Martin H. Sadd

Contact law effects on wave propagation in particulate materials using distinct element modeling

Abstract The microstructural wave propagation behavior of a granular medium is modeled using the distinct element method. This technique simulates the discrete behavior of the medium by assuming that the motion of each particle may be modeled using Newtonian rigid-body mechanics with particular force-deformation and force-deformation rate contact laws. The present work provides a comparison of the effects of various contact laws on the wave propagational behaviors including wave attenuation and dispersion characteristics. Specific cases which have been studied include linear, non-linear and non-linear hysteretic force-deformation contact laws along with velocity proportional damping. Numerical results are compared with experimental data from dynamic photoelastic and strain gage experiments. Since velocity-dependent contact damping is not a reasonable model for dry cohesionless granular media, it was desired to determine if a non-linear hysteretic contact law could be used to replace the velocity damping. Results indicate that such a non-linear law does provide a damping mechanism which can predict experimental attenuation data, and that the dispersion characteristics are modeled more accurately with this hysteretic model.

SIMULATION OF ASPHALT MATERIALS USING FINITE ELEMENT MICROMECHANICAL MODEL WITH DAMAGE MECHANICS

A theoretical and numerical study of the micromechanical behavior of asphalt concrete was undertaken. Asphalt is a heterogeneous material composed of aggregates, binder cement, and air voids. The load-carrying behavior of such a material is strongly related to the local load transfer between aggregate particles, and this is taken as the microstructural response. Numerical simulation of this material behavior was accomplished by developing a special finite element model that incorporated the mechanical load-carrying response between the aggregates. The finite element scheme incorporated a network of special frame elements, each with a stiffness matrix developed from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. A damage mechanics approach was then incorporated within this solution, and this approach led to the construction of a softening model capable of predicting typical global inelastic behavior found in asphalt materials. This theory was then implemented within the ABAQUS finite element code to conduct simulations of particular laboratory specimens. A series of model simulations of indirect tension (IDT) tests were conducted to investigate the effect of variation of specimen microstructure on the sample response. Simulation results of the overall sample behavior compared favorably with experimental results. Additional comparisons were made of the evolving damage behavior within the IDT test samples, and numerical results gave reasonable predictions.

Propagation and Scattering of SH-Waves in Semi-Infinite Domains Using a Time-Dependent Boundary Element Method

The propagation and scattering of SH-waves of general form is investigated using the boundary element method. Attention is focused on semi-infinite regions containing subsurface cavities. The analysis is carried out within the context of linear elastodynamic theory formulated as an integral equation. Using a time-dependent Green’s function for the half space, the boundary integral equation is developed and is discretized into a finite set of algebraic relations. A time-stepping algorithm is constructed for the solution of general boundary value—initial value problems. Results of this solution technique are presented for the two-dimensional case of transient waves scattering off of buried stress-free cavities.

Prediction of Dynamic Contact Loads in Granular Assemblies

An experimental-numerical hybrid technique has been developed to predict the intergranular contact load trasfer in granular media subjected to explosive loading. The granular media were simulated by assemblies of circular disks in contact

DEM simulation of wave propagation in granular materials

Abstract Transient wave propagation in granular materials is numerically studied using discrete element simulation. Three particular cases are investigated including dry cohesionless material, elastic cemented particulate media and fluid saturated granular material. Primary interest is concerned with linking material microstructure with wave propagational behaviors. The discrete element scheme uses several different inter-particle contact laws appropriate to the material modeling application. Simulation results yield information on wave speed and amplitude attenuation for two-dimensional, meso-domain model assembly systems of circular disks and spherical particles. Particulate models were numerically generated using a biasing scheme whereby partial control of particular fabric measures could be achieved. Particular fabric measures which were used to characterize the granular material models include branch and cementation vectors . Discrete element modeling (DEM) wave simulation results indicated that wave speed is dependent upon the stiffness of the interparticle contacts and distribution of branch vectors along the propagation direction. Wave amplitude attenuation is also dependent on the number of branch vectors in the direction of propagation.

Influence of loading pulse duration on dynamic load transfer in a simulated granular medium

Abstract An experimental and numerical investigation was conducted to study the dynamic response of granular media when subjected to impact loadings with different periods or wavelengths. The granular medium was simulated by a one-dimensional assembly of circular disks arranged in a straight single chain. In the experimental study, the dynamic loading was produced using projectile impact from a gas gun onto one end of the granular assembly, and the measured wave signal was collected using strain gages. The numerical simulations were conducted using the distinct element method. It was found from the experiments and numerical simulations that input waves with a short period (τ ≈ 90 μs) will propagate in this granular medium with little waveform change under steady amplitude attenuation ; whereas longer waves (τ

PARAMETRIC MODEL STUDY OF MICROSTRUCTURE EFFECTS ON DAMAGE BEHAVIOR OF ASPHALT SAMPLES

200 μs) will propagate with significant waveform dispersion. For these longer wavelength signals, the smooth waveform will undergo separation into a series of short oscillatory signals, and this rearrangement of energy allows a portion of the transmitted signal to increase in amplitude during the initial phases of propagation. Thus the granular medium acts as a nonlinear wave guide, and local microstructure and contact nonlinearity will allow input signals of sufficiently long wavelength to excite resonant sub-units of the medium to produce this observed ringing separation. Following a modeling scheme originally proposed by Nesterenko [J. Appl. Mech. Tech. Phys. 5,733 (1983)], a nonlinear wave equation model was developed which is related to soliton dynamics and leads to travelling wave solutions of specific wavelength found in our experimental and numerical studies.

The effect of microstructural fabric on dynamic load transfer in two dimensional assemblies of elliptical particles

This paper presents a computational modeling study of the microstructural influence on damage behavior of asphalt materials. Computer generated asphalt samples were created for numerical simulation in indirect tension and compression testing geometries. Our previously developed micromechanical finite element model was used in the simulations. This model uses a special purpose finite element that incorporates the mechanical load-carrying response between neighboring aggregates. The element was developed from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. The computational model establishes a network of such elements to simulate an asphalt mass. Continuum damage mechanics was then incorporated within this scheme leading to the construction of a micro-damage model capable of predicting typical global inelastic behavior found in asphalt materials. A series of model asphalt samples have been generated and simulated with controllable microstructure variation in an effort to determine the effects of particular microstructural variables on the material response. These simulations explored the relationship between microstructure parameters and damage behavior of particular asphalt samples.

Axisymmetric non-fourier temperatures in cylindrically bounded domains

Abstract Experimental and numerical studies have been conducted to investigate the effect of microstructural fabric, such as major axis orientation, contact normal and branch vector distributions, on dynamic load transfer behavior in two dimensional granular material. The granular medium was simulated by assemblies of elliptical particles. The experimental method utilizes the combination of high speed photography and photoelasticity to study the local load transfer behavior in granular assemblies subjected to explosive loading. Numerical studies employed a computational scheme based on the discrete element method. Results indicate that the microstructural fabric has significant effect on the load transfer phenomena, such as stress wave velocity, load pulse wavelength and contact load attenuation.

A DISCRETE ELEMENT STUDY OF THE RELATIONSHIP OF FABRIC TO WAVE PROPAGATIONAL BEHAVIOURS IN GRANULAR MATERIALS

Abstract Heat conduction solutions are presented for the case where the material obeys a non-Fourier conduction law. In contrast to the Fourier law which predicts an infinite speed of heat propagation, the non-Fourier theory implies that the speed of thermal signals are finite. Axisymmetric problems for regions interior and exterior to a circular cylinder are investigated by using methods of Laplace transformation and asymptotic analysis. Comparisons of the temperature profiles are made with Fourier theory for the case of step function temperature boundary conditions.