Academic literature on the topic 'Granular materials; Bulk systems; Shape change'

Size limitations of geotechnical testing equipment often require that samples of coarse granular materials have to be scaled in order to be tested in the laboratory. Scaling implies a convenient modification of the particle size distribution (PSD) to reduce particle sizes. However, it is well known that particle size and shape may be correlated in nature, due to geological factors (as an example). By means of two-dimensional contact dynamics simulations, we analyzed the effect of altering the size span on the shear strength of granular materials when particle size and shape are correlated. Two different systems were considered: one made of only circular particles, and the second made of size-shape correlated particles. By varying systematically the size span we observed that the resulting alteration of material strength is not due to the change in particle sizes. It results instead from the variation of the particle shapes induced by the modification of the PSD, when particle size and particle shape are correlated. This finding suggests that particle shape distribution is a higher order factor than PSD for the shear strength of granular materials. It also highlights the importance of particle shape quantification in soil classification and the case for its consideration in activities such as sampling, subsampling, and scaling of coarse materials for geotechnical testing

Fe–Pd films with Pd content varying between 26 and 30 at.% have been deposited by means of magnetron sputtering of elemental Fe and Pd targets. As-deposited films are highly supersaturated solid solutions of Pd in Fe that have a body-centered-cubic crystal structure and a very fine grain size. Substrate curvature measurements indicate that the films undergo an irreversible densification when heated above 100 °C. This densification is attributed to a structural change that is also observed in other supersaturated systems with a substantial atomic size difference between the constituents. It is possible to retain the high-temperature austenite phase at low temperature by annealing the films at 900 °C followed by rapid cooling. Depending on film composition, this metastable austenitic phase transforms to either a body-centered tetragonal (bct) or a face-centered tetragonal (fct) martensite around room temperature. Substrate curvature measurements show that formation of the fct martensite is reversible, while that of bct martensite is not. The fct transformation occurs at lower Pd content and higher temperature than reported for bulk materials. Both the fct and the fcc phase show a strong Invar effect at lower temperature and Pd content than observed in the bulk.

The physical and mechanical properties of soft coal body constitute one of the most important factors inducing coal wall spalling. In order to explore the mechanical essence of coal instability disaster and stability enhancement of water injection, the 7# coal in Huainan mining area is taken as the research object. Firstly, the distribution characteristics of coal particle size, point-load strength, original water content, microstructure characteristics, and shear strength of coal under different water contents are measured by laboratory tests. Then, based on the test results, the cementation morphology and force evolution law of granular coal water in coal body are analyzed using liquid bridge theory. The results show the following: (1) With the increase of particle size, the mass ratio of granular coal increases gradually. The percentage of particle coal with particle size less than 2.5 mm accounts for 47.157%, fractal dimension is 2.172, and uniaxial compressive strength and tensile strength are 3.822 MPa and 0.165 MPa, respectively. (2) The coal body is dry (the original moisture content is 1.336%), containing a large number of loose particles, pores, fissures, and other microfabrics. This “low water content and multiporosity” feature is the essential reason for its low strength, fragmentation, and instability and disaster. (3) In the process of water content increasing from 0.966% to 26.580%, the shear stress-displacement curve of coal body gradually changes from softening type to hardening type, and the failure type transitions from brittleness to ductility. The cohesive force increases first and then decreases, while the angle of internal friction almost has no change. (4) After reasonable water injection, the shape of liquid bridge in coal body changes into capillary tube, and the liquid bridge force reaches the maximum value, which transforms from a highly unstable bulk to a stable continuum. The research results have important theoretical significance and practical value for the safe and efficient mining of soft coal seams.

High-Mn steels attract attention because of their various technological properties. These are mainly mechanical and functional, such as the shape-memory effect, high damping capacity, high strength with simultaneous large ductility, the TRIP/TWIP (transformation- and twinning-induced plasticity) effect, low cycle fatigue and high work hardening capacity. All these phenomena are associated with the face-centered cubic (f.c.c.)–hexagonal close-packed (h.c.p.) martensitic transformation which takes place in these alloys. During this phase transition defects are introduced, mainly due to the large volume change between austenite and martensite. Knowing this volume change is key to understanding the mechanical behavior of these metallic systems. In the present article, a full-pattern refinement method is presented. The proposed method uses data obtained by means of conventional X-ray diffraction from regular bulk samples and allows a high-precision calculation of the lattice parameters of both phases, f.c.c. and h.c.p., under conditions very different from randomly oriented (powder) materials. In this work, the method is used to study the effect of chemical composition on the volume change between the two structures. By applying empirical models, the results enabled the design and fabrication of Fe–Mn-based alloys with a small volume change, showing the potential of this new tool in the search for improved materials.

An accurate modeling of the piezoelectric effect of coupled structures is essential to application of piezoelectric materials as sensors and actuators in engineering structures, such as Micro-Electro-Mechanical Systems and Interdigital Transducer for health monitoring of structures. This paper presents a simulation for the shear horizontal wave propagation in an infinite metal plate surface bonded by a piezoelectric layer with open electrical circuit, with focus on the dispersion characteristics of a metal core bonded by a layer of piezoelectric material to be used in health monitoring of structures. The dispersive characteristics and mode shapes of the deflection, electric potential, and electric displacement of the piezoelectric layer are theoretically derived. The results from numerical simulations show that the phase velocity of the piezoelectric coupled plate approaches the bulk-shear wave velocity of the substrate at high wavenumbers. The mode shapes of electric potential and deflection of the piezoelectric layer with steel substrates change from a shape with few zero nodes to one with more zero nodes at higher wavenumbers and with thicker piezoelectric layer. For the coupled plate with gold substrates at higher wavenumbers, the electric potential is found to jump from null at the interface of the piezoelectric layer and the substrate to a constant at the surface of the piezoelectric layer along the thickness direction. These findings are useful to the design of sensors using the piezoelectric coupled structures.

Abstract The flow of granular materials and metallic glasses is governed by strongly correlated, avalanche-like deformation. Recent comparisons focused on the scaling regimes of the small avalanches, where strong similarities were found in the two systems. Here, we investigate the regime of large avalanches by computing the temporal profile or “shape” of each one, i.e., the time derivative of the stress-time series during each avalanche. We then compare the experimental statistics and dynamics of these shapes in granular media and bulk metallic glasses. We complement the experiments with a mean-field model that predicts a critical size beyond which avalanches turn into large runaway events. We find that this transition is reflected in a characteristic change of the peak width of the avalanche profile from broad to narrow, and we introduce a new metric for characterizing this dynamic change. The comparison of the two systems points to the same deformation mechanism in both metallic glasses and granular materials.

ABSTRACTThis paper is concerned with two related problems: (1) the construction of geometrical models of porous media relevant to granular composites and (2) the description of transport processes in these model systems. We will show that a variety of interesting porous media can be generated by the packing and subsequent modification of spherical grains. This modification may involve a change in either the grain's size, shape, or both. Steady state transport processes such as the flow of electrical current or viscous fluids are controlled by the distribution of pore throat sizes and, within the presentframework, can be studiedefficiently by random walk simulations of diffusion. The techniques developed here are also of interest in connection with dynamic transport processes such as the filtration of fine grained particles into consolidated granular networks. Wediscuss briefly the modeling of such processes as well as the interaction of steady state and dynamic transport.

AbstractBone-like biological materials have achieved superior mechanical properties through hierarchical composite structures of mineral and protein. Geckos and many insects have evolved hierarchical surface structures to achieve superior adhesion capabilities. What is the underlying principle of achieving superior mechanical properties of materials? Using joint atomistic-continuum modeling, we show that the nanometer scale plays a key role in allowing these biological systems to achieve such properties, and suggest that the principle of flaw tolerance and design for robustness may have had an overarching influence on the evolution of the bulk nanostructure of bone-like materials and the surface nanostructure of gecko-like animal species. We illustrate that if the characteristic dimension of materials is below a critical length scale on the order of several nanometers, Griffith theory of fracture no longer holds. An important consequence of this finding is that materials with such nano-substructures become flaw-tolerant, as the stress concentration at crack tips disappears and failure always occurs at the theoretical strength of materials, regardless of defects. The atomistic simulations complement continuum analysis and reveal a smooth transition between Griffith modes of failure via crack propagation to uniform bond rupture at theoretical strength below a nanometer critical length. This modeling resolves a long-standing paradox of fracture theories, and these results have important consequences for understanding failure of small-scale materials. Additional investigations focus on shape optimization of adhesion systems. We illustrate that optimal adhesion can be achieved when the surface of contact elements assumes an optimal shape. The results suggest that optimal adhesion can be achieved either by length scale reduction, or by optimization of the contact shape. Whereas change in shape does not lead to robustness, reducing the dimension results in robust adhesion devices.

: Nanomedicine has emerged as a lucrative option, which attracts the attention of scientific and industrial stalwarts for its enormous potential and new business opportunities in the health sector. Since it encompasses physical, chemical, and biological interventions used for the transformation of bulk materials into nanomaterials (NMs) with particles size ~ 1-100 nm for highly specific medical applications. Therefore its effect on diagnosis, efficacy, treatment, and prevention of diseases may easily be foreseen. The credit goes to nanotechnology, which has emerged with the ability to manipulate the NMs concerning size, shape, composition as well as surface characteristics. Due to the advantages of their tiny size as well as novel properties, NMs are useful for loading more drugs with controlled release and specific targeting. Although full bloom of nanomedicine realization might take years, a recent innovation in formulations of nanotechnology-based smart drug/vaccine delivery systems is beginning to change the landscape of future medicines. They are being designed to overcome biological barriers in the living system by improving the delivery and efficacy of traditional therapeutics and reducing the toxicity by specificity to target cells/tissues. This review focuses on basic understanding and progress in the field of nanomedicine (especially nanocarriers-based drug and vaccine delivery), including nanoformulation of Amphotericin B with functionalized carbon nanotubes for the therapy of visceral leishmaniasis.

The Discrete Element Method (DEM) has been applied to numerical modelling of the bulk compression of low modulus particulates. An existing DE code for modelling the contact mechanics of high modulus particles using a linear elastic contact law was modified to incorporate non-linear viscoelastic contact, real containing walls and particle deformation. The new model was validated against experimental data from the literature and physical experiments using synthetic spherical particles, apple and rapeseed. It was then used to predict particle deformation, optimum padding thickness in a handling line and bulk compression parameters during oilseed expression. The application of DEM has previously been limited to systems of hard particles of high compressive and shear modulii with relatively low failure strain. Material interactions have therefore commonly been modelled using linear contact law. For high modulus particles, particle shape change resulting from deformation is a not a significant factor. Most agricultural particulates however deform substantially before failure and their interaction is better represented with non-linear hysteretic viscoelastic contact relationship. Deformation of geometrically shaped particles in DEM is usually treated as "virtual" deformation, which means that particles are allowed to overlap rather than deform due to contact force. Change to particle shape has not previously been possible other than in the case of particles modelled as 2-D polygons or where each particle is also modelled concurrently with an FE mesh. In this work a new approach has been developed which incorporates a non-linear deformation dependent contact damping relationship and a shape change while maintaining sufficient geometrical symmetry to allow the problem to be handled by the same DE algorithms as used for true spheres. The method was validated with available experimental results on impact behaviour of rubber and the variations with different damping coefficients were simulated for a selected fruit. A fruit handling process dependent on the impact process was then simulated to obtain data required in the design of a fruit processing line. Changes in shape of spherical synthetic rubber particles and rapeseeds under compression were predicted and validated with physical experiments. Images were taken and analysed using image processing techniques with 1: 1 scaling. The method on shape change entails a number of simplifying assumptions such as uniform stress distribution and homogeneous material properties and uniform material distribution when deformed, which are not observed in real agricultural materials and will tend to overestimate the true contact area between particles. In reality for fruits and vegetables, material redistribution is a complex process involving a combination of compaction and movement. However with the new method a better approximation of bed voidage (which standard DEM approaches underestimate) and stress were obtained in the compression of a synthetic material. This is a significant improvement on existing methods particularly with respect to stress distribution within a bulk particulate system comprising deforming elements where the size and orientation of contact surface between particles has a strong influence on the bulk modulus. The new model was used for prediction of mechanical oil expression in four oilseed beds. Similar patterns in the variation of the characteristic parameters were obtained as observed in existing experimental data. The data could not be matched exactly as the quantity and arrangement of seeds in the initial seedbeds were not the same as those used in the experimental work. However the DE model gave approximate oil point data for seedbeds with the same physical properties and initial conditions as in the experiment. This suggests that the new model may be a useful tool in the study of mechanical seed-oil expression and other agricultural particulate compression processes.

Useful prediction of the kinematics, dynamics, and chemistry of a system relies on precision and accuracy in the quantification of component properties, operating mechanisms, and collected data. In an attempt to emphasize, rather than gloss over, the benefit of proper characterization to fundamental investigations of multiphase systems incorporating solid particles, a set of procedures were developed and implemented for the purpose of providing a revised methodology having the desirable attributes of reduced uncertainty, expanded relevance and detail, and higher throughput. Better, faster, cheaper characterization of multiphase systems result. Methodologies are presented to characterize particle size, shape, size distribution, density (particle, skeletal and bulk), minimum fluidization velocity, void fraction, particle porosity, and assignment within the Geldart Classification. A novel form of the Ergun equation was used to determine the bulk void fractions and particle density. Accuracy of properties-characterization methodology was validated on materials of known properties prior to testing materials of unknown properties. Several of the standard present-day techniques were scrutinized and improved upon where appropriate. Validity, accuracy, and repeatability were assessed for the procedures presented and deemed higher than present-day techniques. A database of over seventy materials has been developed to assist in model validation efforts and future designs.

The jamming of granular materials, which indicates how disordered particle systems change from mechanically unstable to stable states, has attracted significant recent interest due, but not limited, to the appearance of jamming transition or similar behavior in a broad variety of systems. Recent experiments on jamming transition have revealed the relationship between mean coordination number and packing fraction for different jammed states. In this paper the jamming states of two dimensional granular materials under cyclic compression using Smooth Particle Hydrodynamics (SPH) approach is numerically investigated. The SPH method allows one to study the stress developed within individual granular particles of arbitrary shape. In this study the granular system is cyclically and isotropically compressed or expanded. The system undergoes a range of jamming states over a large number of cycles. We measure the evolution of global pressure, mean coordination number, and packing fraction. The force chains and probability density function of force for different compression cycles are also investigated.

In future applications, materials will need to extend beyond that of the bulk response or even simple engineered behavior. This paper attempts to articulate an integrated vision and even push it further to the next realm of materials defined here as Nexus materials, the synergistic connection that weaves it all together. The applications in mind will demand complex functionality such as higher order actuation across surfaces/volumes, distributed conformal sensing, and full-spectrum instantaneous color change. Looking beyond the horizon, the level of complexity needs to be raised to radical state change (not just by a few percent) where the material properties can be grossly changed on command by electrical, magnetic, thermal, optical or chemical control stimuli. While it is desirable to have multiple states, the materials should be situationally functional with the ability to change key properties over time to economically accommodate changing situations- in contrast to current multifunctional materials that are time invariant. The research in nanomaterials holds the promise that one day it will be possible to design and build up materials from the bottom up into ultrahierachial systems like nature. It may be possible to achieve truly intrinsic intelligence with control and supporting elements such as power onsite via energy harvesting. However, the level of integration has to evolve beyond the discrete laminated structures of the 1990's and the discretely integrated composites of the 2000's to continuously integrated materials where the phases are indistinguishable. This doesn't mean no interfaces, but does implies minimizing the parasitic interfaces (typically extrinsic) and maximizing the helpful symbiotic interfaces (typically intrinsic). To achieve this vision, a new wholistic approach will be required that synergistically uses the design space, coupling multiple fields (piezoelectric, magnetostriction, shape memory, etc.). This alternative engineering paradigm will hopefully lead to new classes of material systems - Nexus materials - that make the ultimate, synergistic connection between the past and the future and are just beyond the horizon.

Enclosed helical screw systems are used as screw feeders and conveyers for handling dry bulk solids. The design of such systems is based on the feed load, the properties of the bulk solid and the shape and parameters of the screw features. Similar designs are used as extruders in manufacturing deformable materials with help of pressure and a combination of pressure and temperature. Furthermore, identical arrangements also are used in mixing and transporting viscous fluid. The screw pitch, shape and size are some of the factors that determine the flow dynamics between the center screw (core shaft) and the outer cylinder (barrel), and also are found to determine dry bulk solids transport. The transport processes of helical screw systems for bulk solids, heat exchangers, passive mixers and high viscosity fluids are widely published. However, there are limited studies available that have investigated horizontally placed partially filled screw reactors. In the food processing industry, partially filled horizontal helical screw reactors (PFHSR) are used to transport and mix fluids and slurries and also to chill food products. Thus, understanding the flow dynamics of PFHSRs will lead to the design of effective PFHSR systems as well as open new areas of application. Our main objective is to understand the flow dynamics of a PFHSR system. The test system is a closed end horizontal cylinder with a screw agitator in the middle. The tests are designed to replicate the current industrial process and to investigate the flow dynamics change due to flow path and different screw rotational speeds. Water-based rheoscopic fluid is used to visualize the flow profile, and several replications of the same test were conducted. The PFHSR system length, outside cylinder diameter, screw (auger) pitch and clearance were fixed for all tests. The rotational speed and the flow return path were varied. The flow return path varied by opening and closing the auger’s core. The flow results indicate the annular flow between the auger core and outside cylinder is affected by the rotational speed and the flow return rate related to flow path. The open core region simulates the flow condition where the flow is pumped into and out of the PFHSR system. On the other hand, the closed core region simulates the effect of pressure (slip) flow as well as the implication of slip flow in mixing. The flow process has been studied by observing the flow pattern from different viewpoints. The experimental results are presented by relating the flow field with a Reynolds number (Re) that is defined using the rheoscopic fluid viscosity, the auger rotational speed and diameter. The bulk fluid flow is found to be the result of the moving surfaces and other boundary conditions in addition to the slip-flow through the flight and barrel clearance. Vortices appear at the trailing side of the screw flight and also show a periodic pattern. The flow fields observed from both open and closed core show the flow profile, as well as the flow type significantly affected by the flow path.