Abstract

The present work introduces a new application of the Particle Finite Element Method (PFEM) for the modeling of excavation problems. Lagrangian descriptions of motion of the continuum medium are the natural way of describing motion in solid mechanics. Particle finite element methods are based in these solid mechanics settings that can treat large material deformations and rapidly changing boundaries.

These capabilities are very suitable for modeling fluid motions and moving free surfaces. That is the reason that the most of the research and applications of PFEM can be found in the context of computational fluid dynamics (CFD) instead of solid mechanics.

The satisfying results in modeling fluids have been the motivation to use this method in dynamic solid mechanics.
The simulation of an excavation process is a non-linear dynamic problem. It contains geometrical, material and contact non-linearities. Modeling the contact process can be classiffied as the main difficulty. The simulation face the problems of: the detection of a changing geometry, the detection of contact between several solid domains, the estimation of correct interacting forces, the computation of the wear related to this contact forces and the removing of the material that has been excavated from the model.

The PFEM have its fundamentals in the classical non linear finite element analysis. The formulation provides a foundation in the updated lagrangian formulation for solids.

The dynamic problem is integrated using an implicit scheme. Remeshing strategies are employed in order to identify the boundary surfaces. A remeshing of the domain is introduced for the detection of rapidly changing boundaries. The Delaunay tessellation and the alpha-shape concept together, are used as a methodology to de_ne the boundary from a cloud of points. At the same time an interface mesh is created for contact recognition. A particular constitutive contact law has been developed for capturing contact normal and frictional forces using this mesh. By means of an Archard-type law, the excavation and damage caused in the ground is quantified. The erosion and wear parameters of the grounds under study determine the excavability. The behavior of the geomaterials is described using a Damage Model which model fracture in the continuum. The update and storage of the variables in particles have especial schemes for its correct treatment. The method and the meshing process is adapted for the simulation of an excavation problem.
PFEM is presented as a very suitable tool for the treatment of excavation problem.
The method gives solution for the analysis of all processes that derive from it. The method has a high versatility and a reasonable computational cost. The obtained results are really promising.

Abstract

Comparative studies of different discrete element models of a rock-type material are presented. The discrete element formulation employs spherical particles with the cohesive interaction model combining linear elastic behaviour with brittle failure. Numerical studies consisted in simulation of the uniaxial compression test. Two cylindrical specimens with particle size distributions yielding different degree of heterogeneity have been used. Macroscopic response produced by different discrete element models has been compared. The main difference between the compared models consists in the evaluation of micromechanical constitutive parameters. Two approaches are compared. In the first approach, the contact stiffness and strength parameters depend on the local particle size, while in the second approach, global uniform contact parameters are assumed for all the contacting pairs in function of average geometric measures characterizing the particle assembly. The size dependent contact parameters are calculated as functions of geometric parameters characterizing each contacting particle pair. As geometric scaling parameters, the arithmetic and harmonic means, as well as the minimum of the radii of two contacting particles are considered. Two different models with size dependent contact parameters are formulated. The performance of these models is compared with that of the discrete element model with global uniform contact parameters. Equivalence between the models with size dependent and uniform contact parameters has been checked. In search of this equivalence, different methods of evaluation of global uniform parameters have been studied. The contact stiffness has been evaluated in terms of the average radius of the particle assembly or in terms of the averages of the arithmetic and harmonic means of the contact pair radii, the geometric parameters used in the evaluation of the contact stiffness in the size-dependent models. The uniform contact strengths have been determined as functions of the averages of radii squares, squares of arithmetic radii means or squares of minimum radii of the contacting pairs.

For the more homogenous specimen, the models with local size dependent parameters and models with global uniform parameters give similar response. The models with uniform parameters evaluated according to the averages of the geometric parameters used in the evaluation of local parameters ensure better agreement with the respective models with size-dependent parameters than the models with uniform parameters evaluated according to the particle radii. Simulations using the more heterogenous specimen reveal differences between the considered models. There are significant differences in stress–strain curves as well as in the failure pattern. The models with local size-dependent parameters are more sensitive to the change of heterogeneity than the model with global uniform parameters.

Abstract

A numerical model that accounts for the hydration and aging phenomena during the early ages of concrete curing is presented in a format suitable for a finite element implementation. Assuming the percolation of water through the hydrates already formed as the dominant mechanism of cement hydration, the model adopts an internal variable called hydration degree, whose evolution law is easily calibrated and allows an accurate prediction of the hydration heat production. Compressive strength evolution is related to the aging degree, a concept that accounts for the influences of the hydration and curing temperature on the final mechanical properties of concrete. The model capabilities are illustrated by means of a wide set of experimental tests involving ordinary and high performance concretes, and through the simulation of the concrete curing on a viaduct deck of the Öresund Link.

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