock-compressed polymethylmethacrylate were determined in the range 1390–1900 K for shock pressures of 22–39 GPa. Similar measurements performed for epoxy resin in the pressure range of 18–40 GPa showed values of 940–1900 K, and the temperatures of polytetrafluoroethylene in the pressure range of 30–50 GPa were equal to 2000–3200 K. The equation of state for the three polymers with a nonspherical strain tensor was constructed to describe shock-wave and high-temperature processes in a wide range of thermodynamic parameters. In the proposed model, two Gruneisen parameters were used: the thermodynamic parameter corresponding to intrachain vibrati...

High-velocity impact of steel particles on targets made of porous copper

Results of studying the normal impact of small-diameter steel spheres on the surface of semi-infinite targets made of porous copper are reported. Characteristics of craters being formed are compared with results of other investigations of the impact on high-porosity targets. New experimental data form the basis for continuing these studies and can be incorporated into models that describe a high-velocity impact on porous media.

Simulation of the dynamic compression of porous Al2O3

A model describing the behavior of Al2O3 ceramics under dynamic and shock-wave compression is developed. It is based on the model of a Maxwell-type viscoelastic body, which has been previously used with advantage to simulate the compression of metal porous media. Shock adiabats, including those for the case of high porosity, are calculated, and the evolution of finite-duration compression pulses propagating in a porous half-space are analyzed. The calculations confirm the assumption that the grain (pore) size has an effect on the shock-wave process and the final result.