Dorival M. Pedroso

Breaking processes in three-dimensional bonded granular materials with general shapes

The paper presents an extension to the spheropolyhedra method for the simulation of granular materials comprising particles of general shapes with bonding. A bonding, cement, or cohesion model for particles sharing common faces is introduced. The bonding force is elastic and has a strain-based breaking threshold for modelling fracture. An initial study is conducted based on the Brazilian tensile test to check how the parameters of the proposed model affect the principal variables measured in this test. Afterwards, solid cubic blocks are then subjected to a triaxial test to explore the mathematical macroscopic failure model. It is found that the peak strength envelope is the product of the superposition of frictional and fracture failure mechanisms. The fracture failure is mainly produced by an avalanche of broken cohesive bonds. The intensity of the avalanche exhibits a power law distribution, as reported in previous studies. The method allows for random divisions of solid bodies without any pre-existing internal voids. It offers a natural, effective tool to model, simulate and study fragmentation processes in 3D

A Lattice Boltzmann model for studying transient effects during imbibition–drainage cycles in unsaturated soils

This paper presents a numerical model based on the Lattice Boltzmann Method (LBM), developed for studying dynamic responses of an unsaturated porous medium to periodic imbibition and drainage induced by a cyclic water table movement. The model includes gravity which helps defining an hydraulic head. The model predicted an incremental increase of the overall water content in the medium over each cycle prior to a quasi-steady oscillatory state, a hydraulic ratcheting effect that has been previously observed in laboratory experiments. An empirical model was proposed to combine the transient and harmonic variations of the volumetric water content. The parameters of this empirical model were examined against physical quantities including the frequency of the driving water table oscillations and the porosity of the porous medium. The findings presented here may help to improve the formulation of constitutive models that are able to describe hydraulic processes of unsaturated soils.

Solution of Liquid–Gas–Solid Coupled Equations for Porous Media Considering Dynamics and Hysteretic Retention Behavior

AbstractThis paper presents a solution procedure based on the finite element method for the three-constituent coupled problem representing the behavior of unsaturated porous media. A formulation is...

Parallel evolutionary algorithm for single and multi-objective optimisation: Differential evolution and constraints handling

Abstract This paper presents an evolutionary algorithm employing differential evolution to solve nonlinear optimisation problems with (or without) constraints and multiple objectives. New decision strategies to compare candidate solutions are developed that take into account all constraints and objective functions. The new constraint handling strategy uses the concept of Pareto dominance to rank the candidate solutions based on their constraint violation value. In order to improve the performance of the algorithm, a set of genetic operators and differential evolution operators are combined. In addition, the paper proposes an algorithm to perform parallel evolution in a way that the diversity of the final population is preserved after migrations. Another goal of the algorithm is to handle problems with a mix of integers and real-valued variables. Numerical experiments investigate the robustness and the performance of the algorithm through multiple benchmark optimisation problems. Finally, two engineering applications are studied, namely: (1) the topology optimisation of trusses; and (2) the economical dispatch problem in power generation. Results show that the algorithm is capable of handling optimisation problems with a mix of integer and real-valued variables with constraints and multiple objectives.

Assessment of the Mechanical Behaviour of Granular Media by DEM-Based True Triaxial Tests

This study presents the numerical simulation of a true triaxial test by means of the discrete element method (DEM). Experimental results performed on Toyoura sand are employed as reference and the calibration methodology is explained. Physical aspects of the real soil, such as the grain size distribution and the relative density, are considered during the generation of the virtual sample. It is shown that the main aspects of the macro-mechanical behaviour of granular soils during compression loading can be fairly represented by the idealised simulations with particles.

Simulations of Fall Cone Test in Soil Mechanics Using the Material Point Method

The material point method is a particle-based method that uses a double Lagrangian-Eulerian discretisation. This approach has proved its functionality for the simulation of large deformation problems. Such problems are frequent in geotechnical engineering, more specifically those related to penetration during pile driving and conventional in situ tests such as the Cone Penetration Test. The shallow laboratory fall cone test is considered in this paper. This test is widely used for the determination of the liquid limit of clays, but it is also used to study the relationship between penetration (h) and the undrained shear strength of clays (su). Simulations are verified against laboratory vane shear tests and fall cone tests performed on samples of kaolin clay at different moisture contents. Calibrations using a simple penetration-strength (h-su) model are made based on a single coefficient named the cone factor (K). The numerical results closely match both the experimental data and analytical solutions available in the literature.

Efficient isogeometric shell element with through-thickness stretch: application to incremental sheet forming

An isogeometric shell element with through-thickness stretch is applied to a two-point incremental forming problem. The shell element supports full three-dimensional constitutive laws and therefore does not make the plane stress assumption. An anisotropic material model is implemented to account for the sheet rolling direction. Automatically adjusting penalty stiffness is proposed for modeling the contact between the stylus tool and the sheet, whereas the die contact algorithm uses traditional constant penalty stiffness. A comparison is made between experimental results as well as results from a conventional shell formulation.

Automatic calibration of 3D anisotropic yield criteria using a parallel evolutionary algorithm

Advanced 3D non-quadratic anisotropic yield criteria are usually required to describe highly anisotropic materials such as aluminium alloys. One issue related to the advanced anisotropic yield criteria is that they often require the identification of many parameters that are difficult to calibrate. An automatic and reliable technique for the determination of the coefficients of 3D anisotropic yield criteria is presented here. The error between predictions and experimental data is minimised by finding the yield criteria coefficients using a modification of the evolutionary algorithm described in (Pedroso et al. 2017 Applied Soft Computing 61 995-1012). The method is implemented for parallel computation to both speed up calculations and to make sure the results are consistent after several runs. The results show that the proposed method can produce good coefficients for the 3D anisotropic yield criteria.

Physical and mathematical modeling of erosion processes across the scales

Internal erosion is the interaction of micro-scale hydraulic and mechanic processes taking place within the soil structure leading to macro-scale damages causing failures of whole constructions. While the geometrical and hydraulic conditions on the macro-scale supporting erosion are well investigated, the processes involved in the onset and continuation of erosion, the conditions influencing its temporal evolution are not well understood. The investigation of the processes involved in internal erosion requires a bijective or one-to-one approach with simultaneously implemented physical experiments and computational modelling on both relevant scales, micro- or pore-scale and macro- or continuum-scale. The aim of this approach is to accomplish an improved understanding of the underlying physics to be able to transfer this new knowledge into computational models enabling the solving of problems on the technical scale.

Probabilistic Reliability Assessment of Slope Stability Problems with Genetic Algorithms

Uncertainty estimation and consideration in engineering is an important practice to design reliable structures especially in geotechnical engineering since the level of control with regards to the material parameters is much lower. Research has been conducted in order to assess the reliability of geotechnical works using probabilistic methods where challenges in computing the probability density function and predicting the critical failure region must be first overcome. One method to solve this problem is the Monte Carlo simulation; however it requires a high computational effort. Alternatively, reliability indices such as the Hasofer-Lind (HL) index can approximate the probability of failure Pf with fewer computations. Nonetheless, yet an optimisation problem needs to be solved. In this work, a genetic algorithm is developed to compute the HL index using the limit equilibrium method to search for the critical failure surface. Study cases and the analysis of the Vajont landslide are presented in order to illustrate the method.