Geometrical derivation of frictional forces for granular media under slow shearing

We present an alternative way to determine the frictional forces at the contact between two particles. This alternative approach has its motivation in a detailed analysis of the bounds on the time integration step in the discrete element method for simulating collisions and shearing of granular assemblies. We show that, in standard numerical schemes, the upper limit for the time integration step, usually taken from the average time t _c of one contact, is in fact not sufficiently small to guarantee numerical convergence of the system during relaxation. In particular, we study in detail how the kinetic energy decays during the relaxation stage and compute the correct upper limits for the time integration step, which are significantly smaller than the ones commonly used. In addition, we introduce an alternative approach based on simple relations to compute the frictional forces that converges even for time integration steps above the upper limit.     

Dilatant double shearing theory applied to granular chute flow

Summary Although the steady flow of a granular material down a plane inclined slope has been exhaustively examined from both theoretical and experimental points of view, there is still no general agreement concerning the basic flow properties such as density and velocity profiles. The majority of studies assume that the velocity component of the material perpendicular to the inclined plane is sufficiently small to assume that it is everywhere zero. However, recent dynamical modelling of granular chute flow indicates that this component of velocity, although small, is actually non-zero. In this paper, we examine a dilatant double shearing theory for chute flow assuming that the perpendicular component of velocity is non-zero. An explicit analytical form for the perpendicular velocity profile is deduced which gives rise to an integral expression for the chute stream velocity. Assuming a linear decreasing density profile, numerical integration for the chute stream velocity predicts a non-linear profile which is concave in shape and which is in agreement with recent results from computer simulation and existing experimental data in the literature.     

Effect of cyclic shearing on shear localisation in granular bodies

Cyclic shearing of an infinite narrow layer of dry and cohesionless sand between two very rough boundaries under constant vertical pressure is numerically modelled with the finite element method using a polar hypoplastic constitutive relation. The constitutive relation was obtained through an extension of a non-polar model by polar quantities, viz. particle rotations, curvatures, couple stresses using the mean grain diameter as a characteristic length. The proposed model captures the essential mechanical features of granular bodies in a wide range of densities and pressures with a single set of constants. The material constants can be easily determined from granulometric properties and laboratory tests. The attention of numerical simulations is laid on the influence of number of cycles on the thickness of an induced shear zone for both an initially dense and loose granular specimen. In addition, the effect of a stochastic distribution of the initial void ratio on shear localisation is demonstrated.KeywordsGranular material, Cyclic shearing, Polar hypoplasticity, Finite element method, Shear localisation     

Anisotropy in particle contacts associated with shearing in granular media

Summary Work on directional properties in granular media is briefly reviewed with an emphasis on the angular frequency of normals to particle contact tangents. Data from a two dimensional model material is compared with that of sand sheared in plane strain with a similar stress path. This stress path included a chosen sudden change in major principal stress direction.Directional stress-strain behaviour in granular media is related to the changing angular frequency of particle contacts. Measurements of this changing frequency are related to new model predictions and actual experimental data for sand sheared in plane strain.With 5 Figures     

Stimulated acoustic emissions from coupled strings

We consider traveling transverse waves on two identical uniform taut strings that are elastically coupled through springs that gradually decrease their stiffness over a region of finite length. The wave system can be decomposed into two modes: an in-phase mode ( $+$ ) that is transparent to the coupling springs, and an out-of-phase mode ( $-$ ) that engages the coupling springs and can resonate at a particular location depending on the excitation frequency. The system exhibits linear mode conversion whereby an incoming ( $+$ ) wave is reflected back from the resonance location both as a propagating ( $+$ ) wave and an evanescent ( $-$ ) wave, while both types emerge as propagating forward through the resonance location. We match a local transition layer expansion to the WKB expansion to obtain estimates of the reflection and transmission coefficients. The reflected waves may be an analog for stimulated emissions from the ear.     

Identification of the physical mechanisms underlying acoustic emissions during the cracking of concrete

The investigation presented concerns the use of acoustic emission as a tool for the interpretation and explanation of crtain physical mechanisms involved in mechanical damage to concrete. From assumptions made concerning the existence, for a given acoustic emission test and appratus, of ‘critical’ inclusion sizes that lead to irreversible mechanisms that can be detected by acoustic emission, it is shown that it is possible to distinguish cracks at the matrix-inclusion interface and cracks propagating in the matrix during mechanical tests.     

DEM investigation on the evolution of microstructure in granular soils under shearing

The mechanical behaviors of granular soils at different initial densities and confining pressures in the drained and undrained triaxial tests are investigated micromechanically by three-dimensional discrete element method (DEM). The evolutions of the microstructure in the numerical specimen, including coordination number, contact force and anisotropies of contact normal and contact force, are monitored during the shearing. The typical shear behaviors of granular soils (e.g. strain softening, phase transformation, static liquefaction and critical state behavior) are successfully captured in the DEM simulation. It is found that the anisotropies of contact normal, normal and tangential contact forces comprise the shear resistance and show different evolution features during shearing. After large strain shearing, the microstructure of the soil will finally reach a critical state, although the evolution path depends on the soil density and loading mode. Similar to the macroscopic void ratio $e$ and deviatoric stress $q$ , the coordination number and anisotropies of contact normal and contact force at the critical state also depend on the mean normal effective stress $P^{\prime }$ at the critical state.     

Electricity generation: options for reduction in carbon emissions

Historically, the bulk production of electricity has been achieved by burning fossil fuels, with unavoidable gaseous emissions, including large quantities of carbon dioxide: an average-sized modern coal-burning power station is responsible for more than 10 Mt of CO(2) each year. This paper details typical emissions from present-day power stations and discusses the options for their reduction. Acknowledging that the cuts achieved in the past decade in the UK CO(2) emissions have been achieved largely by fuel switching, the remaining possibilities offered by this method are discussed. Switching to less-polluting fossil fuels will achieve some measure of reduction, but the basic problem of CO(2) emissions continues. Of the alternatives to fossil fuels, only nuclear power represents a zero-carbon large-scale energy source. Unfortunately, public concerns over safety and radioactive waste have still to be assuaged. Other approaches include the application of improved combustion technology, the removal of harmful gases from power-station flues and the use of waste heat to improve overall power-station efficiency. These all have a part to play, but many consider our best hope for emissions reduction to be the use of renewable energy. The main renewable energy contenders are assessed in this paper and realistic estimates of the contribution that each could provide are indicated. It appears that, in the time-scale envisaged by planners for reduction in CO(2) emission, in many countries renewable energy will be unlikely to deliver. At the same time, it is worth commenting that, again in many countries, the level of penetration of renewable energy will fall short of the present somewhat optimistic targets. Of renewable options, wind energy could be used in the short to medium term to cover for thermal plant closures, but for wind energy to be successful, the network will have to be modified to cope with wind's intermittent nature. Globally, hydroelectricity is currently the largest developed source of renewable electricity, but future large-scale projects will probably be limited to the less-developed world: the best schemes in the developed countries have already been exploited. Wave and tidal can be looked on as medium- to long-term generators of electricity, as their respective industries are not as mature as competing renewable resources. Municipal solid-waste combustion and landfill gas technologies can also be seen as short term, as can their rural equivalents, agriculture and forestry waste. Any widespread exploitation of renewable energy will depend on being able to transmit the energy from source to point of use, so the implications for the electrical network from the penetration of substantial levels of renewable energy are presented. Effective management of renewable energy installations will require technical assessment of the range of exploitation strategies, to compare local production of, say, hydrogen and the more traditional transmission of electricity. Such resources will have to compete with others in any national, or grid, system and detailed economic analysis will be necessary to determine the deployment that best fits the trading regime under which the energy will be sold. Consideration will also be necessary to determine how best to control the introduction of this radically new resource such that it does not attract punitive cost overheads until it is mature enough to cope. Finally, it is inescapable that nuclear power is a proven technology that could take its place in any future generation portfolio. Unfortunately, suspicion and mistrust surround waste management and radioactivity release. Unless this is overcome, the lack of confidence engendered by this public mistrust may result in few, if any, new nuclear power stations being built. In the event of that decision, it is difficult to see how CO(2) levels can be significantly reduced: the irony is that nuclear energy may emerge as environmentally essential.     

Passive acoustic emissions from particulates in a V-blender

Context: Regulatory agencies are recommending the development of process analytical technologies (PAT) to improve the efficiency and product quality during pharmaceutical manufacturing.

Objective: The objective of the research was to investigate the potential application of passive acoustic emission monitoring of a V-blender.

Materials and methods: Trials were conducted with sugar spheres, lactose or MCC in a V-blender. Vibrations from acoustic emissions were measured using PCB Piezotronics accelerometers with ICP signal conditioners.

Results and discussion: A wavelet filter was applied to the measured acoustic emissions to remove vibrations from the tumbling motion of the V-shell, allowing a focus on information about particle motion and interactions within the V-shell. The ideal sensor location was determined to be the lid of one of the V-shell arms due to the impact of the tumbling particles on the lid and transmission of the vibrations from other particle motion within the V-shell. The amplitude of vibrations increased with particle size due to larger particle momentum before a collision. The fill level and the V-shell scale also influenced the measured vibrations as particle motion was affected which in turn affected momentum. Changes in particle flowability could be detected through variations in the measured acoustic emissions.

Conclusion: The measured vibrations from passive acoustic emissions reflected particle motion and interactions within a V-blender demonstrating potential as a monitoring method.     

Detector for liquid chromatography based on acoustic emissions from an oscillating flame

The acoustic flame detector (AFD) is examined as a novel detector for liquid chromatography (LC). It is based upon the acoustic emission frequency of an oscillating hydrogen/oxygen premixed flame and produces a universal response toward organic molecules. A stable frequency near 1000 Hz, which further depends on mobile-phase composition, is achieved for flow rates in the microliter per minute range. The mass flow sensitivity of the AFD demonstrates a linear response over 3 orders of magnitude and a detection limit (S/sigma = 3) of approximately 15 ng of C/s for a series of alcohols. For cyclopentanol, this amounts to an injected mass of approximately 77 ng based on a 0.5-microL injection of a 196 ppm solution in methanol (flow rate 20 microL/min methanol; peak width 30 s). Similar sensitivity is observed using a water mobile phase. Low-frequency (1/f ) noise contributions are dominant with or without mobile phase present. The AFD demonstrates a uniform molar sensitivity toward carbon compounds independent of their optical properties or volatility. Results suggest the device might serve as a simple, inexpensive universal LC detector.     

A cellular automaton for segregation during granular avalanches

Segregation is a complex and poorly understood phenomenon that is prevalent in many industrial and natural granular flows. When grains flow down a slope [1–5], are spun in a rotating drum [6–8] or shaken in a box [9], we observe those grains organising into intriguing patterns. Kinetic sieving is the dominant mode of segregation in granular avalanches, where separation of particles occurs according to size. Using a cellular automaton we have modelled kinetic sieving as the swapping of particles in a one-dimensional system. From the cellular automaton we have deduced a continuum model to describe the segregation.     

Ultrafast generation of acoustic waves in copper

The ultrafast generation of acoustic waves in copper films is investigated with a femtosecond optical pump and probe technique. By studying the generation at times before the electrons and the lattice come into equilibrium, the strength of their interaction can be measured and the dynamics of ultrafast electron diffusion can be studied. The acoustic strain pulses observed are bipolar in shape with exponential tails that are much broader than expected from simple thermoelastic stress generation. This can be explained by the supersonic diffusion of electrons over distances larger than the optical skin depth. The nonequilibrium diffusion equations governing stress generation are nonlinear, and are solved numerically. Using a linearized formulation, we also solve them analytically to a good approximation. The acoustic strain profile provides a `snapshot' of the initial spatial temperature distribution of the lattice, thus allowing a sensitive probe of the nonequilibrium dynamics of the diffusion. The electron-phonon coupling constant can be estimated directly from the acoustic pulse duration, provided that the sound velocity and thermal conductivity are known. In general, the relaxation and diffusion of carriers is specific to the sample in question, whether metal or semiconductor, suggesting the use of this method for thin film characterization     

基于模态理论的有源声学结构控制机理研究

有源声学结构是近年来提出的一种控制结构低频声辐射的有效方案，它是智能结构在噪声控制领域的具体应用，其控制机理是一个亟待解决的关键问题，文中结合声辐射模态和结构振动模态理论对该问题进行了研究并作了定量解释。研究结果表明：有源控制的机理在于减小结构主导辐射模态的幅度（声功率），同时保证低阶非主导辐射模态的幅度不会有大的升高；在控制高阶辐射模态幅度的同时，低阶辐射模态幅度也被有效控制；四个次级板能够抵消各种类型的振动模态对应的主导辐射模态，从而有效控制结构所有振动模态的声辐射。     

金属颗粒膜巨磁电阻效应的机制和影响因素

由于巨磁电阻效应在磁记录设备和传感器的潜在应用，具有该效应的材料得到了广泛地研究。特别是继金属多层膜之后的金属磁性颗粒膜，由于其制备简单，巨磁电阻效应的机理较为复杂，受到人们的青睐。本文综述了金属磁性颗粒膜中巨磁电阻效应的自旋相关散射机制和影响因素。在此基础上，指出了当前值得注意的研究方向。     

Effects of loading and sample geometry on acoustic emission generation during fatigue crack growth: Implications for structural health monitoring

The reliability of traditional non-destructive methods for crack detection is well understood and characterised using Probability of Detection (POD) curves. Structural Health Monitoring (SHM) techniques in contrast remain largely unquantified. The performance of the Acoustic Emission (AE) technique for damage detection and location in potential SHM applications is underpinned by the intensity of AE signal generation from the damage site. In this paper, factors influencing the rates of emission of Acoustic Emission (AE) signals from propagating fatigue cracks were investigated. Fatigue cracks were grown in specimens made from 2014 T6 aluminium sheet while observing the effects of changes in crack length, loading spectrum and sample geometry on rates of acoustic emission. Significant variation was found in the rates of AE signal generation during crack progression from initiation to final failure with a number of distinct phases identified in that progression implying different failure mechanisms operating at particular stages in the failure process. A new ‘probability of hit’ method for quantifying crack detecting capability using AE is also presented.     

Investigation of granular surface roughness effect on electrostatic charge generation

Electrostatic charge generation is a multivariable and complex issue whose working mechanism has never been fully understood. The objective of this paper is to investigate the effect of granule surface roughness on electrostatic charge generation. Two kinds of granule material, Polyvinyl chloride (PVC) and polypropylene (PP) were used with the granule size of 4 mm diameter, 2 mm height and the shape was cylinder or semi-cylinder. The working surfaces were grounded and roughness ranged from 0.140 to 8.600 μm. It was found that uneven surfaces tended to give rise to voids between two solids, where air stored in the voids was able to accelerate discharging. With the same roughness, PVC tended to generate more electrostatic charge than PP by one order of magnitude. For both materials, electrostatic charge generation first increased with surface roughness and then decreased. The maximum electrostatic charge generated was found to occur when the effects of interaction, contact area and voids discharging were at equilibrium. With the combined effect of humidity, surface roughness and contact area, highest electrostatics generation occurred near the mid-roughness tested in this work. Humidity had more effect on electrostatic charge generation as the granule working surface had lower roughness.     

A method for the generation of 3D representative models of granular based materials

The morphology of many naturally occurring and man‐made materials at different length scales can be modelled using the packing of correspondingly shaped and sized particles. The mechanical behaviour of this vast category of materials – which includes granular media, particle reinforced materials and foams ‐ depends strongly upon the shape and size distribution of the particles. This paper presents a method for the generation and packing of arbitrarily shaped polyhedral particles. The algorithm for the generation of the particles is based on the Voronoi tessellation technique, whilst the packing is performed using a geometrical approach, which guarantees the non‐overlapping of the bodies without relying upon any, otherwise typically computationally expensive, contact detection and interaction algorithm. The introduction of three geometrical parameters allows to control the shape, size and spacial density of the polyhedral particles, which are used to build numerical models representative of densely packed granular assemblies, granular reinforced materials and closed‐cell foams. Copyright © 2017 John Wiley & Sons, Ltd.