Laurent Ulmet

Numerical simulation of local temperature evolution in bituminous materials under cyclic loading

Asphalt concrete is a heterogeneous material containing a viscoelastic bituminous matrix and elastic aggregates. During fatigue testing in the laboratory, the material stiffness decreases as a result of increase in temperature due to self-heating. The objective of this study was to quantify such self-heating, during fatigue testing, as one of the biases affecting the fatigue life estimation of bituminous materials. A heterogeneous approach, which consists of separating the viscoelastic matrix from the elastic aggregates, has been adopted. According to a complex domain approach, a finite element simulation of a cyclic mechanical loading is proposed by taking into account the dissipated energy, internal thermal evolution, temperature dependence of the matrix stiffness and the heat transfer process. In considering a thermomechanical coupling, the numerical simulation results indicate that dissipated energy in the bituminous matrix is influenced by material heterogeneities. A higher dissipated energy can be observed in thin matrix films, where the strain level exceeds that of thicker films. An estimation of temperature evolution using dissipated energy as a heat source is in a good agreement with experimental results. Local temperature variations are dependent on the local heat source, the thermal properties of each phase and aggregate distribution.

Experimental and numerical aspects in diffusion process characterization in tropical species

The development of timber structures in tropical regions using local species requires characterization of mechanical properties in respect to moisture content. In this context, the estimation of moisture content distribution relies on knowledge of diffusion properties in terms of sorption hysteresis and diffusion kinetics. This paper deals with experimental protocols and numerical approaches used in the 3D orthotropic diffusion characterization for Moabi and Ozigo species.

Numerical and experimental approaches to characterize the mass transfer process in wood elements

The scope of this paper is an experimental characterization of diffusion parameters for wood material. Based on a nonlinear mass transfer algorithm, the present study focuses on the need to capture experimental moisture profiles in the sample, along with its evolution in weighting during both the desorption and adsorption phases, especially when the moisture content of the samples is far from the equilibrium state inducing a great gradient between heart and exchange surfaces of specimen. These moisture profiles are derived by means of a gammadensimetry laboratory method based on the water adsorption of gamma rays. Determination of the diffusion parameters is obtained through optimizing a simulation by means of implementing the mass transfer kinetics into a finite difference method. Both the diffusion coefficient and convective exchange coefficient are deduced by considering a Nelder–Mead simplex inversion method. This work highlights the efficiency of the approach dedicated to uncoupling nonlinear diffusion in the cross sections from boundary conditions in terms of convective exchanges and equilibrium moisture. Scale effects and boundary conditions are also investigated herein.

Approaches to the Moisture Content Monitoring Intimber Elements: Development of a Resistive Method

The scope of this work is based on the use of resistive method to quantify the water content in timber elements. For an in-depth mapping, we adapted a multiplexed technique derived from geophysics based on a maximum crossing of current lines to obtain topography of measures sweeping the space boundary conditions being defined by the sample. The calibration of these measures is completed by a gammadensimetry laboratory method which allows access to water profiles along a preferred direction. Nevertheless, the resistive method is penalized by logarithmic laws linking moisture and resistivity. So, we develop a hybrid method for coupling the data obtained to diffusion models: it will provide complementary information where resistivity and gammadensimetry are no longer effective. The developed experimental protocol allows employing the selected method and optimizing the diffusion properties (diffusion coefficient and convective exchange coefficient) injected into a characterization algorithm (Nelder-Mead’s simplex inversion method) based on a finite difference method for the Fick’s diffusion law determination integrating orthotropic and non- linear properties. Overlap between electric field and density measurements and the numerical simulation tool are possible.

Application of a Regularized SGBEM Formulation to FEM-BEM Coupling in Elastostatics

This paper concerns a procedure, based on the symmetric Galerkin boundary element equations, for computing the stiffness matrix of an elastic region without body forces in terms of boundary nodal displacements. This domain can thus be treated as a macro-element in a FEM calculation. In the present case, the implementation has been done in the Castem 2000 FEM environment. Sample numerical examples are presented; they yield numerical results in good agreement with the corresponding exact solutions.