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International audience; The degradation and failure of brittle or quasi-brittle geomaterials are analyzed in this Lecture. The main emphasis is put on the discrimination between discrete and continuum approaches to describe damage and fracture. The materials studied herein are cement, high-performance concrete and rocks.

We investigate the role of carbon nanotubes structure on their optical limiting properties. Samples of different and well-characterized structural features are studied by optical limiting and pump-probe experiments. The influence of the diameter's size on the nano-object is demonstrated. Indeed, both nucleation and growth of gas bubbles are expected to be sensitive to diameter.

Aqueous suspensions of bundles and individual single-wall carbon nanotubes were prepared with the help of surfactants. We study the changes in the Raman spectra of the suspensions with respect to powders, and of exfoliated tubes with respect to bundles. The radial breathing modes (RBM) upshift in suspensions because of the internal pressure of the liquid. By contrast, no shift is observed in the RBM spectra after exfoliation in the suspensions. However, we demonstrate a selectivity of the exfoliation process for tubes of small diameters., Soumis le 3 D\'{e}cembre 2003

Nanotube-based systems are good candidates for optical limiting against broadband laser pulses. We explore new routes to improve their limiting performances. We show that the diameter of the nanotubes is a key factor to control the performances. On the other hand, we demonstrate that chemically modified nanotubes can be mixed with organic chromophores, leading to high performance composite limiting systems which are particularly efficient in the nanosecond regime due to the cumulative effects of nonlinear scattering and multiphoton absorption.

New systems are required for optical limiting against broadband laser pulses. We demonstrate that the association of non-linear scattering from single-wall carbon nanotubes (SWNT) and multiphoton absorption (MPA) from organic chromophores is a promising approach to extend performances of optical limiters over broad spectral and temporal ranges. Such composites display high linear transmission and good neutral colorimetry and are particularly efficient in the nanosecond regime due to cumulative effects., Comment: 5 avril 2004

International audience; An experimental procedure is developed to determine the scatter of the macroscopic toughness of brittle materials. First, samples are precracked to obtain a sharp precrack. The toughness is then determined by using a standard three-point flexural test. Digital image correlation is used to analyze displacement fields of cracked samples. Based upon the resolution and the spatial resolution of the measurement technique, a detection criterion is proposed and validated. It allows for an accurate estimate of the crack tip location so that the presence of a crack and its size at arrest can be monitored. As an example, the toughness distribution of 18 samples made of silicon carbide is evaluated. By using a simple macro-micro transition, an analysis of the scatter in toughness is related to that in strength for the material with no macrocracks.

International audience; The threat of small to medium calibre armor piercing projectiles requires efficient protections that can be achieved by using bilayered configurations. They consist in a front face made of a hard material and a backing made of a ductile material. These solutions are among the most interesting in terms of mass efficiency. To design such bilayered concepts, one needs to understand and model fragmentation of the ceramic during the first microseconds after impact. This cracking pattern may significantly reduce the multi-hit capability of the armor. A new material made of porous silicon carbide infiltrated with aluminium is considered. The presence of an aluminum skeleton may improve the residual strength after impact. The present study deals a comparison of fragmentation properties of this new material with those of porous silicon carbide. Quasi-static experiments and edge-on-impact tests are performed. The role of aluminum is discussed. An anisotropic damage model coupled with a fragmentation study is used to analyze the experimental observations.

Nowadays, it seems evident that a unique nonlinear optical (NLO)material cannot offer simultaneously linear transparency,colour neutrality and broadband optical limiting efficiency at the performance levels required for sensor and eye protection against all laser threats.Several combinations of NLO materials were investigated last few years, including multicell or multilayer geometries. The approach presented here combines multiphoton absorption with nonlinear scattering. For that purpose, singlewall carbon nanotubes are suspended in various solutions of multiphoton absorbing chromophores. Such combinations allow us to obtain optical limiters of high linear transmittance and excellent colour neutrality. Broadband optical limiting is expected from the association of these two broadband materials,and enhanced optical limiting efficiency is expected from cumulative effects in the nanosecond regime. We report here on the optical limiting studies performed with nanosecond laser pulses on several families of multiphoton absorbers in chloroform,with carbon nanotubes suspended in the solutions. The performances of these samples are compared with those of simple multiphoton absorber solutions and carbon nanotube suspensions, and the differences observed are interpreted in terms of cumulative NLO effects and adverse aggregation phenomenon. Ways to optimise stability of the suspensions are also experimented and discussed.

International audience; Dynamic loadings produce high stress waves leading to the fragmentation of brittle materials such as ceramics, concrete, glass and rocks. The main mechanism used to explain the change of the number of fragments with stress rate is a shielding phenomenon. However, under quasi-static loading conditions, a weakest link hypothesis may be applicable. Therefore, depending on the local strain or stress rate, different fragmentation regimes are observed. One regime corresponds to single fragmentation for which a probabilistic approach is needed. Conversely, the multiple fragmentation regime may be described by a deterministic approach. The transition between the two fragmentation regimes is discussed for high performance concrete, glass and SiC ceramics.

International audience; We report the synthesis, characterization and optical limiting behavior of a nanohybrid built by grafting C60-fullerenes on carbon nanotubes (CNTs). The nonlinear optical limiting properties of the CNT-C60 complex were investigated at wavelengths where C60 does not absorb. We found that the nanohybrid had superior performances to those of CNTs and fullerenes, either taken individually or as a mixture. This enhanced optical limitation of the nanohybrid suggests not only cooperative but also synergistic effects between the two carbon forms. A mechanism involving higher excitonic states of the CNTs formed by Auger recombination of low energy excitons is proposed.

International audience; Impact produces high stress waves leading to the degradation of brittle materials such as ceramics. Based upon a probabilistic approach, single and multiple fragmentation regimes are exhibited. The deterministic nature of the numerical simulation is discussed with respect to stress rate and volume. A damage model is proposed to account for dynamic loadings. Characteristic parameters are proposed and used to choose the mesh size. The mesh sensitivity is studied on a spalling configuration. Numerical predictions are compared with experimental data obtained on edge-on impact configurations.

We present a multispectroscopic structural study of various nanotube samples with different tube diameters. We determine for each sample the mean bundle and tube diameter as well as the tube diameter distribution. The possibility to work on SWNT of various structural characteristics opens new opportunities to correlate the nanotube structure and their physical properties., Conf\'{e}rence : 8 au 15 mars 2003