Daniel E. Eakins

Shock compression of reactive powder mixtures

Abstract The shock compression of reactive powder mixtures can yield varied chemical behaviour with occurrence of mechanochemical reactions in the timescale of the high pressure state, or thermochemical reactions in the timescale of temperature equilibration, or simply the creation of dense packed highly reactive state of material. The principal challenge has been to understand the processes that distinguish between mechanochemical (shock induced) and thermochemical (shock assisted) reactions, which has broad implications for the synthesis of novel metastable or non-equilibrium materials, or the design of highly configurable next generation energetic materials. In this paper, the process of shock compression in reactive powder mixtures and the associated role of various intrinsic and extrinsic characteristics of reactants in the triggering of ultrafast shock induced chemical reactions are discussed. Experimental techniques employing time resolved diagnostics and results which identify the occurrence of shock induced reactions are reviewed. Conceptual and numerical models used to describe the heterogeneous nature of such reactions through mesoscopic details of shock compression are presented. Finally, a discussion of the application of recent results for the design of reactive material systems with controlled reaction initiation and energy release characteristics is provided.

Discrete particle simulation of shock wave propagation in a binary Ni+Al powder mixture

Numerical simulations of shock wave propagation through discretely represented powder mixtures were performed to investigate the characteristics of deformation and mixing in the Ni+Al system. The initial particle arrangements and morphologies were imported from experimentally obtained micrographs of powder mixtures pressed at densities in the range of 45%–80% of the theoretical maximum density (TMD). Simulations were performed using these imported micrographs for each density compact subjected to driver velocities (Up) of 0.5, 0.75, and 1km∕s, and the resulting shock velocity (Us) was used to construct the Us-Up equation of state. The simulated equation of state for the 60% TMD mixture was validated by matching results obtained from previous gas-gun experiments. The details of shock wave propagation through the Ni+Al powder mixtures were explored on several scales. It is shown that the shock compression of mixtures of powders of dissimilar densities and strength is associated with heterogeneous deformatio...

A dynamic discrete dislocation plasticity method for the simulation of plastic relaxation under shock loading

In this article, it is demonstrated that current methods of modelling plasticity as the collective motion of discrete dislocations, such as two-dimensional discrete dislocation plasticity (DDP), are unsuitable for the simulation of very high strain rate processes (106 s−1 or more) such as plastic relaxation during shock loading. Current DDP models treat dislocations quasi-statically, ignoring the time-dependent nature of the elastic fields of dislocations. It is shown that this assumption introduces unphysical artefacts into the system when simulating plasticity resulting from shock loading. This deficiency can be overcome only by formulating a fully time-dependent elastodynamic description of the elastic fields of discrete dislocations. Building on the work of Markenscoff & Clifton, the fundamental time-dependent solutions for the injection and non-uniform motion of straight edge dislocations are presented. The numerical implementation of these solutions for a single moving dislocation and for two annihilating dislocations in an infinite plane are presented. The application of these solutions in a two-dimensional model of time-dependent plasticity during shock loading is outlined here and will be presented in detail elsewhere.

Shock-induced reaction in a flake nickel + spherical aluminum powder mixture

Mixtures of flake-shaped nickel and spherical aluminum powders have been subjected to shock-loading up to 6 GPa to investigate the occurrence of shock-induced chemical reactions. The high-pressure Hugoniot state is determined through time-resolved measurements of the input stress and shock transit time through the specimen. An increase in shock velocity is observed above stress levels of 3.5 GPa, suggesting the initiation of an ultrafast shock-induced chemical reaction and formation of Ni-Al compound(s) occurring in the time scale of the high-pressure shock state.

The shock-densification behavior of three distinct Ni+Al powder mixtures

The shock-densification response of equivolumetric mixtures of Ni+Al powders of varying particle size and morphology has been determined through instrumented parallel-plate impact experiments. The results reveal a variation in the densification response, with crush strengths (stress at full density) ranging from 0.5 to nearly 6GPa. A modified Fischmeister–Artz contact model was proposed to predict the crush strength of configurationally varying Ni+Al powder mixtures.

Instrumented Taylor anvil-on-rod impact tests for validating applicability of standard strength models to transient deformation states

Instrumented Taylor anvil-on-rod impact tests have been conducted on oxygen-free electronic copper to validate the accuracy of current strength models for predicting transient states during dynamic deformation events. The experiments coupled the use of high-speed digital photography to record the transient deformation states and laser interferometry to monitor the sample back (free surface) velocity as a measure of the elastic∕plastic wave propagation through the sample length. Numerical continuum dynamics simulations of the impact and plastic wave propagation employing the Johnson-Cook [Proceedings of the Seventh International Symposium on Ballistics, 1983, The Netherlands (Am. Def. Prep. Assoc. (ADPA)), pp. 541–547], Zerilli-Armstrong [J. Appl. Phys. C1, 1816 (1987)], and Steinberg-Guinan [J. Appl. Phys. 51, 1498 (1980)] constitutive equations were used to generate transient deformation profiles and the free surface velocity traces. While these simulations showed good correlation with the measured free ...

Dynamic densification behavior of nanoiron powders under shock compression

The dynamic densification behavior of nanoiron powder (∼25nm particle size) prepressed to ∼35% and ∼45% of solid density was determined based on measurements of shock input stress and wave velocity by using piezoelectric stress gauges. The experimentally determined shock densification response is observed to be sensitive to the initial density (or porosity) of prepressed nanoiron powder compacts. Hugoniot measurements show an obvious densification-distension transition at ∼2GPa for the ∼35% dense and ∼6GPa for the ∼45% dense powder compacts. The densification and shock compression responses of the nanoiron powders are also calculated by using isobaric and isochoric models. Correlations of the model calculations with the measured data indicate that the shock Hugoniot of nanoiron powders cannot be correctly described by the currently available analytical models that are otherwise capable of predicting the Hugoniot of highly porous materials (prepressed compacts) of micron-sized powders.

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research. There exists very little experimental data probing WDM states at the atomic level to test current models and those performed up to now are limited in quality. Here, we report a proof-of-principle experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states of WDM Fe. Structural and electronic changes in Fe are clearly observed for the first time at such extreme conditions. The amplitude of the EXAFS oscillations persists up to 500 GPa and 17000 K, suggesting an enduring local order. Moreover, a discrepancy exists with respect to theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models.

Plate-impact loading of cellular structures formed by selective laser melting

Porous materials are of great interest because of improved energy absorption over their solid counterparts. Their properties, however, have been difficult to optimize. Additive manufacturing has emerged as a potential technique to closely define the structure and properties of porous components, i.e. density, strutwidthandporesize; however, thebehaviourofthesematerialsatveryhigh impact energies remains largely unexplored. We describe an initial study of the dynamic compression response of lattice materials fabricated through additive manufacturing. Lattices consisting of an array of intersecting stainless steel rods were fabricated into discs using selective laser melting. The resulting discs were impacted against solid stainless steel targets at velocities ranging from 300 to 700ms −1 using a gas gun. Continuum CTH simulations were performed to identify key features in the measured wave profiles, while 3D simulations, in which the individual cells were modelled, revealed details of microscale deformation during collapse of the lattice structure. The validated computermodelshavebeenusedtoprovideanunderstandingofthedeformation processes in the cellular samples. The study supports the optimization of cellular structures for application as energy absorbers.

X-ray imaging of subsurface dynamics in high-Z materials at the Diamond Light Source

In this paper, we describe a new approach enabling study of subsurface dynamics in high-Z materials using the unique combination of high-energy synchrotron X-rays, a hybrid bunch structure, and a new dynamic loading platform. We detail the design and operation of the purpose-built, portable small bore gas-gun, which was installed on the I12 high-energy beamline at the Diamond Light Source and used to drive compression waves into solid and porous metal targets. Using a hybrid bunch structure and broadband X-ray pulses of up to 300 keV, radiographic snapshots were captured during various dynamic deformation processes in cm-scale specimens, thereby contributing to a more complete understanding of the evolution of mesoscale damage. Importantly, we highlight strategies for overcoming the challenges associated with using high-energy X-rays, and suggest areas for improvement needed to advance dynamic imaging through large-scale samples of relevance to engineering scenarios. These preliminary measurements demonstrate the feasibility of probing highly transient phenomena using the presented methodology.