The residual stresses have been measured by X-ray and neutron diffraction on PM green bodies manufactured by conventional and high speed compaction of iron powder with and without added copper and brass powder. Compressive residual stresses are present in a thin layer in both top and side surfaces. They are largest in the side surfaces due to plastic deformation of the surface material caused by the friction forces during ejection out of the die. In the interior of the green body residual stresses exist with certain region under compression (periferical regions) and other under tension (more central regions). It is unclear whether mixing iron powder with brass or copper powder leads to considerable phase stresses between the two phases.

Many sand dunes – at least seven in the United States – make loud booming noises when they avalanche. Records of the sound are centuries old, but the cause remains a mystery. This study examines properties of both the sand and the sound.

Properties of the sand reveal clues about the source of the booming. Sand must be extremely dry to boom, but low moisture content alone is not sufficient to facilitate booming. Although the mean grain diameters of both booming and silent dune sands range from 0.20 – 0.40mm, the booming samples have smaller standard deviations. However, synthetic sands with similar size distributions do not boom, so a narrow size distribution cannot be solely responsible for the booming. Studies of the roundness and sphericity of the grains are currently underway.

Air microphone and geophone recordings of the booming indicate that the fundamental frequency varies between 80–105 Hz depending on the dunes. This is consistent with previous measurements. Laboratory recordings of the “burping” sound that booming sand makes when shaken in a jar reveal a broad peak between 150–300 Hz.

We discuss our recent work on simulational studies of impulse propagation and backscattering for the detection and imaging of shallow buried objects in close packed granular beds.

Shock absorption traditionally exploits visco-elastic devices, fluid viscous dampers and friction dampers, or “soft” materials. Recent work on impulse propagation in granular assemblies suggests that it may be possible to absorb shock waves using a different variety of shock absorbers and thus unlock the possibility of building structures that are significantly more shock absorbent than is currently possible, an issue of relevance to concerns of security. These new shock absorbers are composed of granular materials and exploit the nonlinear energy propagation properties in assemblies of granular materials. They have the potential to partially recover the energy of the absorbed shock waves for useful purposes, a property that might allow one to design devices to convert the incident energy to other useful forms. The basic physics of the “tapered chain shock absorber” is briefly discussed in this work.

Discrete element method (DEM) is one of the excellent procedures for model simulation of behavior of granular assemblies. A two-dimensional DEM simulation of powder compaction by a complex mold was conducted by considering the deformation behavior of individual particles measured by a compression test of free particles. Spherical copper particles were used as a model material in this study. The particles (particle size ranges from 38 to 125microns) were individually compressed between parallel platens to measure compressive load and displacement in the loading direction. From the measured load and displacement data, force-displacement relations at contacts between the particles, and between the particles and mold wall were determined. In the simulation, the two-dimensional complex mold consisting of three stages (upper stage:3-mm wide and 1-mm deep, middle stage:2-mm wide and 1-mm deep, lower stage:1-mm wide and 1-mm deep) was charged with the copper powder consisting of spherical particles with size range of 50 to 200microns and was pressed under four press conditions. Forces at all contacts were calculated by using the determined force-displacement relations and each particle's position was traced by solving equations of motion for each particle. Pressure at each wall of the mold and density distribution in the powder compact were calculated as results of the simulation. It was found that the increase in the pressure at each wall corresponds to the increase in the density of the powder near the wall.

Consolidation of nanostructure magnetic particles is required not only for manufacturing bulk component, it is actually a fundamental requirement for obtaining novel magnetic properties from the material. Consolidation (assembly) of nanoparticles to full density without deteriorating their nanostructure (size and morphology) is a big challenge. Here we present the consolidation experiments of NiFe/SiO2 and Co/SiO2 nanocomposites via detonation consolidation. This approach is based on the explosive pressure created when an acetylene and oxygen mixture gas fires in a sample containing tube, the very high hypersonic propulsion force makes nanoparticles deposit onto the target. Depending on the powder morphology and operation conditions, the density of the consolidated sample can reach over 91% of the theoretical density of the bulk materials. X-ray diffraction experiments on the samples before and after consolidation indicate that the denotation consolidations can be optimized such that it does not cause any phase transition. However, a particle size increase was observed. Static magnetic studies carried out on the samples before and after detonation operation shows that the saturation magnetization does not. This indicates that the operation does not cause an oxidation of the nanopowders. These experiments show that detonation approach is a good candidate for consolidating magnetic nanoparticles.

Sound and pressure wave propagation in a granular material is of interest not only for its intrinsic and practical value, but also because it provides a non-intrusive means of probing the state of a granular material. By examining wave speeds and attenuation, insight can be gained into the nature of the contacts between the particles. In the present paper, wave speeds and attenuation rates are first examined for a static granular bed for a variety of system parameters including particle size, composition and the overburden of the material above the measuring transducers. Agitation of the bed is then introduced by shaking the material vertically. This causes the bed to transition from a static granular state to a vibrofluidized state. The dilation of the bed allows for relative particle motion and this has a significant effect on the measured wave speeds and attenuation. Further, the fluid-like characteristics of the agitated bed distort the force-chain framework through which the waves are thought to travel. The consequences of bed consolidation, a natural result of shaking, are also examined.

The mechanical behavior of explosives subjected to high acceleration has been studied in an ultracentrifuge at –10°C and 25°C. Melt-cast TNAZ and pressed TNAZ, LX-14, Composition A3 Type II, PAX-2A, and PAX-3 have been studied. Failure occurs when the shear or tensile strength of the explosive is exceeded. The fracture acceleration of melt-cast TNAZ is greater than that of pressed TNAZ at –10°C and 25°C. The fracture acceleration of PAX-3 is greater than that of Composition A3 Type II at –10°C and 25°C. The fracture acceleration of melt-cast TNAZ and pressed TNAZ at –10°C is about 10% less than at 25°C. The fracture acceleration of PAX-3 at –10°C is about 2.6 times that at 25°C. The fracture acceleration of Composition A3 Type II at –10°C is about 1.7 times that at 25°C.

This paper reports on three-dimensional, steady discrete element simulations of a single large spherical intruder in a gravity-free granular Couette flow of uniform particles with diameter d. The non-equilibrium nature of the flow is characterized by the depth profile of granular temperature, which decreases inwards toward the center. An intruder of size φ = D/d migrates away from the walls at a rate that increases with φ and wall velocity U. Computations indicated that the intruder's motion is induced by the high pressure near the wall.

During the past several years, we developed a 3-D particle simulation software system to study particle transport problems that occur in research and design of electrophotographic or “laser” printers. Several particle systems have been simulated; they include various singlecomponent electrophotographic image formation systems, transport of charged micron-sized plastic particles through magnetically agitated granular bulk, and magnetic bead chain formation. The software has broader applications including granular hopper flow, and ion generation and tracking.

The software can simulate the motion of tens of thousands of particles through bounded volumes, handling particle-particle and particle-boundary collisions. It tracks individual particle motion in non-uniform ensembles of multiple material types under the influence of various forces: electric and magnetic fields, adhesion, cohesion, air drag, friction, etc. It is easily extended to include other force laws. Excellent agreement has been obtained between simulated and experimental data. When accurate materials characterization experiments are available, the software provides valuable insight into the behavior of particle systems.

The fracture energy of a body containing pores might be expected to decrease linearly in proportion to the area fraction of material in the crack plane. However, there is experimental evidence that the fracture energy of porous materials only decreases when the pore volume fraction exceeds some critical value. To understand this, experiments have been conducted to directly observe the interaction between a growing crack with model distributions of pores. It is seen that cracks do not simply pass through the pores but spread around them causing the crack front to become curved and increase in length. For just two pores (or a line of pores) this is observed to continue until the crack has completely spread around the pores. It is observed that this increase in length increases the energy required for cracking, suggesting that the maximum fracture energy should rise with the volume fraction of pores. However, when this exceeds a certain value, the spreading crack front impinges on the pores ahead of the crack front before the maximum length of crack front due to bowing is reached. Beyond this critical volume of porosity, the resistance to fracture drops rapidly with porosity. Predictions of the relative fracture resistance of bodies containing spherical as well as cylindrical pores give good agreement with experimental observations, and are consistent with observations that the matrix fracture energy and pore size have little effect, provided the pores are much smaller than the sample.

We present experimental measurements on the spontaneous formation of compact spiral structures in vertically-vibrated granular chains. Under weak vibration, when the chain is quasi two-dimensional and self-avoiding, spiral structures emerge from generic initial configurations. We compare geometrical characteristics of the spiral with that of an ideal tight spiral. Globally, the spiral undergoes a slow rotation such that to keep itself wound, while internally, fast vibrational modes are excited along the backbone with transverse oscillations dominating over longitudinal ones. Spirals have an extremely small volume in phase space, and hence, their formation demonstrates how nonequilibrium dynamics can result in a nonuniform sampling of phase space.

PACS numbers: 81.05.Rm, 83.10Nn

We review mechanisms leading to the separation of granular mixtures according to the size or density of the constituent particles. Besides a general overview, the focus of this article is on separation induced by vertical vibrations of the vessel holding the granular material, and specifically on recent results showing a density-dependence to the separation process. A convenient measure of the separation speed is the rise time of a larger “intruder particle” from some fixed initial depth to the free surface, where it will remain. Recent experiments have shown that for large intruder sizes the rise time depends on the density of the intruder particle. Furthermore, in three-dimensional systems we found that this density dependence is highly nonmonotonic. While there is no detailed theoretical understanding available yet, our experiments demonstrate that this surprising behavior is produced by interactions not only between the large intruder particle and the smaller particles of the surrounding granular bed, but also between the particles and the interstitial air.

It is a common point that “soft” condensed matter (like granular materials or foams) can reduce damage caused by impact or explosion. It is attributed to their ability to absorb significant energy. This is certainly the case for quasistatic type of deformation at low velocity of impact widely used for packing of fragile devices. At the same time a mitigation of blast phenomena must take into account shock wave properties of “soft” matter which very often exhibit highly nonlinear, highly heterogeneous and dissipative behavior. This paper considers applications of “soft” condensed matter for blast mitigation using simplified approach, presents analysis of some anomalous effects and suggestions for future research in this exciting area.

Rapid granular flows of two species with different material densities are examined via three-dimensional, hard-sphere simulations of simple shear flow. Simulation results are compared with existing theories for binary systems based on the kinetic theory analogy. The comparison between simulation data and theoretical predictions indicate that although non-equipartition is observed and well-predicted by the theory which accounts for its effects, the influence of non-equipartition on stress predictions is fairly small. The influence of non-Maxwellian effects, however, are critical for accurate stress predictions.

The contact force distribution in stressed grain piles under simple compaction or gravity has been studied intensively in recent years. In the present investigation, by performing discrete element simulations, it is found that the contact force distribution in a stressed granular packing can be described by a generalized form the Second Law of Thermodynamics involving minimization of a free energy functional containing an energy and an entropy component. The relative importance of energy and entropy is controlled by a parameter known as the “mechanical temperature”.

Surfactant-coated spherical iron-based nanoparticles 3.0–9.5 nm in diameter were synthesized and dispersed in hexane. TEM images of dried particle assemblies were taken and analyzed to discover the structure of the assemblies, and to establish the relationship between particle concentration, surfactant type, and conditions of drying and array structure.

Die compaction of granulated powder is a common forming process used in the ceramics industry. Glass spheres were used as a model system to investigate granule failure during die compaction. Since glass spheres are brittle, failure results in fragmentation. Particle size analysis of the resulting fragments demonstrates the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while a large fraction persist at even the highest applied loads. The results are discussed in terms of the Andreasen, Furnas and Dinger-Funk particle packing models for continuous size distributions.

The material-point method is used to examine numerically the macroscopic stress-strain response of a granular sample under compression. The simulations reproduce experimental observations of the large stiffening that occurs as the granular bed becomes packed. We also show the network of force chains that forms and how the character of contacts between grains changes for large deformations. Finally, we examine the probability distribution of forces and observe exponential distributions above the mean with a small peak at the mean for small deformations, and a transition to a larger peak at larger deformations.

Using hard-disk simulations of relatively dense packs of mono-sized system in an annular Couette geometry the formation of dilute regions inside the granular media, namely shear bands, are investigated. The results represent the influence of entire system characteristics such as solid area fraction and shear rate on the development of shear bands as well as the local properties of grains that cause them to participate in the formation of a shear band. Moreover, simulations have been performed for binary-sized system, which revealed that the formation of such diluted shear bands is unlikely.