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The purpose of this paper is to analyse the development and the evolution of the fracture process zone during fracture and damage in quasi-brittle materials. A model taking into account the material details at the mesoscale is used to describe the failure process at the scale of the heterogeneities. This model is used to compute histograms of the relative distances between damaged points. These numerical results are compared with experimental data, where the damage evolution is monitored using acoustic emissions. Histograms of the relative distances between damage events in the numerical calculations and acoustic events in the experiments exhibit good agreement. It is shown that the mesoscale model provides relevant information from the point of view of both global responses and the local failure process.

This study is focused on the tightness of containment walls of nuclear power plants in case of loss of coolant accident. This kind of accident results in the rise from ambient conditions to a temperature of 140 °C and an air–steam mixture pressure of 5.2 bars. This rise creates high hydric, thermal and mechanical stress in concrete, impairing the tightness integrity of concrete wall. In this work, we propose to highlight experimentally the influence of initial water content of the sample, the compressive load and the percolating fluid on the hydraulic behavior of concrete. The tests are carried out on hollow cylindrical samples, using a device which allows to generate a gas mixture at the inlet and to measure the leakage rate at the outlet of the samples. As the tests are performed in isothermal conditions, the results show that at stress levels lower than 70–80 % of the peak stress, the leakage rate of the mixture is slightly influenced by both parameters (degree of saturation and stress). However, it can be noted that as the load exceeds this threshold value, the damage effect due to mechanical loading remains more important in the leakage rate evaluation.

International audience; Many researchers have proposed hypotheses concerning the physical mechanisms that govern creep and among them the development of microcracks is well recognized. For high load levels, microcracking may initiates at the moment of load application and begins to grow to form a time-dependent crack path. An experimental investigation is proposed here in order to provide interesting insight into the coupling between creep and damage with specimens loaded in flexure. The acoustic emission (AE) technique is used as a tool to provide information on the pertinence of the physical hypothesis that microcracks appear during creep. An original test is performed to accelerate the creep phenomenon by submitting concrete beams to desiccation after a basic creep period. The results show a good proportionality between the creep deformation and the AE activity and thus the efficiency of acoustic measurements for the estimation of the state of damage. In addition, an unsupervised pattern recognition analysis is used as a tool for the classification of the monitored AE signatures. The cluster analysis shows two clusters during basic creep and three clusters during desiccation creep indicating different damage mechanisms.

International audience; In this paper, the fracture process zone (FPZ) is investigated on unnotched and notched beams with different notch depths. Three-point bending tests have been realized on plain concrete under crack mouth opening displacement (CMOD) control. Crack growth is monitored by applying the acoustic emission (AE) technique. The comparison with a numerical model is also realized by using a mesoscopic approach. Such an approach is of particular interest in the analysis of interactions between the cementitious matrix and aggregates. Several AE parameters are examined during the entire loading process, and show that the relative notch depth influences the AE characteristics, the process of crack propagation, and the brittleness of concrete. The numerical load-CMOD curves show that the mesoscopic modelling reproduces well the notch effect and concrete failure. In order to improve our understanding of the FPZ, the width and length of the FPZ are followed based on the AE source locations maps in parallel with the numerical damage fields. An important energy dissipation is observed at the crack initiation in unnotched beams.

International audience; The fracture process zone (FPZ) was investigated on unnotched and notched beams with different notch depths. Three point bending tests were realized on plain concrete under crack mouth opening displacement (CMOD) control. Crack growth was monitored by applying the acoustic emission (AE) technique. In order to improve our understanding of the FPZ, the width and length of the FPZ were followed based on the AE source locations maps and several AE parameters were studied during the entire loading process. The bvalue analysis, defined as the log-linear slope of the frequency-magnitude distribution of acoustic emissions, was also carried out to describe quantitatively the influence of the relative notch depth on the fracture process. Theresults show that the number of AE hits increased with the decrease of the relative notch depth and an important AE energy dissipation was observed at the crack initiation in unnotched beams. In addition, the relative notch depth influenced the AE characteristics, the process of crack propagation, and the brittleness of concrete.

International audience; Results of railway traffic, concerning with the environmental disturbances (vibrations, noises, various pollutions, ...) are all the more important as the train velocities are higher. For vibrations, the effect of this velocity is directly linked to its relative value against that of waves propagating in the soil (particularly for Rayleigh surface waves). For soft soils (clay, peat, ...), a Rayleigh wave velocity very lower than 100 m/s is quite possible, and in this case, the reach of a super-Rayleigh regime can be envisaged for trains moving at high velocity. This situation includes important displacements in the rail (sometimes higher than 12 mm), not acting on the passengers comfort but troubling for the life duration of structure. This paper describes the numerical approach with a semi-analytical model for the waves propagation in the case of moving loadings. Also, sets of tests were able to be performed on two specific sites (peaty soil) with variable parameters (type and composition of trains, train velocity, traffic weight). The comparison achieved between displacements and velocities, obtained on the one hand through the numerical simulations and on the other hand by means of test measurements, must pass for a relatively satisfactory result, taking into account the numerous uncertainties in the two approaches. This agreement is enough correct to consider that the proposed model allows to obtain accurate informations for the railway behaviour and that of the surrounding soil.; Les conséquences de la circulation ferroviaire, en termes de nuisances environnementales (vibrations, bruits, pollutions diverses, ...) sont d'autant plus importantes que les vitesses des trains sont élevées. En ce qui concerne les vibrations, l'effet de cette vitesse est directement lié à son positionnement relatif vis-à-vis de celle des ondes se propageant dans le sol (en particulier, celle des ondes superficielles de Rayleigh). Pour des sols assez mous (argile, tourbe, ...), une vitesse d'onde de Rayleigh bien inférieure à 100 m/s est tout-à-fait possible, et dans ce cas, l'atteinte d'un régime super-Rayleigh est bien envisageable pour des trains circulant à grande vitesse. Cette situation se traduit par des déplacements importants au niveau du rail (supérieurs éventuellement à 12 mm), ne jouant pas sur le confort des passagers mais préoccupant pour la durée de vie de la structure. Cet article présente la mise en œuvre numérique d'un modèle semi-analytique de propagation d'ondes soumis à un ensemble de charges mobiles. Des séries d'essais ont pu également être opérées sur deux sites spécifiques (sol tourbeux) avec des paramètres variables (type et composition des trains, vitesse du train, poids du convoi). La comparaison qui a pu être réalisée entre les déplacements et les vitesses obtenus, d'une part, grâce aux simulations numériques et, d'autre part, par les mesures expérimentales, doit être considérée comme relativement satisfaisante compte tenu des différentes incertitudes présentes dans les deux démarches. Cet accord est suffisamment significatif pour estimer que le modèle qui est proposé permet d'obtenir des informations crédibles sur le comportement de la voie et du sol environnant.

This paper introduces a two-dimensional model for the response of the ground surface due to vibrations generated by a railway traffic. A semi-analytical wave propagation model is introduced which is subjected to a set of harmonic moving loads and based on a calculation method of the dynamic stiffness matrix of the ground. In order to model a complete railway system, the effect of a simple track model is taken into account including rails, sleepers and ballast especially designed for the study of low vibration frequencies. The priority has been given to a simple formulation based on the principle of spatial Fourier transforms compatible with good numerical efficiency and yet providing quick solutions. In addition, in situ measurements for a soft soil near a railway track were carried out and will be used to validate the numerical implementation. The numerical and experimental results constitute a significant body of useful data to, on the one hand, characterize the response of the environment of tracks and, on the other hand, appreciate the importance of the speed and weight on the behaviour of the structure.

Flexible piezoelectric cantilever beam has been realized by depositing lead zirconate titanate (PZT) thin film (4.5 μm) by chemical solution deposition (CSD) onto very thin aluminium foil (16 μm). The tip deflection of the beam has been measured as a function of the frequency of the applied sinusoidal voltage to the PZT film for different amplitudes. Resonance curves have been compared to a classical model of an oscillating system under sinusoidal stress with a very good agreement. Despite of weak ferroelectric properties (remnant polarization: 13 μC/cm2), ultra-large deflection amplitudes have been measured under very moderate applied voltage values: 750 μm@10 V for quasi-static mode and 5 mm@10 V at the resonance frequency (~12 Hz), which makes this PZT/aluminium composite film very promising for highly flexible actuation applications where large displacements are wanted.