Photoelastic investigation of bolted joints in composites

In this paper, the methods of photo-orthotropic elasticity are applied to the study of bolted joints in composites. The photoelastic models are E-glass fibre-reinforced epoxy strips loaded through a cylindrical pin, simulating bolt loading without lateral pressure. A special loading arrangement is devised so that the photoelastic response around the hole is not obscured. Quasi-isotropic and unidirectionally-reinforced specimens are tested, with ratios of end distance to hole diameter varying from 2 to 6. Photoelastic isochromatic fringe patterns are presented along with the shear stress distribution for the quasi-isotropic models.     

Photoelastic investigation of the double-torsion specimen

The stress distribution of the double-torsion fracture mechanics specimen geometry is presented using a photoelastic “frozen stress” technique. Isochromatic fringes of a plastic double-torsion specimen under load were photographed and analyzed in the top, middle and bottom planes parallel to the top surface. Principal stress directions for these planes are also given. The relative magnitudes of the principal stresses perpendicular to the crack plane at the crack tip, crack origin and a point midway between the origin and crack tip are shown.     

Experimental investigation of heat transfer through porous media in superfluid helium

An experimental investigation of heat transfer through porous media in superfluid helium has been conducted in the framework of the development of porous electrical insulations for superconducting magnet cables cooled by superfluid helium. Several types of porous media with different characteristics were tested and, in particular, samples with pore size diameters of 0.1 μm, 1 μm, 2 μm, 10 μm and 20 μm. Temperature and pressure were measured between an insulating inner bath and the cryostat bath, communicating only through the porous medium. The cryostat bath is held constant all along the measurement and, for each sample, the tests are performed for bath temperature from 1.4 K to 2.1 K with 0.1 K increment. Depending on the porous medium average pore size diameter, different flow regimes are observed: for porous media with a pore diameter of 0.1 and 1 μm, only the Landau regime is observed whereas for porous media with a pore diameter of 2 μm, we observed the Landau regime and the Gorter-Mellink regime. For samples with a pore diameter of 10 and 20 μm, measurements only permitted to detect the Gorter-Mellink regime. In the laminar regime, the permeability of the samples was determined and it was found that the permeability is constant for bath temperature above 1.9 K whereas it increases as the bath temperature decreases from 1.8 K to 1.4 K. For samples with a pore size diameter of 10 and 20 μm, measurement permits only to observe the turbulent regime and the analysis exhibits a constant average tortuosity for each samples, independently of the bath temperature.     

基于动态横向转移载荷的轮式装载机侧倾稳定性研究

为研究轮式装载机动态侧倾稳定性,在考虑车身侧倾和动臂俯仰的基础上,构建基于拉格朗日法和包含动坐标系的轮式装载机十自由度侧倾动力学模型,求解出非平稳路面与发动机耦合激励下的侧倾响应,推导出表征侧倾稳定性的动态横向转移载荷方程,并分析了横向坡度角、动臂结构参数和转弯速度变化对侧倾角与动态横向转移载荷的影响趋势。结果表明:车身侧倾角和动态横向转移载荷随横向坡度角、动臂质量和举升角以及转弯速度的增加而不断增大,但横向坡度角和动臂举升角对侧倾稳定性的影响灵敏度较高;当车身侧倾角超过临界横向坡度角时,两侧车轮垂向载荷发生完全动态横向转移,使一侧车轮产生离地现象进而导致侧倾;上述模型和动态横向载荷转移表征方法可为后续的性能优化提供研究方法和理论基础。     

Theoretical investigation of colloid separation from dilute aqueous suspensions by oppositely charged granular media

A theoretical investigation of removal of colloidal particles from aqueous suspensions by oppositely charged granular (porous) media is presented in th     

Microstructural mechanics of granular media

A geometrically and physically linear micromechanical theory for elastic granular media is presented, based on the identification of the constituent grains with the nodes of a Bravais lattice. Adjacent particles are permitted to displace normally and transversely to each other, and to rotate with respect to the doublet axes. Thus, microstrains of the axial, torsional, and shear type are generated. The conjugate microstresses are then defined. Through a variational formulation, the microstress equations of motion are derived, together with natural boundary conditions and the transition from the microstresses to the macrostresses. The principles of thermodynamics are employed to derive the most general, invariant, and appropriately symmetric microconstitutive equations, and to close the system of field equations for the granular medium, subject to both adiabatic and non-adiabatic processes. The problem of a granular semispace loaded by compressive boundary force is solved as an application, and the existence of locally tensile microstresses is determined, while the associated macrostresses are computed to coincide with the well-known Flamant's solution and thus to be compressive everywhere.     

Jamming in weakly perturbed granular media

A granular system is weakly vibrated and we observe how it reaches a static configuration when the perturbation intensity is decreased. This behavior is investigated using an immersed torsion oscillator whose angular deflections are analyzed to give information on the granular agitation. The oscillator can thus be seen as a low-frequency “thermometer” sensing the granular noise. The observed diffusive noise appears to approach the final static (or jammed) state according to the same expressions that describe the temperature dependence of viscosity or diffusivity in supercooled liquids, thus showing strong analogies with the vitrification process. We then discuss a simple model that relates the microscopic dynamics to the macroscopic rearrangements and accounts for that dependence.     

Transport coefficients in porous granular media

Transport coefficients are calculated as functions of porosity using an ordered model of granular porous media. We consider grain-spheres situated at the nodes of various crystalline lattices. The behavior of the transport coefficients at low porosity values is considered. A comparison with experimental data is performed.ranslated from Inzhenerno-Fizicheskii Zhurnal, Vol. 61, No. 6, pp. 964–970, December, 1991.     

Photoelastic determination of dynamic stress intensity factors in portal frames

Dynamic photoelasticity has been applied to the determination of the dynamic stress intensity factor associated with cracks in the central beam of a portal frame, next to the junction with a column. Results are presented for cracks extending over 12% of the beam depth. The value of photoelastic measurements, in conjunction with numerical analysis, for the assessment of the severity of defects in structures loaded under impact is discussed.     

Avalanches in anisotropic sheared granular media

We study the influence of particle shape anisotropy on the occurrence of avalanches in sheared granular media. We use molecular dynamic simulations to calculate the relative movement of two tectonic plates. Our model considers irregular polygonal particles constituting the material within the shear zone. We find that the magnitude of the avalanches is approximately independent of particle shape and in good agreement with the Gutenberg–Richter law, but the aftershock sequences are strongly influenced by the particle anisotropy yielding variations on the exponent characterizing the empirical Omori’s law. Our findings enable one to identify the presence of anisotropic particles at the macro-mechanical level only by observing the avalanche sequences of real faults. In addition, we calculate the probability of occurrence of an avalanche for given values of stiffness or frictional strength and observe also a significant influence of the particle anisotropy.     

Planar deformation of cohesionless granular media

A theory for planar deformation of cohesionless granular media is presented. Although this study follows closely the methodology used by Mandel (1947), Spencer (1964) and Mehrabadi and Cowin (1978) it differs in many respects from the contents of these papers. Both the angle of dilatancy and the orientation of the family of the velocity characteristics (slip lines) with respect to the direction of the principal axes of stress are assumed variant and dependent on the loading history of the element. The velocity and stress characteristics are assumed not to be coincidental (necessarily) and it is reasoned that deformation may proceed along one family of the slip lines only. A hypothesis regarding the symmetry of orientation of slip lines with respect to the principal stress directions and a second one regarding the minimal value for a so called ‘energy correction factor’ involving the ratio of the first invariant of the strain rate tensor to the second invariant strain rate deviation tensor is stated. It is shown that the adaptation of these hypotheses leads directly to St. Venant's principle: the coaxiality of the stress and the strain rate tensors. It should be stressed that coaxiality conclusion reached in this paper is in variance with the conclusions reached by the above authors (see also Spencer, 1981) and the conclusions of Christoffersen et al. (1981) who approached the problem more fundamentally using micromechanical modeling.     

DEM analysis of swelling behaviour in granular media

Many granular materials change their volume as they absorb fluids. This phenomenon is called swelling and can be observed in a variety of solids, such as soils, wood, absorbent hygiene products (AHPs) and pharmaceutical excipients. Therefore, an in-depth understanding of grain swelling is of great importance. Since experimental investigations can often provide only limited information, while great insight could be gained from numerical modelling, rigorous numerical models for predicting particle swelling are required. Hence, the objective of this research is to develop and validate a Discrete Element Method (DEM) for swelling of particles. A first order kinetic model was employed to predict the volume expansion of a single grain and subsequently implemented in DEM. The validation of the model was accomplished by comparing the expansion with time of a packed bed made of super absorbent polymer (SAP) particles obtained numerically and experimentally. It was shown that the DEM model can accurately predict the bed expansion. The model was then employed to simulate the swelling of three different materials: superabsorbent polymer (SAP), rice and microcrystalline cellulose (MCC Avicel PH102). As expected, it is demonstrated that the material properties play a significant role on the swelling; the fastest to reach its maximum expansion is the granular bed made of MCC PH102, followed by SAP and rice. However, the highest swelling capacity is achieved with SAP. Moreover, a preliminary DEM analysis of the segregation in a swelling binary mixture is presented in this work. Results suggest that systems which contain a small number of particles, and thus are looser, are more prone to segregation. Future study could advance the developed model to analyse consequences of swelling phenomena in granular materials, such as segregation and heat generation.     

Properties of force networks in jammed granular media

Discrete element method simulations are conducted to investigate the effects of applied uniaxial compressive load, and bidisperse particle size distributions on force networks within jammed granular media. The differences between the strong and weak networks are examined through investigating the spatial correlation and distribution of contact angles, and emergence of chainlike structures. The simulation results show that the chainlike structures are more prevalent in the strong network due to the larger cumulative probabilities of contact angles, but not all the contacts belonging to the strong or weak networks are able to constitute the chainlike structures. Although the contacts of coarse-fine particles are dominant for the bidisperse systems, the contacts of coarse–coarse particles dominate the strong network, as well as the linear chainlike structures. Upon increasing the pressure from very low to high, the probability of contact orientations with respect to the compression direction in the strong network increases for contact orientation less than \(60^{\circ }\) and decreases for contact orientation greater than \(60^{\circ }\), while the opposite trends are observed in the weak network. The tails of normalized normal contact forces distributions are quantified by \(\hbox {P}(\hbox {f}) = \hbox {exp}(-\hbox {cf}^{\mathrm{n}})\), and it is found that the value of n depends on the applied pressure and particle size distribution. Statistical analysis shows that the degree of homogeneity of contact force increases with increasing pressure, which is also validated by participation number.     

Features of static pressure in dense granular media

It is shown that the partial pressure, i.e. the contribution of contacts with a given force to the total average pressure, in a granular packing in quasistatic flow increases linearly from zero with the force level both in two and three dimensions. It reaches its maximum for the average force and decays for larger forces. We found that a well-defined sub-texture, composed of the contacts carrying a force below the average force, does not contribute to the shear stress, so that its contribution to the average pressure is mechanically similar to a hydrostatic pressure.     

动载下矿岩-充填体能量传递规律及破坏特性

为研究嗣后充填采矿矿柱爆破回采时相邻胶结充填体内能量传递规律及破坏特性,利用霍普金森压杆装置,对矿岩与不同配比充填体组合试件进行单轴冲击实验。实验结果表明:能量传递过程中,大部分能量以反射波形式消散;随着应变率增加,吸能密度增大。同等吸能密度下,应变率■;冲击荷载下矿岩破碎形态呈简单块状。充填体破碎情况为,随应变率增加,呈块状分布减少,呈粉末状增多。当应变率在60 s^(-1)左右时,ZH330较ZH250与ZH180,充填体粉末状明显减少;计算充填体平均粒径与分形维数发现,随应变率增大,平均粒径减小,分形维数增加。实验研究对嗣后充填采矿矿柱回采爆破参数选择及胶结充填体保护具有一定意义。     

Recording performance characteristics of granular perpendicular media

The recording performance of CoCrPtO granular-type perpendicular media was examined with two types of perpendicular heads to demonstrate the importance of matching head and media designs in perpendicular recording. Shielded-pole heads with high write field gradients, field angles, and sufficient write field magnitude yielded superior writability and signal-to-noise ratio as compared to mono-pole heads. The recording performance dependences on head-to-medium spacing, interlayer thickness, and soft-underlayer (SUL) thickness were also weaker with the shielded-pole heads. In addition, the effect of stray fields on the SUL domain noise was investigated for a synthetic antiferromagnetically-coupled (SAF) SUL. A radial field close to the exchange field of the SAF SUL was found to induce domain noises that could potentially cause errors in recording systems.     

Development of micromechanical models for granular media

Micromechanical analysis has the potential to resolve many of the deficiencies of constitutive equations of granular continua by incorporating information obtained from particle-scale measurements. The outstanding problem in applying micromechanics to granular media is the projection scheme to relate continuum variables to particle-scale variables. Within the confines of a projection scheme that assumes affine motion, contact laws based on binary interactions do not fully capture important instabilities. Specifically, these contact laws do not consider mesoscale mechanics related to particle group behaviour such as force chains commonly seen in granular media. The implications of this are discussed in this paper by comparison of two micromechanical constitutive models to particle data observed in computer simulations using the discrete element method (DEM). The first model, in which relative deformations between isolated particle pairs are projected from continuum strain, fails to deliver the observed behaviour. The second model accounts for the contact mechanics at the mesoscale (i.e. particle group behaviour) and, accordingly, involves a nonaffine projection scheme. In contrast with the first, the second model is shown to display strain softening behaviour related to dilatancy and produce realistic shear bands in finite element simulations of a biaxial test. Importantly, the evolution of microscale variables is correctly replicated. This paper is dedicated to Professor Ching S. Chang on the occasion of his 60th birthday.     

On the apparent particle dispersion in granular media

The apparent particle dispersion in a granular medium due to the combined effects of random granular arrangements and interstitial fluid flow was studied. The particle motion was a two-dimensional random walk on the transverse plane. The corresponding dispersion coefficient was found by sampling all possible trajectories with the aid of two granular media models. The theoretical results were verified by numerical simulation data obtained with commercial CFD software. Reasonably good agreement between the theory and simulation suggests that the present theory may be applied to practical granular system applications.