Joseph Absi

Characterization of mixed-mode fracture based on a complementary analysis by means of full-field optical and finite element approaches

This paper focuses on the characterization of mixed-mode fracture parameters through use of two formalisms based on Crack Relative Displacement Factors and Stress Intensity Factors, respectively. The evaluation of Crack Relative Displacement Factors is based on a kinematic approach that integrates the experimental displacement field measured by a digital image correlation method. In parallel with this step, the stress intensity factor is calculated from a finite element analysis. The coupling between these two approaches allows for the identification of fracture parameters in terms of an energy release rate without any prior knowledge of material elastic properties. Depending on the mixed-mode configuration, the proportion of the energy release rate corresponding to opening and shear modes can be calculated. Moreover, the proposed formalism allows determining, in addition to fracture parameters, the local elastic properties in terms of reduced elastic compliance directly from the test sample. Experimental protocols are carried out using a Single-Edge notched specimen made from a rigid Polyvinyl Chloride polymer loaded at various mixed-mode ratio values.

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.

Determination of the polymerization degree of various alkaline solutions: Raman investigation

Alkaline silicate solutions are used for different industrial applications. Structural studies of these solutions performed by fourier transform infrared and nuclear magnetic resonance spectroscopies have shown the presence of cyclic species. However, these techniques do not allow determining the variations of chain and ring size depending on the content of alkali cations and water. The objective of this study was thus to use Raman spectroscopy to obtain additive information, such as variations of ring and chain size to evaluate the reactivity of the silicate solutions. For this, several solutions were prepared according to different preparation methods (commercial or laboratory) with different cations and Si/M ratios (M = Na and/or K). The structural results obtained by Raman spectroscopy showed that the preparation method has an effect on the variation of the amount of rings with 3 and 4 tetrahedra. It has also been demonstrated that the cation had a stronger influence on the variations of these rings for commercially available solutions compared to their laboratory-made counterparts. Moreover, in the case of mixed solutions (M = Na + K), the alkali cation influenced the formation of this type of rings. Then, the effect of the Si/M ratio on the different species was highlighted and, in particular, the formation of rings at the expense of chains. Finally, this study has demonstrated that Raman spectroscopy is a crucial technique for understanding the structure of silicate solutions and their reactivity.Graphical Abstract

Modelling self-heating and thixotropy phenomena under the cyclic loading of asphalt

Asphalt concrete is a heterogeneous material containing a viscoelastic bituminous matrix and elastic aggregates. When testing asphalt materials under cyclic loading, various phenomena (so-called biasing effects) can decrease the modulus. This effect has been explained by an increase in the temperature of materials due to energy dissipation (self-heating), thixotropy and damage. The aim of this study is to analyse a uniaxial cyclic tension-compression test performed on bitumen and asphalt mixes, in modelling self-heating as one of the biasing effects. To quantify the self-heating and dissipated energy (as a heat source), a heterogeneous thermomechanical approach is introduced by separating the viscoelastic bituminous matrix from the elastic aggregates. According to this approach, various processes such as energy dissipation in the matrix due to viscoelastic properties, the thermal sensitivity of the matrix as well as its capacity to develop a heat source and diffuse heat through aggregates can all be studied. Local temperature variations are calculated by considering the heterogeneous dissipated energy field as a heat source. The complex modulus variation can then be calculated by taking into account both the temperature field and thermal sensitivity of the material. Simulation results show that as opposed to bitumen, in which 100% of complex modulus variations observed during a strain sweep test are due to self-heating, the results on asphalt mixes indicate that thixotropy varies with mechanical properties to a greater extent than self-heating. This fact is probably correlated with a higher strain level in thin bituminous matrix films, a higher load velocity in thin matrix films, material heterogeneity, and the 3D characteristic of matrix loading during the tension-compression test on asphalt mixes.

Effect of thermal treatment of a clay-based raw material on porosity and thermal conductivity: experimental approach, image processing and numerical simulation

This work aims to reveal the influence of thermal treatment on thermal conductivity of a clay-based raw material. The analysis of the effect of the porous network and solid skeleton on thermal conductivity has been performed by numerical simulation. Three steps were combined: (i) elaboration, (ii) characterisation, (iii) validation and comparison with experimental results. In filigree, the objective is to make an insulating material for the building field while decreasing energy of fabrication. After the step of elaboration, the porosity was evaluated experimentally by pycnometer and mercury intrusion porosimeter and numerically by means of micrographs observations. The effective thermal conductivity was evaluated using the laser flash technique and compared to results obtained by numerical simulation using ABAQUS software. The influence of the microstructure characteristics was highlighted by the numerical study which enables to overcome the limitation of the classical analytical models. As results, we observe that when the temperature of fabrication decreases of about 200 °C, the pore volume fraction is multiplied by six while the thermal conductivity decreases four times. The mechanical properties remain acceptable for a construction field. These tendencies were confirmed by both experimental and numerical approaches.

Accounting for respiratory motions in online mechanical impedance estimation

The aim of this work is to improve the safety and control robustness of robots interacting physically with humans by enhancing their contact perception. More specifically in the abdominal area, respiratory motions are clearly influencing the contact dynamics but have not yet been accounted for in the estimation of the contact impedance. We propose here a combined mechanical-respiratory model of impedance along with its on-line identification. Numerical and practical experiments with living subjects validate the identification process and show the relevance in accuracy of accounting for the respiratory beats.

Micromechanical modelling of bituminous materials’ complex modulus at different length scales

Abstract The aim of this work is to establish a multi-scale modelling technique usable in the study of the complex viscoelastic properties of asphalt mixes. This technique is based on a biphasic approach. At each scale, the heterogeneous media is considered as a two-phase material composed of granular inclusions with linear elastic properties and a matrix of bituminous materials exhibiting linear viscoelastic behaviour at small strain values. In this approach, the homogeneous equivalent properties of biphasic composites are transferred from one scale of observation to the next, higher scale of observation. The viscoelastic properties of the matrix and the elastic properties of the aggregates serve as the input parameters for the numerical models. The generalised Maxwell rheological model is used to describe the viscoelastic behaviour of the matrix. Thanks to the rheological properties of bitumen and the elastic properties of the aggregates, the viscoelastic properties of mastic, mortar and hot mix asphalt (HMA) as bituminous composites can be, respectively, estimated using a micromechanical finite element model. Random inclusions of varying sizes and shapes are generated in order to construct the granular skeleton. A cyclic loading was imposed on the top layer of the digital model. The dynamic modulus of the pre-cited bituminous composites, obtained from the presented multi-scale modelling process while passing from the bitumen to the HMA scale, is validated by comparison with experimental measurements when possible. Concerning our results, we have found that at low temperature (−10 °C), the predicted dynamic modulus is satisfactorily comparable to the experimental measurements. On the other hand, an acceptable gap between predicted numerical results and experimental data takes place when the temperature increases.

Micromechanical modeling of the interfacial zone in hot mix asphalt through use of a heterogeneous numerical method

Abstract This work focuses on the use of a heterogeneous generation method in order to develop a micromechanical model of Hot Mix Asphalt (HMA) that takes into account the effects of characteristics at the interfacial zone (IZ), where aggregate surfaces come into contact with the mortar matrix. Numerical calculations are conducted to investigate the effect of IZ mechanical characteristics on both the overall viscoelastic response of HMA and the local scale behavior. First of all, the HMA is treated as a biphasic material composed of mortar and granular inclusions. Next, a third phase is added between these two at each point of aggregate/mortar contact. Elastic and viscoelastic behaviors are assigned to the aggregates and mortar, respectively. The mechanical behavior of IZ varies between that of the inclusions and the matrix. At the global scale, these numerical results demonstrate that the use of triphasic micromechanical models improves the predictive accuracy of the dynamic modulus. At the local scale, a significant strain variation was found in each model. These results justify our triphasic modeling approach whenever local zone characteristics are the subject of interest.

Mechanical behavior of asphalt mixture based on x-ray computed tomography images and random generation of aggregates skeleton

The microstructure of an asphalt mix is influenced by the aggregate content, orientation and contacts. In several previous studies, the asphalt concrete is treated as homogeneous material and its microstructures are ignored. However, it has been reported that the deformation and strength of asphalt concrete are not only influenced by volume fraction of its components but also affected by the spatial distribution of its microstructures. Thus, to investigate the mechanical behaviors of asphalt concrete, it is imperative to notice more accurately a micromechanical model containing information of components and microstructures. The objective of this study is to investigate the influence of the microstructure characteristics on the mechanical behaviour of asphalt mixes. The details of the microstructure are obtained by X-ray computed tomography (CT). A comparison with results issues from digital models of random aggregate generation will be considered. The dynamic modulus of the matrix and elastic properties of aggregates were introduced as input parameters into numerical models qualified by heterogeneous microstructure. Finally, the influence of the microstructure on the mechanical response of material at local level is also highlighted.

Addition of Ammonium Molybdate in Geopolymer Formulation

Geopolymers are inorganic materials obtained by the alkaline activation of aluminosilicate sources. The ammonium molybdate could be used as a complexant for silica in order to complex the siliceous species in the alkaline solution. According to this, the aim of this work is to control the siliceous species and to understand the role of ammonium molybdate as a complexing agent acting on the formation of the different networks. To do this, additions of ammonium molybdate (up to 0.32% molar) in the silicate solution were realized along the formulation of geopolymer using two metakaolins. The results highlight that the addition of ammonium molybdate in geopolymer results in a decrease of the shrinkage at high temperature. Moreover, X-ray diffraction data and SEM after calcination show that geopolymers without ammonium molybdate form two phases (KAlSi2O6 and KAlSiO4) while with additions of molybdate, there were only the phase KAlSi2O6 associated with Al2O3 doped Mo and K2Mo2O7. Finally, SEM observations show that additions of ammonium molybdate seem to favor crystallization. The results allow to evidence the role of molybdate in the control of the polycondensation reaction in order to influence the formation of specific network