Settlement of a granular material: boundary versus volume loading

Throughout this paper, the compressibility of granular soils is investigated by considering different loading modes. In particular, the difference in the response of the soil along an œdometric loading path, according to the nature of the axial loading, is explored. It is shown that the application of a boundary axial loading gives rise to a more pronounced nonlinear response with more important settlements than what is observed with a volume (downward hydraulic flow) loading. A theoretical approach is developed and the predicted results are confronted to experimental results obtained from an original apparatus allowing both volume and boundary loadings be applied. A satisfying agreement is obtained, validating the theoretical model proposed.     

Microfracturing characteristics in brittle material containing structural defects under biaxial loading

The purpose of this study is to specify the influence of structural defects and confining pressure on microfracturing characteristics in rocklike brittle materials. The stress field and microfracturing events of rock samples containing structural defects under biaxial loading are simulated by the finite element software, Rock Failure Progress Analysis (RFPA^2D). The simulated rock samples are compressed by an increasing vertical displacement of 0.001 mm/step and confined with different constant horizontal lateral pressures 0, 5, 10 and 15 MPa, respectively. The characteristics of microfracturing events with stress field evolution in rock failure process are visually represented by damaged elements. The results show that rock failure with great structural effects is greatly dependent on the arrangement of structural defects. As the outer loading increases, the stresses in rock mass are built up gradually. When the stress of an element reaches a certain critical value, a microfracturing event occurs. Two neighboring damaged elements owning one same edge are considered in the same damaged element group. Based on the newly-developed statistic function of RFPA^2D on damaged element group, the scales (element number in a group) and counts of damaged groups (micro cracks) are recorded. The distribution of microfracturing events for all rock samples under different lateral pressures represents self-similar fractal features. However, rescaled range analysis indicates that microfracturing events do not exhibit the similar scale-invariant property strictly under higher lateral pressures.     

Statistical properties of a 2D granular material subjected to cyclic shear

This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, P, and the mean contact number, Z. This relationship applies to the shear stress τ, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as P, τ and Z are strongly hysteretic functions of the strain, ε. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.     

Physical processes within a 2D granular layer during an impact

In this paper, the impact of a block on a coarse granular soil corresponding to rockfall events is investigated using the Discrete Element Method. Different impacting particle and medium characteristics (impact point, impacting particle size and shape, sample height, etc.) are considered. The numerical results first exhibit the physical phenomena involved in the interaction between the impacting particle and the granular medium. The impact process starts with the partial energy exchange from the impacting particle to the soil. This phase is followed by the propagation of a shockwave from the impact point and a wave reflection on the bottom wall of the sample. A second energy exchange from soil particles to the impacting particle can occur if the reflected wave reaches the soil surface before the end of the impact. Based on these investigations, the impacting particle bouncing occurrence diagram is defined for various impacting particle sizes, incident kinematic parameters and sample heights. The bouncing occurrence diagram brings out three impact regimes. For a small impacting particle, the impact is mainly determined by the first interaction between the impacting particle and the soil, whereas for an intermediate-sized impacting particle, the shockwave propagation through the sample is the leading phenomenon. For a large impacting particle, bouncing is associated with the formation of a compact layer below the impacting particle.     

3D meso-mechanical analysis of concrete specimens under biaxial loading

In recent years, the mechanics of materials group at ETSECCPB‐UPC has developed an approach for meso‐mechanical analysis of concrete using zero‐thickness interface elements in 2D and more recently in 3D. In this methodology, the meso‐structure is generated with in‐house developed computer programs based on Voronoï/Delaunay theory. In the analysis, continuum elements are assumed linear elastic. Non‐linearity and fracture phenomena are made possible by the systematic use of zero‐thickness interface elements inserted on a priori determined potential fracture planes. In this paper, the results obtained for a 3D specimen under biaxial loading are presented. The results turn out to be very satisfactory and, in particular, it is observed that even the specimens which contain a reduced number of aggregates (14 in the present calculations) lead to a realistic failure envelope under biaxial loading, and they also capture the tendencies of cracking and fracture orientations observed in experiments for different rates of biaxial loading. The special limit case of biaxial loading under restrained out‐of‐plane deformations is also analysed, leading to practically elastic behaviour as shown by available experimental evidence.     

Moisture distribution in granular material during drying in a fluidized bed

This paper gives the results of a determination of the moisture content gradient and an investigation of the moisture distribution in a granular material during drying in a fluidized bed.     

Predicting evolution of ply cracks in composite laminates subjected to biaxial loading

An energy-based model is developed to predict the evolution of sub-critical matrix crack density in symmetric multidirectional composite laminates for the case of multiaxial loading. A finite element-based numerical scheme is also developed to evaluate the critical strain energy release rate, G_Ic, associated with matrix micro-cracking, a parameter that previously required fitting with experimental data. Furthermore, the prediction scheme is improved to account for the statistical variation of G_Ic within the material volume by using a two-parameter Weibull distribution. The variation of G_Ic with increasing crack density is also accounted for based on reported experimental evidence. The simulated results for carbon/epoxy and glass/epoxy cross-ply laminates demonstrate the ability of the improved model to predict the evolution of multidirectional ply cracking. By integrating this damage evolution model with the synergistic damage mechanics approach for stiffness degradation, the stress-strain response of the studied laminates is predicted. Finally, biaxial stress envelopes for ply crack initiation and pre-determined stiffness degradation levels are predicted to serve as representative examples of stiffness-based design and failure criterion.     

Distinct element analysis of the microstructure evolution in granular soils under cyclic loading

Granular Matter - The article proposes functions linking the standard deviation of a particle distribution in a porous bed consisting of spherical particles with various parameters characterising...     

Micromechanical behavior of granular materials with inherent anisotropy under cyclic loading using 2D DEM

This paper presents the micromechanical behavior of granular materials due to different initial inherent anisotropic conditions during cyclic loading using the discrete element method (DEM). Oval particles were used to model the samples. Three samples, with three different inherent anisotropic conditions based on the particle’s bedding direction, were prepared and subjected to biaxial cyclic loading. The differences in the inherent anisotropic conditions of the samples affect the stress–strain-dilative behavior of granular materials. The width of the stress–strain cyclic loops decreases as the preferred bedding angle changes from vertical to horizontal. Contact fabric evolution is found to be dependent on the inherent anisotropic fabric of the sample during loading and unloading. The fabric anisotropy is dominant for horizontal particle bedding at the end of loading and for vertical particle bedding at the end of unloading. A change in fabric anisotropy is observed only for the first few loading–unloading cycles for the given conditions depicted in the present study.     

Force fluctuations in a pushed granular material

We analyse the time evolution of stresses in a noncohesive granular bed cut by the motion of a tool. Our numerical simulations both in two and three dimensions reveal large fluctuations of the force acting on the tool. These fluctuations have a decreasing exponential distribution. We find that, in spite of fluctuations, the mean force is well fitted by an analytical form obtained from a limit analysis with Coulomb's failure criterion. Received: 7 January 2000     

Triaxial behavior of granular material under complex loading path by a new numerical true triaxial engine

A new numerical true triaxial engine based on discrete element method accounting for rolling resistance contact is developed. By this engine, we have simulated mechanical behavior of granular materials under complex stress loading path in this study. Stress-strain responses of a kind of typical granular sand under several stress loading path in meridian and deviatoric stress space are provided. The results show that the three dimensional effects like the intermediate principal stress play an important role in the modeling processes. Theoretical analysis in strength characteristic implies the strength criteria with three parameters such as unified strength criterion and van Eekelen strength criterion are capable of describing cohesionless granular material behaviors in three dimensional stress states. Moreover, the case study for Chende sand further demonstrates the numerical true triaxial engine, is a potential tool. As compared to conventional triaxial compression test, this new developed apparatus could be widely used to “measure” elastic-plastic behavior in three dimensional stress space for finite element analysis in geotechnical problems.     

Microstructural evolution during creep of a hot extruded 2D70Al-alloy

Microstructural evolution during creep of a hot extruded Al–Cu–Mg–Fe–Ni (2D70) Al-alloy was investigated in this study using transmission electron microscopy (TEM). The samples for creep test were carried out two-stage homogenization, followed by extruding. The creep ultimate strength dropped and the temperature increased gradually from 312 to 117 MPa and from 423 to 513 K, respectively. The microstructural observation for the crept samples showed that the S′ phase coarsened with increased creep temperature and the aging precipitates transformed from S″ phase to S′ phase during creep process. Meanwhile, excess solute atoms in supersaturated solid solution dynamically precipitated to further form finer S′ phase and S″ phase, which pinned the dislocations and impeded the dislocation movements. Large amount of dislocations piled up around the micron-scale Al₉FeNi phase, and a lot of dislocation walls were generated along 〈220〉 orientation. S phase accumulates around these defects. The interaction between dislocations and precipitates was beneficial for the improved performances at elevated temperature.     

A closed-form solution of a slant crack under biaxial loading

A closed-form solution for the stress field in an infinite plate which contains a slant crack was presented in this paper. The exact solution derived is valid all-over the stress field and its accuracy is independent of the normalized to the crack-length distance from the crack tips. Therefore, the solution yields with the same accuracy the sum and the difference of principal stresses, which can he compared with whole-field experimental solutions. A comparison of the exact solution with two-term approximations, given by Eftis and Subramonian[11], which constitute the most accurate solutions of the problem up-to-now, indicated that significant errors exist in the approximate solutions varying with distance from the crack tip and type of quantity studied.     

Large-strain dynamic cavity expansion in a granular material

The dynamic problem of the symmetric expansion of a cylindrical or spherical cavity in a granular medium is considered. The constitutive behaviour of the material is governed by a hypoplasticity relation for granular soils capable of describing both monotonic and cyclic deformation. The problem is solved numerically by a finite-difference technique. A nonreflecting boundary condition used at the outer boundary of the computational domain makes it possible to model a continuous multi-cycle loading on the cavity wall. The solution is illustrated by numerical examples. Possible geomechanical applications to the modelling of the vibratory compaction and penetration in granular soils are discussed.     

Large-strain dynamic cavity expansion in a granular material

The dynamic problem of the symmetric expansion of a cylindrical or spherical cavity in a granular medium is considered. The constitutive behaviour of the material is governed by a hypoplasticity relation for granular soils capable of describing both monotonic and cyclic deformation. The problem is solved numerically by a finite-difference technique. A nonreflecting boundary condition used at the outer boundary of the computational domain makes it possible to model a continuous multi-cycle loading on the cavity wall. The solution is illustrated by numerical examples. Possible geomechanical applications to the modelling of the vibratory compaction and penetration in granular soils are discussed.     

The strength of a filament wound composite under biaxial loading

This paper describes experiments in which filament wound glass/polyester tubes were loaded by internal pressure and simultaneous axial tension or compression. Failure envelopes are presented which describe the strength of ± 35° and ± 55° winding angle composites under a variety of biaxial stresses. The experimental results are compared with a semi-empirical failure criterion.     

Propagation of a through crack in a sheet material under biaxial tension

We present the results of investigation of the effect of the form of the stressed state and low temperatures on the growth of plastic strains at the crack tip in sheet materials subjected to repeated static biaxial loading. Information of this sort enables one to obtain adequate estimates of the load-carrying capacity of plates with cracklike concentrators by determining the plastic component of strains at the crack mouth and to introduce the required corrections in the existing models of fracture mechanics. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 52–59, July–August, 1998.