@article {588, title = {From cellulose to kerogen: molecular simulation of a geological process}, journal = {Chemical Science}, volume = {8}, year = {2017}, month = {Dec-01-2017}, pages = {8325-8335}, abstract = {

The process by which organic matter decomposes deep underground to form petroleum and its underlying kerogen matrix has so far remained a no man\&$\#$39;s land to theoreticians, largely because of the geological (Myears) timescale associated with the process. Using reactive molecular dynamics and an accelerated simulation framework, the replica exchange molecular dynamics method, we simulate the full transformation of cellulose into kerogen and its associated fluid phase under prevailing geological conditions. We observe in sequence the fragmentation of the cellulose crystal and production of water, the development of an unsaturated aliphatic macromolecular phase and its aromatization. The composition of the solid residue along the maturation pathway strictly follows what is observed for natural type III kerogen and for artificially matured samples under confined conditions. After expulsion of the fluid phase, the obtained microporous kerogen possesses the structure, texture, density, porosity and stiffness observed for mature type III kerogen and a microporous carbon obtained by saccharose pyrolysis at low temperature. As expected for this variety of precursor, the main resulting hydrocarbon is methane. The present work thus demonstrates that molecular simulations can now be used to assess, almost quantitatively, such complex chemical processes as petrogenesis in fossil reservoirs and, more generally, the possible conversion of any natural product into bio-sourced materials and/or fuel.

}, issn = {2041-6520}, doi = {10.1039/C7SC03466K}, url = {http://xlink.rsc.org/?DOI=C7SC03466K}, author = {Atmani, Lea and Christophe Bichara and Roland Jean-Marc Pellenq and Henri Van Damme and Adri CT Van Duin and Raza, Zamaan and Truflandier, Lionel A. and Ama{\"e}l Obliger and Kralert, Paul G. and Franz-Josef Ulm and Jean-Marc Leyssale} } @article {248, title = {Fracture toughness anomalies: Viewpoint of topological constraint theory}, journal = {Acta Materialia}, volume = {121}, year = {2016}, month = {Dec-2016}, pages = {234 - 239}, abstract = {

The relationship between composition, structure, and resistance to fracture remains poorly understood. Here, based on molecular dynamics simulations, we report that sodium silicate glasses and calcium\–silicate\–hydrates feature an anomalous maximum in fracture toughness. In the framework of topological constraint theory, this anomaly is correlated to a flexible-to-rigid transition, driven by pressure or composition for sodium silicate and calcium\–silicate\–hydrates, respectively. This topological transition, observed for an isostatic network, is also shown to correspond to a ductile-to-brittle transition. At this state, the network is rigid but free of eigen-stress and features stress relaxation through crack blunting, resulting in optimal resistance to fracture. Our topological approach could therefore enable the computational design of tough inorganic solids, which has long been a \“holy grail\” within the non-metallic materials chemistry community.

Image 1

}, issn = {13596454}, doi = {10.1016/j.actamat.2016.09.004}, author = {Mathieu Bauchy and Wang, Bu and Wang, Mengyi and Yu, Yingtian and Mohammad Javad Abdolhosseini Qomi and Smedskjaer, Morten M. and Christophe Bichara and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {249, title = {Free Volume Theory of Hydrocarbon Mixture Transport in Nanoporous Materials}, journal = {The Journal of Physical Chemistry Letters}, volume = {7}, year = {2016}, month = {Jun-10-2016}, pages = {3712 - 3717}, abstract = {
Despite recent focus on shale gas, hydrocarbon recovery from the ultraconfining and disordered porosity of organic matter in shales (kerogen) remains poorly understood. Key aspects such as the breakdown of hydrodynamics at the nanoscale and strong adsorption effects lead to unexplained non-Darcy behaviors. Here, molecular dynamics and statistical mechanics are used to elucidate hydrocarbon mixture transport through a realistic molecular model of kerogen [Bousige, C.; et al. Nat. Mater. 2016, 15, 576]. Owing to strong adsorption effects, velocity cross-correlations between the mixture components and between molecules of the same species are shown to be negligible. This allows estimation of each component permeance from its self-diffusivity, which can be obtained from single-component data. These permeances are found to scale with the reciprocal of the alkane length and decrease with the number of adsorbed molecules following a simple free volume theory, therefore allowing mixture transport prediction as a function of the amount of trapped fluid.
Abstract Image
}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.6b01684}, author = {Ama{\"e}l Obliger and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Benoit A. Coasne} } @article {137, title = {Fracture toughness of calcium{\textendash}silicate{\textendash}hydrate from molecular dynamics simulations}, journal = {Journal of Non-Crystalline Solids}, volume = {419}, year = {2015}, month = {Jul-01-2015}, pages = {58 - 64}, abstract = {

Concrete is the most widely manufactured material in the world. Its binding phase, calcium\–silicate\–hydrate (C\–S\–H), is responsible for its mechanical properties and has an atomic structure fairly similar to that of usual calcium silicate glasses, which makes it appealing to study this material with tools and theories traditionally used for non-crystalline solids. Here, following this idea, we use molecular dynamics simulations to evaluate the fracture toughness of C\–S\–H, inaccessible experimentally. This allows us to discuss the brittleness of the material at the atomic scale. We show that, at this scale, C\–S\–H breaks in a ductile way, which prevents one from using methods based on linear elastic fracture mechanics. Knowledge of the fracture properties of C\–S\–H at the atomic scale opens the way for an upscaling approach to the design of tougher cement paste, which would allow for the design of slender environment-friendly infrastructures, requiring less material.

}, issn = {00223093}, doi = {10.1016/j.jnoncrysol.2015.03.031}, author = {Mathieu Bauchy and Hadrien Laubie and Mohammad Javad Abdolhosseini Qomi and Christian G. Hoover and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {313, title = {Fl{\"u}gge{\textquoteright}s Conjecture: Dissipation- versus Deflection-Induced Pavement{\textendash}Vehicle Interactions}, journal = {Journal of Engineering Mechanics}, volume = {140}, year = {2014}, month = {Aug-2014}, pages = {Article Number: 04014053 }, abstract = {

The dissipation occurring below a moving tire in steady-state conditions in contact with a viscoelastic pavement is expressed using two different reference frames: a fixed observer attached to the pavement and a moving observer attached to the pavement\–tire contact surface. The first approach is commonly referred to as dissipation-induced pavement\–vehicle interaction (PVI), the second as deflection-induced PVI. Based on the principle of frame independence, it is shown that both approaches are strictly equal, from a thermodynamic point of view, and thus predict the same amount of dissipated energy. This equivalence is illustrated through application to two pavement systems: a viscoelastic beam and a viscoelastic plate both resting on an elastic foundation. The amount of dissipated energy in the pavement structure needs to be supplied by the vehicle to maintain constant speed, thus contributing to the rolling resistance, associated excess fuel consumption, and greenhouse gas emissions. The model here proposed can be used to quantify the dissipated energy and contribute to the development of engineering methods for the sustainable design of pavements.

}, issn = {0733-9399}, doi = {10.1061/(ASCE)EM.1943-7889.0000754}, author = {Arghavan Louhghalam and Mehdi Akbarian and Franz-Josef Ulm} } @article {301, title = {Fracture scaling relations for scratch tests of axisymmetric shape}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {60}, year = {2012}, month = {Mar-2012}, pages = {379 - 390}, abstract = {

Scratch testing and scratch test analysis continues to gain momentum in Applied Mechanics, due to the possibility offered by this method to assess fracture properties at very fine scales. In this paper, we derive general scratch force scaling relations for axisymmetric scratch probes defined by single variable monomial functions. These relations are used to define fracture criteria with and without consideration of the development of shear stresses at the probe\–material interface. The approach is illustrated for common scratch probe geometries: conical probe, flat punch, and hemi-spherical probe. Application of the proposed method to micro-scratch tests on two materials (an aluminum alloy and a thermoplastic polymer) using a Rockwell probe (a conical probe ending in a hemi-spherical shape) illustrates the versatility of the approach: First, the scratch force-depth scaling relations provide a means to determine the degree of the homogeneous function characterizing the scratch probe. Second, the fracture criteria enable an experimental assessment of the fracture toughness. The good agreement between the fracture toughness determined by scratching and values reported in the open literature show the potential of the proposed method for determining fracture properties of materials at even smaller scales.

Fig.1. Axisymmetric scratch probe geometry

Fig.2. Scaling of the normalized fracture force and the perimeter function, β, of the{\textellipsis}

Fig.3. Forces in three scratch tests on aluminum alloy 2024 (AA 2024) obtained with a{\textellipsis}

Fig.4. Fracture toughness determination from the scratch response of AA 2024 with a{\textellipsis}

Fig.5. Response and analysis of three scratch tests on a thermoplastic polymer (Delrin{\textellipsis}

}, issn = {00225096}, doi = {10.1016/j.jmps.2011.12.009}, author = {Ange-Therese Akono and Franz-Josef Ulm} }