Ali Daouadji

In this study, a large-scale laboratory test was developed to analyze the contribution of geogrid reinforcement to the improvement of unpaved roads. A detailed experimental protocol was established regarding the road base soil preparation, installation, and soil compaction procedure. Eight tests were performed under a circular plate load test in a box 1800 mm in width, 1900 mm in length, and 1100 mm in height. Under this configuration, three repeat tests were performed to verify the developed protocol. Furthermore, the plate load tests were used to compare different road base thicknesses and geogrid types. Once the developed protocol was confirmed, three tests, using the developed simulator accelerator of traffic (SAT), were performed. Indeed, the same road base configuration was tested, but this time it was placed in a box 5000 mm in length. The prepared road base was subjected to two different loadings: a specific plate load and a traffic load. The results showed the efficiency of...

Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the prestressed surrounding medium. The geological fault gouge, coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms are influenced, if not controlled, by the characteristics and environment of this tribological “third body”. A 2D granular fault (mm scale) is implemented with Discrete Element Modelling (DEM). A displacement-driven model with dry contact is studied to observe kinematics and properties of the slipping zone. Increasing the length of the granular media increases the slip needed to weaken the friction from friction peak to steadystate. Low-angle Riedel shear bands are mostly observed. Their number increases with the inter-particle friction coefficient, which also influences shear bands formation in their orientation angle (higher friction leads to higher angle with the main slip direction).

An experimental method has been carried out for analyzing the thermo-mechanical characteristics of concrete based on treated and untreated red-wood chips and concrete based on treated and untreated chips of steamed beech wood. The obtained results of experimental study for concrete characteristics confirm the superior performances of studied concrete. The treatment improves relatively wood-matrix adherence. Wood chips can upgrade the thermal-conductivity of ordinary concrete. This innovative building material, based on renewable resources, can provide a good thermal insulation.

One of the best ways to manage and reduce noise is the design of advanced structural ma terials that are able to reduce sound radiations and propagation by minimising th e total acoustic power radiated from the vibrating structure surface into a sur rounding acoustic medium. Among these materials are composites containing inclusions, v oids, fibres or particulates in the matrix. The most adopted strategy is the one that consider adding a viscoelastic inhomogeneities either as thin layers or as dilute inclus ions depending on the practical considerations. The focus of the present work is to propose an intuitive numerical proce dure for the vibration analysis of heterogeneous materials using micro to macro app roach as the alternative to the direct simulation which requires enormous computing resources. A multilevel finite element methodology is introduced where the hierarchical character of model descriptions and simulation results are exploited to expedite a nalysis of problems. Thi...

In this paper we study the reflective properties of a 2D interface separating a homogeneous solid from a band-gap metamaterial by modeling it as an interface between a classical Cauchy continuum and a relaxed micromorphic medium. We show that the proposed model is able to predict the onset of Stoneley interface waves at the considered interface both at low and high-frequency regimes. More precisely, critical angles for the incident wave can be identified, beyond which classical Stoneley waves, as well as microstructure-related Stoneley waves appear. We show that this onset of Stoneley waves, both at low and high frequencies, strongly depends on the relative mechanical properties of the two media. We suggest that a suitable tailoring of the relative stiffnesses of the two media can be used to conceive "smart interfaces" giving rise to wide frequency bounds where total reflection or total transmission may occur.

The conception of new metamaterials showing unorthodox behaviors with respect to elastic wavepropagation has become possible in recent years thanks to powerful dynamical homogenization techniques. Such methods effectively allow to describe the behavior of an infinite medium generated by periodically architectured base materials. Nevertheless, when it comes to the study of the scattering properties of finite-sized structures, dealing with the correct boundary conditions at the macroscopic scale becomes challenging. In this paper, we show how finite-domain boundary value problems can be set-up in the framework of enriched continuum mechanics (relaxed micromorphic model) by imposing continuity of macroscopic displacement and of generalized traction when non-local effects are neglected. The case of a metamaterial slab of finite width is presented, its scattering properties are studied via a semi-analytical solution of the relaxed micromorphic model and compared to numerical simulations ...

This paper presents an approach to the analysis of free and forced vibrations of strengthened sandwich beam with viscoelastic core and composite coats by considering geometrical asymmetry. A higher-orde theory is used considering longitudinal and rotational inertias as well as the asymmetry of sandwich beam. The formulation of the equation of motion is carried out by Hamilton principle. A comparative study to validate the proposed numerical approach is performed comparing the obtained results with other findings. Moreover, a parametric analysis is carried out with different configurations of the sandwich beams in order to analyze the influence of different parameters on the dynamic behavior. The analysis highlighted from the study of fiber orientation influence on dynamic behavior, that the structure damping can be improved by adopting a better composite configuration. However, the obtained results from the thickness ratio effect showed that the sandwich structure has more dissipati...

It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results highlight that these collapses occur under a diffuse mode of failure. We present and discuss experimental data on constant shear drained tests. A non-associated elastoplastic multimechanism model is briefly presented and numerical results are compared with our data obtained

The influence of three loading paths on the collapse of loose sand is analyzed with a particular attention paid to the onset of collapse and the mode of failure exhibited. Experimental results on conventional undrained triaxial compression tests, constant shear drained tests, as well as quasi-constant shear undrained path are presented, compared, and analyzed. It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results obtained with the new arrangement built up highlight that these collapses occur under a diffuse mode of failure. An extensive experimental series of tests shows that the first negative value of the second-order work computed using experimental data corresponds to the loss of controllability. Moreover, it is shown that the stress ratios at collapse and the corresponding mobilized angles of friction are very close for all types of tests. For similar void ratios, the onset of collapse is thus largely independent of the loading path under drained and undrained conditions but depends on a stress state to bring the material inside the unstable domain and also on the current direction of the stress increment. Indeed, it appears that the orientations of the stress increments at collapse for all tests are the same, what explains, according to the second-order work criterion, that collapse occurs at the same stress ratio. A potentially unstable domain, depending on the stress increment direction, can thus be defined.

<p>How do earthquakes start? What are the parameters influencing fault evolutions? What are the local parameters controlling the seismic or aseismic character of slip?</p><p>To predict the dynamic behaviour of faults, it is important to understand slip mechanisms and their source. Lab or in-situ experiments can be very helpful, but tribological experience has shown that it is complicated to install local sensors inside a mechanical contact, and that they could disturb the behaviour of the sheared medium. Even with technical improvements on lab tools, some interesting data regarding gouge kinematics and rheology remains very difficult or impossible to obtain. Numerical modelling seems to be another way of understanding physics of earthquakes.</p><p>Fault zone usually present a granular gouge, coming from the wear material of previous slips. That is why, in this study, we present a numerical model to observe the evolution and behaviours of fault gouges. We chose to focus on physics of contacts inside a granular gouge at a millimetre-scale, studying contact interactions and friction coefficient between the different bodies. In order to get access to this kind of information, we implement a 2D granular fault gouge with Discrete Element Modelling in the software MELODY (Mollon, 2016). The gouge model involves two rough surfaces representing the rock walls separated by the granular gouge.</p><p>One of the interests of this code is its ability to represent realistic non-circular grain shapes with a Fourier-Voronoï method (Mollon et al., 2012). As most of the simulations reported in the literature use circular (2D)/spherical (3D) grains, we wanted to analyse numerically the contribution of angular grains. We confirm that they lead to higher friction coefficients and different global behaviours (Mair et al., 2002), (Guo et al., 2004).</p><p>In a first model, we investigate dry contacts to spotlight the influence of inter-particular cohesion and small particles on slip behaviour and static friction. A second model is carried out to observe aseismic and seismic slips occurring within the gouge. As stability depends on the interplay between the peak of static friction and the stiffness of the surrounding medium, the model includes the stiffness of the loading apparatus on the rock walls.</p><p>The work presented here focuses on millimetre-scale phenomena, but the employed model cannot be extended to the scale of the entire fault network, for computational cost reasons. It is expected, however, that it will lead to a better understanding of local behaviours that may be injected as simplified interface laws in larger-scale simulations.</p>

This paper presents a numerical study of the free and damped forced vibration of simply-supported beams with composite coats subjected to a moving load by use of finite elements method. Three types of beam configurations, aluminum, composite and strengthened beam are investigated. The equation of motion of the beam is solved using the modal superposition method and integrated by applying the Newmark time integration procedure. Good agreements were achieved between the FEM and analytical solutions. The damped dynamic response, critical velocities and the dynamic amplification factor of the beam are calculated for different parameters such as the thickness ratio, the fiber orientation of the coat and damping ratio.

ABSTRACT Nous examinons dans cet article le concept de liquéfaction statique. Nous rappelons pour cela une interprétation communément avancée basée sur l'augmentation de la pression interstitielle ainsi que la définition d'une ligne d'instabilité joignant les pics. Nous présentons ensuite des résultats expérimentaux qui semblent conforter cette analyse. Cependant, nous mettons en évidence la similarité des chemins de chargement et expliquons la raison pour laquelle cette ligne semblait fondée. Nous mettons en évidence le rôle primordial joué par la direction des incréments de contraintes et notamment une fois atteint le domaine de bifurcation. A partir d'un point situé sur cette ligne, un chargement dans une direction non incluse dans les cônes de direction instable engendre un comportement stable jusqu'au critère plastique limite. Ces résultats sont conformes aux travaux analytiques et numériques publiés très récemment.