Micromechanics, imaging and multiscale modeling

Axis 2 is devoted to the relationship between microstructure and macroscopic material properties, within the framework of solid mechanics (complex fluids are addressed in Axis 3). In most studies carried out in this Axis, continuum mechanics is supposed to apply down to the smallest length-scales considered. This is however by no means a requirement, and atomistic simulations are also encouraged.

Civil engineering materials covered by this Axis are notoriously multi-scale and multi-physics and require upscaling techniques (theoretical or numerical) of coupled phenomena backed by experimental studies to characterize not only the macroscopic behaviour, but also the microstructure as well as the mechanical local response through 2D or 3D imaging techniques. Projects that combine both sides are particularly encouraged

Presently (Spring 2016), 12 PhDs and 5 post-docs are attached to Axis 2. Together, they led to 20 communications and 10 publications, with additional papers under preparation/review.

Materials targeted by this Axis cover the whole range of construction materials. A significant number of projects are dedicated to innovative materials. Erica Gea RODI (PhD, started oct. 2014) is attempting to improve chemically the adhesion between matrix and reinforcement (miscanthus giganteus in the present case) in green wood plastic composites. Two procedures are proposed: thiol-ene reaction and reactive extrusion.

Van Son VO (PhD, started oct. 2013) is studying the elaboration and characterization of bio-based epoxy/clay nanocomposites. He showed that photopolymerization was an efficient method to control the surface chemistry of clay nanofillers. Cement-based innovative composites are also represented.

Observing that phase change materials provide efficient storage of thermal energy, Jérôme KODJO (PhD, started oct. 2015) embeds microcapsules of phase change materials into a standard concrete matrix and studies the effect of these microscapsules on the overall mechanical properties. The durability of these capsules is also investigated.

Additive construction is an increasingly active topic in civil engineering, and Nicolas DUCOULOMBIER will study in his PhD work (starting oct. 2016) new ways to reinforce concrete with long fibers (effectively treating concrete as a standard composite).

Acoustic metamaterials will be designed and optimized by Navid NEMATI (post-doc, starting sept. 2016).

More traditional construction materials and geomaterials are naturally represented, as in the PhD work of Sara BAHAFID (started oct. 2014), who investigates the effects of hydration temperature on microstructure and macroscopic properties of hardened cement pastes. Understanding these effect is of paramount importance for the cement sheath of oil-wells.

Also, in the PhD work of Youssouf ABDALLAH (starting oct. 2016), the behavior of carbonate rocks under high pressures and temperatures will be investigated, with optimization of the performances of geothermal systems in mind.

While most research projects funded through Axis 2 are concerned with bulk properties of materials, surface/interface properties are also investigated. Karim HOUANOH (PhD, started oct. 2013) is studying unilateral contact between rough surfaces. Yang XU (PhD, started oct. 2015) is working on sprayed surface coatings for corrosion protection.

Due to the multi-physics nature of the mechanisms under consideration and the complexity of the microstructures, multi-technique experimental investigations combining direct observation at the microscopic level and macroscopic measurements are used. Computed micro-tomography is used by Mohamed Hassan KHALILI (PhD, started oct. 2013, funded by IFSTTAR and MMCD) to measure the displacement of individual grains in granular materials, as well as Erica Gea RODI and Nicolas DUCOULOMBIER to assess the distribution of fibers. Other microscopic investigation techniques are also used, such as SEM (Erica Gea RODI), XRD, thermogravimetric analysis, mercury intrusion porosimetry and 1H NMR relaxometry (Sara BAHAFID). In-situ mechanical testing combined with 2D and 3D imaging and full-field measurement techniques are used to get access to local mechanical responses of materials, in view of identification of active micromechanisms (M. H. KHALILI), and possibly to compare these observations with predictions of multiscale full-field modelling techniques (PhD Thanh Tung NGUYEN).

Most projects also include a modelling side, ranging from simple, mean-field homogenization techniques (Erica Gea RODI) to complex, multi-physics numerical simulations (Yang XU: free-surface flows with phase changes). From this perspective, it should be noted that simulation of cracking and damage propagation has progressively emerged in Axis 2 as a common research topic. Sabri SOUGUIR (PhD, started oct. 2015) is studying the initiation of failure in brittle materials by means of atomistic simulation techniques. Selection of the appropriate (macroscopic) failure criterion is still highly debated, and it is hoped that atomistic simulations will shed new light on this issue. Within the framework of continuum mechanics, a numerical tool based on the phase field method was initially proposed by Thanh Tung NGUYEN (PhD, ended oct. 2015) to study the propagation of cracks in heterogeneous materials. This tool can handle initiation and propagation of cracks that occur in the bulk of each phase as well as interfacial cracks. Its predictions have satisfactorily been compared to experimental measurements of evolving crack networks in heterogeneous cement based materials. It is currently being extended to hydraulic cracking by Liang XIA (post-doc, started febr. 2016), and will be central to the work of Jérôme KODJO to assess the durability of phase-change microcapsules embedded in a concrete structure. Cracking and damage is now identified as a structuring topic within LabEx MMCD, which led to the construction of a transverse project (to be discussed elsewhere).

Methodological developments are also welcome in Axis 2. From the experimental point of view, Dac Loi NGUYEN (PhD, started oct. 2012) develops models based on limit analysis to analyze nano-indentation experiments. Mohamed Hassan KHALILI proposes a reconstruction-free method to measure the displacements of grains from few tomographic projections. Thanh Tung NGUYEN has developed a digital volume correlation-based image subtraction technique to extract evolving networks of cracks able to detect micro-cracks with sub-voxel openings in heterogeneous microstructures. From the numerical point of view, Liang XIA is extending the phase-field approach to hydraulic cracking within the framework of poromechanics. Vinh Phuc TRAN (PhD, started oct. 2013, shared with Axis 4) has proposed a probabilistic prior model for the generation of mesoscale materials (obtained by filtering of the underlying microstructure).

A post-doctoral position is also currently open (PI: Virginie EHRLACHER) regarding the full-field simulation of very large microstructures by means of a domain decomposition approach. Finally, Michaël BERTIN (post-doc, ended febr. 2016, shared with Axis 4) investigated a variance-reduction technique for the Monte-Carlo simulation of heterogeneous materials. Regarding this last project, the role of LabEx MMCD as a catalyst should again be underlined. Indeed, the idea for this project came from discussions that were initiated between members of the LabEx by a previous project (PhD of William MINVIELLE, ended sept. 2015).

To close this description a few theoretical works should also be mentioned. Navid NEMATI will be building on a generalized nonlocal theory of sound propagation in porous media to design and optimize acoustic metamaterials. Vinh Phuc TRAN  has developed a theoretical framework for the homogenization of stress-gradient materials. Finally, Bo YU (post-doc, started may 2016) is revisiting the homogenization of saturated, poroelastic media.

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