Mechanical properties of ferrochromium slag in granular layers of flexible pavements

Ferrochromium slag is a waste material obtained from the manufacture of ferrochromium (FeCr). This article reports the results of experiments, aiming to identify the properties of ferrochromium slag when it is used as an artificial aggregate for preparation of granular layers of flexible pavements. The experimental program consisted of two stages: (1) study of the physical and chemical properties of slag; (2) study of the mechanical properties of specimens made with ferrochromium slag and limestone as aggregate. Laboratory prepared cylindrical specimens were tested in repeated load triaxial (RLT) test apparatus, developed at Süleyman Demirel University, Turkey. In addition, particle size analysis, abrasion test, frost resistance, compaction test, California Bearing Ratio (CBR) test and leaching test were performed on the materials prior to RLT testing. The results indicate that the physical and mechanical properties of air-cooled ferrochromium slag are as good or better than those of natural aggregates. Therefore, FeCr slag and SiFeCr slag have potential to be used as a pavement base layer material in applications where crushed limestone aggregate materials are traditionally used.     

Mechanical properties of diffused chromized layers

The mechanical properties of thermo-diffused layers obtained by vacuum chromizing of steel were studied. It was established that to produce strong and ductile chromized layers by this method it is necessary to decarburize the substrate material before or after the treatment.     

Mechanical properties of electron-beam-melted AlSi12 surface layers

The melting parameters for an AlSi12 alloy, which lead to surface-melted layers with constant melting depth and smooth surface have been discovered using the electron beam melting technique. The main mechanical properties of the melted layer were determined with different testing methods, which were adapted for this specific use. Examination under quasistatic stresses showed an increase of the hardness of 60% compared with the base material. The tensile strength increased in the melted layer by about 50%, whereas the elongation was not reduced drastically. Examination under fatigue stress resulted in a reduced fatigue strength of the surface-melted samples. The reason for this was crack initiation at pores at the boundary of melted layer and base material. For a poreless melted AlSi12 alloy, however, the same fatigue strength as for the base material was found.     

Impact response of granular layers

Motivated by the wave tailoring potential of granular media, this study aims at evaluating force transmission through granular layers made of spherical particles. 2D simulations based on Hertzian contact law between adjacent particles are performed on two distinct systems: (1) layers consisting of ordered bimaterial lattices, and (2) single material layers with random packing. For the ordered systems, force transmission properties are found to vary with material mismatch and layer thickness. Transmitted force-decay in random configurations is substantially higher than those in the ordered systems.

     

Mechanical properties of freely suspended atomically thin dielectric layers of mica

We have studied the elastic deformation of freely suspended atomically thin sheets of muscovite mica, a widely used electrical insulator in its bulk form. Using an atomic force microscope, we carried out bending test experiments to determine the Young’s modulus and the initial pre-tension of mica nanosheets with thicknesses ranging from 14 layers down to just one bilayer. We found that their Young’s modulus is high (190 GPa), in agreement with the bulk value, which indicates that the exfoliation procedure employed to fabricate these nanolayers does not introduce a noticeable amount of defects. Additionally, ultrathin mica shows low pre-strain and can withstand reversible deformations up to tens of nanometers without breaking. The low pre-tension and high Young’s modulus and breaking force found in these ultrathin mica layers demonstrates their prospective use as a complement for graphene in applications requiring flexible insulating materials or as reinforcement in nanocomposites.      

Jamming in granular shear flows of frictional,polydisperse cylindrical particles

In this work, frictional, cylindrical particle shear flows with different size distributions (monodisperse, binary, Gaussian, uniform) are simulated using the Discrete Element Method (DEM). The influences of particle size distribution and interparticle friction coefficient on the solid phase stresses, bulk friction coefficient, and jamming transition are investigated. In frictional dense flows, shear stresses rise rapidly with the increasing solid volume fraction when jamming occurs. The results suggest that at the jamming volume fraction, stress fluctuation and granular temperature achieve the maximum values, and the rate of the stress increase with increasing solid volume fraction approaches the peak value. Meanwhile, the degree of cylindrical particle alignment approaches a valley value. In the polydisperse flows, the jamming volume fraction exhibits significant dependences on the fraction of the longer particles and the particle size distribution. Two models considering the effect of particle size distribution are discussed for predicting the jamming volume fractions of polydisperse flows with frictional, cylindrical particles.     

Investigating mechanism of inclined CPT in granular ground using DEM

This paper presents an investigation on mechanism of the inclined cone penetration test using the numerical discrete element method (DEM). A series of penetration tests with the penetrometer inclined at different angles (i.e., \(0^{\circ },\,15^{\circ },\,30^{\circ },\,45^{\circ }\) and \(60^{\circ }\) ) were numerically performed under \(\mu =0.0\) and \(\mu =0.5\) , where \(\mu \) is the frictional coefficient between the penetrometer and the soil. The deformation patterns, displacements of soil particles adjacent to the cone tip, velocity fields, rotations of the principal stresses and the averaged pure rotation rate were analyzed. Special focus was placed on the effect of friction. The DEM results showed that soils around the cone tip experienced complex displacement paths at different positions as the inclined penetration proceeded, and the friction only had significant effects on the soils adjacent to the penetrometer side and tip. Soils exhibited characteristic velocity fields corresponding to three different failure mechanisms and the right side was easier to be disturbed by friction. Friction started to play its role when the tip approached the observation points, while it had little influence on rotation rate. The normalized tip resistance \((q_{c}=f/\sigma _{v0})\) increased with friction as well as inclination angle. The relationship between \(q_{c}\) and relative depth \((y/R)\) can be described as \(q_{c}=a\times (y/R)^{-b}\) , with parameters \(a\) and \(b\) dependent on penetration direction. The normalized resistance perpendicular to the penetrometer axis \(q_{p}\) increases with the inclination angle, thus the inclination angle should be carefully selected to ensure the penetrometer not to deviate from its original direction or even be broken in real tests.     

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.     

Fully developed flow of granular materials down a heated inclined plane

Summary The mechanics of flowing granular materials such as coal, sand, metal ores, etc., and their flow characteristics have received considerable attention in recent years as it has relevance to several important technological problems. In a number of instances, these materials are also heated prior to processing, or cooled after processing. The governing equations for the flow of granular materials, taking into account the heat transfer mechanism by conduction, are derived using a continuum model (cf. Goodman and Cowin [1], [2], Rajagopal and Massoudi [3]). For a fully developed flow of these materials down an inclined plane, the equations reduce to a system of coupled non-linear ordinary differential equations. The resulting boundary value problem is solved numerically and the results are presented for cases where the viscosity and thermal conductivity are assumed to be functions of the volume fraction. It is shown that the equations admit multiple solutions for certain values of the parameters.List of symbols D Symmetric part of the velocity gradient - K thermal conductivity - L velocity gradient - T Cauchy stress tensor - b body force - h characteristic height - q heat flux - r radiating heat - u velocity vector - angle of inclination of the inclined plane with the horizontal - specific internal energy - distributed mass density - temperature - volume fraction - bulk mass density     

Sound propagation in isotropically and uni-axially compressed cohesive, frictional granular solids

Using an advanced contact model in DEM simulations, involving elasto-plasticity, adhesion, and friction, pressure-sintered tablets are formed from primary particles and prepared for unconfined tests. Sound propagation in such packings is studied under various friction and adhesion conditions. Small differences can be explained by differences in the structure that are due to the sensitivity of the packing on the contact properties during preparation history. In some cases the signals show unexpected propagation behaviour, but the power-spectra are similar for all values of adhesion and friction tested. Furthermore, one of these tablets is compressed uni-axially and under unconfined conditions and the sound propagation characteristics are examined at different strains: (i) in the elastic regime, (ii) during failure, and (iii) during critical flow: the results vary astonishingly little for packings at different externally applied strains.     

The onset of instabilities in discretely shaken granular layers

A simple computational model is introduced to study a system consisting of a discretely shaken granular layer. Despite the simplicity the model shows the formation of instabilities in the layer and the later development of granular heaplets. We show analytically that the onset of instabilities is dependent on the density or thickness of the layer and this result is justified by the subsequent computer simulations. Our simulations also show that the development of heaplets can be divided into three stages: an early stationary stage, an intermediate growing stage and a late-time saturated stage. In the early stage, the average volume of the heaplets remains almost unchanged until the system crosses over to the intermediate growing stage. The average length of time that the system remains in the early stage defines the average onset time of the instabilities, k ₀ and this depends on the shaking intensity, . The onset time k ₀ seems to diverge for values of >_c where ^c14. In the growing stage, the average volume of the heaplets grows with time and can be approximated by a power law with a growth exponent, z which depends on and from our simulation z depends linearly on . The late-time saturated stage is where most of the particles are trapped in a big heap and this big heap is in equilibrium with the surrounding granular gas.     

Analytical stress and velocity fields for gravity flows of highly frictional granular materials

For granular materials the Coulomb-Mohr yield condition characterizes the two physical processes of inter-particle cohesion and inter-particle friction. The latter effect is quantified by the so-called angle of internal friction, denoted here by . The special case arising from zero angle of internal friction corresponds to the standard Tresca yield condition of metal plasticity. For certain materials such as coal, alumina filter cake, waste rock and silica, angles of internal friction occur in the vicinity 70°–80°, and therefore the study of an idealized granular theory with an angle of internal friction equal to ninety degrees has real practical significance. Here for the special case of =90°, the governing second-order nonlinear partial differential equations for the non-dilatant double-shearing model of granular flow are presented for both plane and axially symmetric flows, and a number of simple analytical solutions of these novel equations are determined. Some of these solutions are illustrated graphically by showing the orthogonal grids which give the maximum and minimum principal stress directions and by showing the streamlines which give the particle paths.     

Extended kinetic theory applied to inclined granular flows: role of boundaries

We compare the predictions of extended kinetic theory (EKT), where the roles of surface friction and correlation in fluctuation velocities are taken into account, with discrete element simulations of steady, fully-developed, inclined flows of identical spheres over bumpy bases, in the presence and absence of flat, frictional sidewalls. We show that the constitutive relation for the pressure of EKT must be modified in the proximity of the boundary, because of the influence of excluded volume and shielding associated with collisions of particles with the boundary itself. We also note that currently available boundary conditions for flows over bumpy planes in kinetic theory underestimate the energy dissipation. These two observations explain the lack of agreement of EKT with the simulations, in terms of the maximum angles of inclination for which steady, fully-developed flows are possible. That is, for some high angles of inclination, EKT does not have solutions, while steady flows are predicted in DEM. However, whenever a solution to the system of differential equations of EKT does exist, the predicted distributions of velocity, solid volume fraction and granular temperature satisfactorily match the numerical measurements. The incompressible, algebraic approximation of EKT, which ignores the conduction of energy in the energy balance, admits solutions for a wider range of angles of inclination, as in the simulations, but fails to reproduce the quantitative and qualitative behaviour of solid volume fraction and granular temperature in the two conductive layers at the top and bottom of the flow. When frictional sidewalls are added to the domain, we show that the spanwise ratio of shear stress to pressure is linearly distributed in the dense core region of the flow, confirming that the sidewalls exert, on average, a Coulomb-like resistance to the flow with an effective friction coefficient which is less than half the actual particle-wall friction.     

Effect of base dissipation on the granular flow down an inclined plane

The effect of base dissipation on the granular flow down an inclined plane is examined by altering the coefficient of restitution between the moving and base particles in discrete element (DE) simulations. The interaction laws between two moving particles are kept fixed, and the coefficient of restitution (damping constant in the DE simulations) between the base and moving particles are altered to reduce dissipation, and inject energy from the base. The energy injection does result in an increase in the strain rate by up to an order of magnitude, and the temperature by up to two orders of magnitude at the base. However, the volume fraction, strain rate and temperature profiles in the bulk (above about 15 particle diameters from the base) are altered very little by the energy injection at the base. We also examine the variation of h _stop , the minimum height at the cessation of flow, with energy injection from the base. It is found that at a fixed angle of inclination, h _stop decreases as the energy dissipation at the base decreases.     

Molecular dynamics simulation of granular flows: Slip along rough inclined planes

Although the velocity slip of granular flows on an inclined plane has been observed in many experiments, sufficient detail is not available for comparison with theoretical predictions. In this paper, we employ a molecular dynamics simulation technique to model the flow of a granular material down a rough inclined plane, with attention being focused on the effect of the boundary roughness and the angle of inclination of the plane. We find that the parameter θ proposed in the literature, which characterises the extent of flowing particles “penetrating” into the boundary, is not the decisive factor for the measurement of the boundary roughness. Slip velocity is found to be linearly proportional to the angle of inclination, but there is a non-linear relation with the boundary parameter θ. The average shear rate produced depends non-linearly on the inclination angle but the influence of the boundary parameter θ on the shear rate is not obvious.     

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.     

Effect of boundary vibration on the frictional behavior of a dense sheared granular layer

We report results of 3D discrete element method simulations aiming at investigating the role of the boundary vibration in inducing frictional weakening in sheared granular layers. We study the role of different vibration amplitudes applied at various shear stress levels, for a granular layer in the stick-slip regime and in the steady-sliding regime. Results are reported in terms of friction drops and kinetic energy release associated with frictional weakening events. We find that a larger vibration amplitude induces larger frictional weakening events. The results show evidence of a threshold below which no induced frictional weakening takes place. Friction drop size is found to be dependent on the shear stress at the time of vibration. A significant increase in the ratio between the number of slipping contacts to the number of sticking contacts in the granular layer is observed for large vibration amplitudes. These vibration-induced contact rearrangements enhance particle mobilization and induce a friction drop and kinetic energy release. This observation provides some insight into the grain-scale mechanisms of frictional weakening by boundary vibration in a dense sheared granular layer. In addition to characterizing the basic physics of vibration-induced shear weakening, we are attempting to understand how a fault fails in the earth under seismic wave forcing. This is the well-known phenomenon of dynamic earthquake triggering. We believe that the granular physics are key to this understanding.