Tang-Tat Ng

Numerical simulations of granular soil using elliptical particles

Abstract Two-dimensional numerical experiments on a random array of circular and elliptical particles were performed using the discrete element method (DEM) to study the behavior of idealized granular soil. A new program called ELLIPSE2 was developed and used to show the stress-strain behavior of granular soil under monotonic drained compressive loading and cyclic constant volume loading at various shear strains (10 −6 to 10 −3 ). In these numerical simulations, the granular soil was represented by a 2-D random array of elastic, rough quartz particles. The random array consists of a mixture of circular and elliptical particles. The numerical results were compared with available experimental data on actual sands with good agreement. These results are extremely encouraging and clearly show the potential of a program such as ELLIPSE2 for studying granular soil behavior.

Effect of Particle Shape and Fine Content on the Behavior of Binary Mixture

AbstractThe behavior of binary granular mixture is studied by the discrete element method. This granular material contains large and small particles that are similar ellipsoids of different sizes. The size ratio of the major-axes of the large and small ellipsoids is 5. Samples of ellipsoids of different particle shapes have been created by pluviation method and the particle growth technique. Segregation is observed in samples prepared by pluviation method but not particle growth technique. Thus, nine very dense samples (≈ the lowest void ratio) with various fine contents (volume fraction of the small ellipsoids) are generated using the particle growth technique. The result indicates that the packing density is a function of particle shape and fine content. These very dense samples are then sheared under drained triaxial compression to very large strain to determine peak shear strength, critical state void ratio, and ultimate shear strength. Peak shear strength is affected more by particle shape than by fi...

Damping and Particle Mass in DEM Simulations under Gravity

AbstractThis paper presents a study on the input parameters used in the discrete-element simulations with gravity. Input parameters of the discrete-element method (DEM) include particle mass (density), damping, gravity constant, strain rate, and contact relationship. This paper focuses on particle mass and damping. The samples consisted of two kinds of ellipsoidal particles. The unit weight of the particles is constant. Samples are prepared by depositing particles under gravity. The final bulk densities of the samples are different for different damping and particle mass. The effect of the reduction of particle density is similar to that of the reduction of damping in sample preparation. For the samples, static equilibrium can be achieved with a damping ratio greater than 0.24%. When the damping ratio is less than 0.24%, some particles are oscillating. The oscillation cannot be reduced with further relaxation. Undrained simulations are carried out on these samples. Shear strength increases with the increa...

Shape Correction of Inflatable Membranes by Rigidization and Actuation

The desire to see farther into space with increased accuracy is driving the development of large diameter (10-100 m in diameter) inflatable telescope reflectors that can be launched in a compact form and inflated in space. The surface shape of these membrane reflectors must be precise down to the micrometer level. However, it is unlikely that a membrane reflector could reach this ideal shape without the assistance of active and passive shape correction. Two methods of shape correction considered in this study were 1) passive actuation through epoxy-coating shrinkage strains during rigidization and 2) active actuation through a polyvinylidene fluoride (PVDF) piezoelectric film. Laser interferometry shearography and shadow moire interferometry techniques were used to observe how cure stress and PVDF actuation affect the surface shape of sample membranes. The effect of in-plane strains on the reflector shape was analyzed using a finite element model. Results show that a limited amount of shape correction is possible with both techniques. However, the magnitude of shape correction diminishes dramatically with inflation pressure and is not sufficient to overcome the surface errors caused by geometric nonlinearities at high inflation pressures.

Numerical Study of a Granular Material in Various Gravity Environments

AbstractThis paper presents the material response of a granular material under different gravity fields using the discrete element method (DEM). A bidispersive porous medium is prepared and sheared under various gravity conditions. The medium is composed of two kinds (shape and size) of ellipsoids. Each type of particle occupies 50% of the whole mass. The ellipsoids are randomly generated and deposited in lunar gravity (1/6g), Earth’s gravity (1g), and elevated gravity (10g). Various friction coefficients are used during the deposition stage, which yields samples of different initial densities. Afterward, samples are isotropically consolidated to different initial confining pressures, and the friction coefficient between particles is 0.5. A total of 25 numerical samples have been created. Very large strain is applied to the sample under drained or undrained conditions. The granular material comes into the critical state. Overall, the gravity effect on peak friction angle (shear strength of the material) i...

Sample Size Effect on DEM Simulations of Binary Mixture

This paper presents a study of the effect of sample size on packing density and the mechanical behavior of a binary granular mixture of ellipsoids subjected to rigid and periodic boundaries. The binary mixture contains two similar ellipsoids of different sizes. Since they are similar, the particle shape of these binary mixtures can be denoted by elongation, the ratio between the major length and minor length of an ellipsoid. Elongation of 1.5 is chosen in this study. The large particle is 5 times the size of the small particle. Samples of various weight fraction of small particles (fine content) have been created. The generated samples are very dense that are close to the densest packing. The void ratio (packing density) of the binary mixtures is related to fine content as expected. The sample size effect on void ratio was found in rigid boundary but not in periodic boundary as expected. Triaxial simulations were carried out to determine the peak shear strength and the properties at critical state. Sample size affects more on peak shear strength than critical state void ratio. Although small sample size can be used with periodic boundary to avoid boundary effect, the observed unusual peak strength behavior requires further investigation.

Effect of Sample Size and Shape on Critical State

This paper presents a numerical study of the effect of sample size and sample shape on the critical state condition of a granular material. The sample consists of binary-disperse ellipsoids. Samples were created by gravity deposition technique which is similar to the dry pluviation sample preparation method in physical testing. Same generation method was used to generate these samples but the initial sample dimensions were different. The grain size distribution is identical. The samples are either a cube or a rectangular prism. Samples were compressed isotropically with a confining pressure of 100 kPa. Then, conventional triaxial compression tests were performed until the critical state (ultimate state) was reached. Triaxial compression tests along different principal directions were also carried out. We present typical macroscopic result including stress-strain relationship and volumetric behavior. The friction angle and void ratio at critical state were examined. It is found that the conventional defined critical void ratio is related to the sample size. However, the friction angle at critical state is unique.

Rigidization of a Deployable Membrane Reflector

The effect of rigidization of an inflated membrane was studied. Of particular interest was the deformation induced by the curing of an epoxy resin on a thin membrane that is already inflated under vacuum pressure. The membrane material used was a Polyvinylidene Fluoride, (PVDF) film that can be electrically actuated to correct small amounts of shape anomalies in such reflectors. Depending on the required sensitivity ‘Moire Interferometry’ (0.1mm – 1 mm), or laser based ‘Electronic Speckle Interferometry’ (0.2microns – 100 microns) was used to provide the full displacement field over the entire surface. It was shown that the loss of vacuum due to the diffusion of air through polymer membranes makes it impossible to measure deformations induced by shrinkage at the micron level of sensitivity. A large deformation finite element code (ABAQUS) was used to model the inflation of a membrane and to study the subsequent change in deformation due to shrinkage induced by rigidization.

Bridge Foundation Depth Estimation Using Sonic Echo Test

Nondestructive Tests are capable of determining unknown characteristics of the bridge foundations especially the depth of the piles and drilled shafts. Sonic Echo—Impulse Response (SE/IR) is one of the methods used in determining the depth of wooden and concrete piles and drilled shafts. When the top of the pile is inaccessible, sonic waves in the pile would be generated by either horizontally striking the side of the pile or striking vertically on a block attached to the pile surface. In the present research, in order to have reasonable criteria to determine the depth of piles, the data acquired by the sensors should be interpreted properly. In addition, while a sensor is mounted somewhere far from the top of the pile, the sensor receives different echoes at different times and reflection of the waves from the top of the piles and superstructures makes the case more complicated. In order to develop a methodology for testing and Interpretation, tests were conducted and analyzed in the laboratory and on railway bridges in New Mexico.

Study on Micro-Mechanism of Concrete Hydraulic Fracturing Using DEM

The micro-mechanism of hydraulic fracturing of concrete was investigated by using the discrete element method . A bonded-particle model was employed to simulate the mechanical behavior of the cement among aggregates. Coarse aggregates were modeled as unbreakable clusters composed of several particles. The initial bonded contacts is assumed to be a parallel-plate channel that is the fluid flow path, and the element of fluid is characterized by domain, which is defined as a closed chain of particles and receives flows from the surrounding channels. Hydraulic pressure applied to the particles was increasing gradually until micro-cracks were developed. The number of micro-crack caused by normal stress and shear stress were recorded during the hydraulic fracturing test. After the test was finished, the macro-crack propagation direction was obtained. The results showed that the direction of macro-crack is almost in horizontal plane and hydraulic fracturing of concrete is due to tensile cracks between aggregates.