Powders under compression form mesostructures of particle agglomerations in response to both inter- and intra-particle forces. The ability to computationally predict the resulting mesostructures with reasonable accuracy requires models that capture the distributions associated with particle size and shape, contact forces, and mechanical response during deformation and fracture. The following report presents experimental data obtained for the purpose of validating emerging mesostructures simulated by discrete element method and peridynamic approaches. A custom compression apparatus, suitable for integration with our micro-computed tomography (micro-CT) system, was used to collect 3-D scans of a bulk powder at discrete steps of increasing compression. Details of the apparatus and the microcrystalline cellulose particles, with a nearly spherical shape and mean particle size, are presented. Comparative simulations were performed with an initial arrangement of particles and particle shapes directly extracted from the validation experiment. The experimental volumetric reconstruction was segmented to extract the relative positions and shapes of individual particles in the ensemble, including internal voids in the case of the microcrystalline cellulose particles. These computationally determined particles were then compressed within the computational domain and the evolving mesostructures compared directly to those in the validation experiment. The ability of the computational models to simulate the experimental mesostructures and particle behavior at increasing compression is discussed.
The peridynamic theory of solid mechanics is applied to modeling the deformation and fracture of micrometer-sized particles made of organic crystalline material. A new peridynamic material model is proposed to reproduce the elastic–plastic response, creep, and fracture that are observed in experiments. The model is implemented in a three-dimensional, meshless Lagrangian simulation code. In the small deformation, elastic regime, the model agrees well with classical Hertzian contact analysis for a sphere compressed between rigid plates. Under higher load, material and geometrical nonlinearity is predicted, leading to fracture. The material parameters for the energetic material CL-20 are evaluated from nanoindentation test data on the cyclic compression and failure of micrometer-sized grains.
This Laboratory Directed Research and Development project developed and applied closely coupled experimental and computational tools to investigate powder compaction across multiple length scales. The primary motivation for this work is to provide connections between powder feedstock characteristics, processing conditions, and powder pellet properties in the context of powder-based energetic components manufacturing. We have focused our efforts on multicrystalline cellulose, a molecular crystalline surrogate material that is mechanically similar to several energetic materials of interest, but provides several advantages for fundamental investigations. We report extensive experimental characterization ranging in length scale from nanometers to macroscopic, bulk behavior. Experiments included nanoindentation of well-controlled, micron-scale pillar geometries milled into the surface of individual particles, single-particle crushing experiments, in-situ optical and computed tomography imaging of the compaction of multiple particles in different geometries, and bulk powder compaction. In order to capture the large plastic deformation and fracture of particles in computational models, we have advanced two distinct meshfree Lagrangian simulation techniques: 1.) bonded particle methods, which extend existing discrete element method capabilities in the Sandia-developed , open-source LAMMPS code to capture particle deformation and fracture and 2.) extensions of peridynamics for application to mesoscale powder compaction, including a novel material model that includes plasticity and creep. We have demonstrated both methods for simulations of single-particle crushing as well as mesoscale multi-particle compaction, with favorable comparisons to experimental data. We have used small-scale, mechanical characterization data to inform material models, and in-situ imaging of mesoscale particle structures to provide initial conditions for simulations. Both mesostructure porosity characteristics and overall stress-strain behavior were found to be in good agreement between simulations and experiments. We have thus demonstrated a novel multi-scale, closely coupled experimental and computational approach to the study of powder compaction. This enables a wide range of possible investigations into feedstock-process-structure relationships in powder-based materials, with immediate applications in energetic component manufacturing, as well as other particle-based components and processes.
Microstructures and corresponding properties of compacted powders ultimately depend on the mechanical response of individual particles. In principle, computational simulations can predict the results of powder compaction processes, but the selection of appropriate models for both particle–particle interactions and particle deformations across all relevant length scales remain nontrivial tasks, especially in material systems lacking detailed mechanical property information. The work presented here addresses these issues by conducting uniaxial compressions in situ inside of a scanning electron microscope to characterize the mechanical response of individual micron-sized particles of a molecular crystal, hexanitrohexaazaisowurtzitane (CL-20). This experimental approach enabled the collection of quantitative force and displacement data alongside simultaneous imaging to capture morphology changes. The results reveal information about elastic deformation, yield, plastic deformation, creep, and fracture phenomena. Accordingly, this work demonstrates a generalizable approach for assessing the mechanical response of individual micron-sized molecular crystal particles and utilizing those responses in particle-level models. Graphic abstract: [Figure not available: see fulltext.].
Ignition and material response properties of aluminumized HMX heterogeneous explosive mixtures were explored in a series of planar impact experiments performed over multiple years. This work expands on previous work studying material response to impact in single-component HMX granular materials. The addition of nanometric aluminum is shown to affect the ignition sensitivity and growth to reaction from impact. The gas gun test results are presented here varying parameters of particle size, shock strength, and aluminum mass fraction.
Two key mechanical processes exist in the formation of powder compacts. These include the complex kinematics of particle rearrangement as the powder is densified and particle deformation leading to mechanical failure and fragmentation. Experiments measuring the time varying forces across a densifying powder bed have been performed in powders of microcrystalline cellulose with mean particle sizes between 0.4 and 1.2 mm. In these experiments, diagnostics measured the applied and transmitted loads and the bulk powder density. Any insight into the particle behavior must be inferred from deviations in the smoothly increasing stress-density compaction relationship. By incorporating a window in the compaction die body, simultaneous images of particle rearrangement and fracture at the confining window are captured. The images are post-processed in MATLAB® to track individual particle motion during compression. Complimentary discrete element method (DEM) simulations are presented and compared to experiment. The comparison provides insight into applying DEM methods for simulating large or permanent particle deformation and suggests areas for future study.
Particle characteristics can drastically influence the process-structure-property-performance aspects of granular materials in compression. We aim to computationally simulate the mechanical processes of stress redistribution in compacts including the kinematics of particle rearrangement during densification and particle deformation leading to fragmentation. Confined compression experiments are conducted with three sets of commercial microcrystalline cellulose particles nearly spherical in shape with different mean particle size. Experimentally measured compression curves from tall powder columns are fitted with the Kenkre et al. (J. of American Chemical Society, Vol. 79, No. 12) model. This model provides a basis to derive several common two-parameter literature models and as a framework to incorporate statistical representations of critical particle behaviors. We focus on the low-stress compression data and the model comparisons typically not discussed in the literature. Additional single particle compressions report fracture strength with particle size for comparison to the apparent particle strength extracted from bulk compression data.
A novel experimental methodology is presented to study the deviatoric response of powders in shock regimes. The powders are confined to a cylindrical wedge volume, and a projectile-driven shock wave with a sinusoidally varying front propagates through the powder. The perturbed shock wave exhibits a damping behavior due to irreversible processes of viscosity and strength (deviatoric) of the powder with propagation through increasing powder thicknesses. The inclined surface of the wedge is polished and coated to establish a diffuse surface suitable for reflecting incident laser light into a high-speed camera imaging at 5 MHz. Images of the contrast loss upon shock wave arrival at the observation surface are post-processed for qualitative and quantitative information. New data of shock damping behavior with parameters of perturbation wavelength and initial shock strength are presented for powders of copper, tantalum, and tungsten carbide as well as their mixtures. We present the first full-field images showing additional spatial disturbances on the perturbed shock front that appear dependent on particle material and morphology.
Surrogate laboratory experiments were conducted to understand how cavities surrounding an explosive event in a bounded material influence the transfer of energy to the surface of the material. Exploding bridgewires were detonated in a cavity that had been created in an optically accessible polymer material. The shock propagation was investigated through five paths, three through spherical cavities of varying size filled with air, one without a cavity and one in air. Shock propagation was imaged using the schlieren technique. The path effects were determined by measuring energy at the polymer surface using a pressure sensor. All cavities were successful at reducing the peak pressure nearly 100% from the case without a cavity, a reduction close to air. The degree of reduction was not uniform over all frequencies. Overall, our findings suggest that cavities influence both the amount and frequency content of the energy transfer from source to surface.
A multi-point microwave interferometer (MPMI) concept is presented for non-invasively monitoring the internal transit of a shock, detonation, or reaction front in energetic media. The concept utilizes an electro-optic (EO) crystal to impart a timevarying phase lag onto a laser with a microwave signal. Polarization optics convert this phase lag into an amplitude modulation. A heterodyne interferometer compares the modulated laser beam to a constant reference. This enables the detection of changes in the modulating microwave frequency generated by the motion of the measurement surface. The design is scalable and makes use of the established construction and analysis methods employed in photonic Doppler velocimetry (PDV). The technical challenges associated with the concept are the frequency stability of the lasers, the amount of light return after EO modulation, and the frequency uncertainty of fast Fourier transform (FFT) methods.
The combustion of molten metals is an important area of study with applications ranging from solid aluminized rocket propellants to fireworks displays. This work uses digital in-line holography (DIH) to experimentally quantify the three-dimensional position, size, and velocity of aluminum particles during combustion of ammonium perchlorate (AP) based solid-rocket propellants. In addition, spatially resolved particle temperatures are simultaneously measured using two-color imaging pyrometry. To allow for fast characterization of the properties of tens of thousands of particles, automated data processing routines are proposed. Using these methods, statistics from aluminum particles with diameters ranging from 15 to 900 µm are collected at an ambient pressure of 83 kPa. In the first set of DIH experiments, increasing initial propellant temperature is shown to enhance the agglomeration of nascent aluminum at the burning surface, resulting in ejection of large molten aluminum particles into the exhaust plume. The resulting particle number and volume distributions are quantified. In the second set of simultaneous DIH and pyrometry experiments, particle size and velocity relationships as well as temperature statistics are explored. The average measured temperatures are found to be 2640 ± 282 K, which compares well with previous estimates of the range of particle and gas-phase temperatures. The novel methods proposed here represent new capabilities for simultaneous quantification of the joint size, velocity, and temperature statistics during the combustion of molten metal particles. The proposed techniques are expected to be useful for detailed performance assessment of metalized solid-rocket propellants.
High-speed (20 kHz) digital in-line holography (DIH) is applied for 3D quantification of the size and velocity of fragments formed from the impact of a single water drop onto a thin film of water and burning aluminum particles from the combustion of a solid rocket propellant. To address the depth-of-focus problem in DIH, a regression-based multiframe tracking algorithm is employed, and out-of-plane experimental displacement accuracy is shown to be improved by an order-of-magnitude. Comparison of the results with previous DIH measurements using low-speed recording shows improved positional accuracy with the added advantage of detailed resolution of transient dynamics from single experimental realizations. The method is shown to be particularly advantageous for quantification of particle mass flow rates. For the investigated particle fields, the mass flows rates, which have been automatically measured from single experimental realizations, are found to be within 8% of the expected values.
Three-dimensional deformation of rupture discs subjected to gas-dynamic shock loading was measured using a stereomicroscope digital image correlation (DIC) system. One-dimensional blast waves generated with a small-diameter, explosively driven shock tube were used for studying the fluid-structure interactions that exist when incident onto relatively low-strength rupture discs. Prior experiments have shown that subjecting the 0. 64-cm-diameter, stainless steel rupture discs to shock waves of varying strength results in a range of responses from no rupture to shear at the outer weld diameter. In this work, the outer surface of the rupture discs were prepared for DIC using 100–150 _m-sized speckles and illuminated with a Xenon flashlamp. Two synchronized Shimadzu HPV-2 cameras coupled to an Olympus microscope captured stereoimage sequences of rupture disc behavior at speeds of 1 MHz. Image correlation performed on the stereo-images resulted in spatially resolved surface deformation. The experimental facility, specifics of the DIC diagnostic technique, and the temporal deformation and velocity of the surface of a rupturing disc are presented.
A multi-point microwave interferometer (MPMI) concept was developed for non-invasively tracking a shock, reaction, or detonation front in energetic media. Initially, a single-point, heterodyne microwave interferometry capability was established. The design, construction, and verification of the single-point interferometer provided a knowledge base for the creation of the MPMI concept. The MPMI concept uses an electro-optic (EO) crystal to impart a time-varying phase lag onto a laser at the microwave frequency. Polarization optics converts this phase lag into an amplitude modulation, which is analyzed in a heterodyne interfer- ometer to detect Doppler shifts in the microwave frequency. A version of the MPMI was constructed to experimentally measure the frequency of a microwave source through the EO modulation of a laser. The successful extraction of the microwave frequency proved the underlying physical concept of the MPMI design, and highlighted the challenges associated with the longer microwave wavelength. The frequency measurements made with the current equipment contained too much uncertainty for an accurate velocity measurement. Potential alterations to the current construction are presented to improve the quality of the measured signal and enable multiple accurate velocity measurements.
The flash technique was used to measure the thermal diffusivity and specific heat of titanium potassium perchlorate (TKP) ignition powder (33wt% Ti - 67wt% KP) with Ventron sup- plied titanium particles, TKP ignition powder (33wt% Ti - 67wt% KP) with ATK supplied titanium particles, TKP output powder (41wt% Ti - 59wt% KP), and titanium subhydride potassium perchlorate (THKP) (33wt% TiH 1.65 - 67wt% KP) at 25°C. The influence of density and temperature on the thermal diffusivity and specific heat of TKP with Ventron supplied titanium particles was also investigated. Lastly, the thermal diffusivity and specific heats of 9013 glass, 7052 glass, SB-14 glass, and C-4000 Muscovite mica are presented as a function of temperature up to 300° C.
This report summarizes a project in which the authors sought to develop and deploy: (i) experimental techniques to elucidate the complex, multiscale nature of thermal transport in particle-based materials; and (ii) modeling approaches to address current challenges in predicting performance variability of materials (e.g., identifying and characterizing physical- chemical processes and their couplings across multiple length and time scales, modeling information transfer between scales, and statically and dynamically resolving material structure and its evolution during manufacturing and device performance). Experimentally, several capabilities were successfully advanced. As discussed in Chapter 2 a flash diffusivity capability for measuring homogeneous thermal conductivity of pyrotechnic powders (and beyond) was advanced; leading to enhanced characterization of pyrotechnic materials and properties impacting component development. Chapter 4 describes success for the first time, although preliminary, in resolving thermal fields at speeds and spatial scales relevant to energetic components. Chapter 7 summarizes the first ever (as far as the authors know) application of TDTR to actual pyrotechnic materials. This is the first attempt to actually characterize these materials at the interfacial scale. On the modeling side, new capabilities in image processing of experimental microstructures and direct numerical simulation on complicated structures were advanced (see Chapters 3 and 5). In addition, modeling work described in Chapter 8 led to improved prediction of interface thermal conductance from first principles calculations. Toward the second point, for a model system of packed particles, significant headway was made in implementing numerical algorithms and collecting data to justify the approach in terms of highlighting the phenomena at play and pointing the way forward in developing and informing the kind of modeling approach originally envisioned (see Chapter 6). In both cases much more remains to be accomplished.
The effect of vents on the fast cookoff of energetic materials is studied through experimental modifications to the confinement vessel of the Radiant Heat Fast Cookoff Apparatus. Two venting schemes were investigated: 1) machined grooves at the EM-cover plate interface; 2) radial distribution of holes in PEEK confiner. EM materials of PBXN-109 and PBX 9502 were tested. Challenges with the experimental apparatus and EM materials were identified such that studying the effect of vents as an independent parameter was not realized. The experimental methods, data and post-test observations are presented and discussed.
A hybrid closed bomb-strand burner is used to measure the burning behavior of the titanium subhydride potassium perchlorate pyrotechnic with an equivalent hydrogen concentration of 1.65. This experimental facility allows for simultaneous measurement of the closed bomb pressure rise and pyrotechnic burn rate as detected by electrical break wires over a range of pressures. Strands were formed by pressing the pyrotechnic powders to bulk densities between 60% and 90% theoretical maximum density. The burn rate dependance on initial density and vessel pressure are measured. At all initial strand densities, the burn is observed to transition from conductive to convective burning within the strand. The measured vessel pressure history is further analyzed following the closed bomb analysis methods developed for solid propellants.
A multifunctional reactor is a chemical engineering device that exploits enhanced heat and mass transfer to promote production of a desired chemical, combining more than one unit operation in a single system. The main component of the reactor system under study here is a vertical column containing packing material through which liquid(s) and gas flow cocurrently downward. Under certain conditions, a range of hydrodynamic regimes can be achieved within the column that can either enhance or inhibit a desired chemical reaction. To study such reactors in a controlled laboratory environment, two experimental facilities were constructed at Sandia National Laboratories. One experiment, referred to as the Two-Phase Experiment, operates with two phases (air and water). The second experiment, referred to as the Three-Phase Experiment, operates with three phases (immiscible organic liquid and aqueous liquid, and nitrogen). This report describes the motivation, design, construction, operational hazards, and operation of the both of these experiments. Data and conclusions are included.
Digital image correlation (DIC) and the tremendous advances in optical imaging are beginning to revolutionize explosive and high-strain rate measurements. This paper presents results obtained from metallic hemispheres expanded at detonation velocities. Important aspects of sample preparation and lighting of the image will be presented that are key considerations in obtaining images for DIC with frame rates at 1-million frames/second. Quantitative measurements of the case strain rate, expansion velocity and deformation will be presented. Furthermore, preliminary estimations of the measurement uncertainty will be discussed with notes on how image noise and contrast effect the measurement of shape and displacement. The data are then compared with analytical representations of the experiment.
A novel multiphase shock tube to study particle dynamics in gas-solid flows has been constructed and tested. Currently, there is a gap in data for flows having particle volume fractions between the dusty and granular regimes. The primary purpose of this new facility is to fill that gap by providing high quality data of shock-particle interactions in flows having dense gas particle volume fractions. Towards this end, the facility aims to drive a shock into a spatially isotropic field, or curtain, of particles. Through bench-top experimentation, a method emerged for achieving this challenging task that involves the use of a gravity-fed contoured particle seeder. The seeding method is capable of producing fields of spatially isotropic particles having volume fractions of about 1 to 35%. The use of the seeder in combination with the shock tube allows for the testing of the impingement of a planar shock on a dense field of particles. The first experiments in the multiphase shock tube have been conducted and the facility is now operational.
The thrust from a multi-cycle, pulse detonation engine operating at practical flight altitudes will vary with the surrounding environment pressure. We have carried out the first experimental study using a detonation tube hung in a ballistic pendulum arrangement within a large pressure vessel in order to determine the effect that the environment has on the single-cycle impulse. The air pressure inside the vessel surrounding the detonation tube varied between 100 and 1.4 kPa while the initial pressure of the stoichiometric ethylene-oxygen mixture inside the tube varied between 100 and 30 kPa. The original impulse model (Wintenberger et al., Journal of Propulsion and Power, Vol. 19, No. 1, 2002) was modified to predict the observed increase in impulse and blow down time as the environment pressure decreased below one atmosphere. Comparisons between the impulse from detonation tubes and ideal, steady flow rockets indicate incomplete expansion of the detonation tube exhaust, resulting in a 37% difference in impulse at a pressure ratio (ratio of pressure behind the Taylor wave to the environment pressure) of 100.