Capturing material toughness by molecular simulation: accounting for large yielding effects and limits

The inherent computational cost of molecular simulations limits their use to the study of nanometric systems with potentially strong size effects. In the case of fracture mechanics, size effects due to yielding at the crack tip can affect strongly the mechanical response of small systems. In this paper we consider two examples: a silica crystal for which yielding is limited to a few atoms at the crack tip, and a nanoporous polymer for which the process zone is about one order of magnitude larger. We perform molecular simulations of fracture of those materials and investigate in particular the system and crack size effects. The simulated systems are periodic with an initial crack. Quasi-static loading is achieved by increasing the system size in the direction orthogonal to the crack while maintaining a constant temperature. As expected, the behaviors of the two materials are significantly different. We show that the behavior of the silica crystal is reasonably well described by the classical framework of linear elastic fracture mechanics (LEFM). Therefore, one can easily upscale engineering fracture properties from molecular simulation results. In contrast, LEFM fails capturing the behavior of the polymer and we propose an alternative analysis based on cohesive crack zone models. We show that with a linear decreasing cohesive law, this alternative approach captures well the behavior of the polymer. Using this cohesive law, one can anticipate the mechanical behavior at larger scale and assess engineering fracture properties. Thus, despite the large yielding of the polymer at the scale of the molecular simulation, the cohesive zone analysis offers a proper upscaling methodology.MIT Energy InitiativeShell Oil CompanySchlumberger Limite

Role of Interfaces in Elasticity and Failure of Clay–Organic Nanocomposites: Toughening upon Interface Weakening?

Synthetic organic-inorganic composites constitute a new class of engineering materials finding applications in an increasing range of fields. The interface between the constituting phases plays a pivotal role in the enhancement of mechanical properties. In exfoliated clay-organic nanocomposites, individual, high aspect ratio clay sheets are dispersed in the organic matrix providing large interfaces and hence efficient stress transfer. In this study, we aim at elucidating molecular-scale reinforcing mechanisms in a series of model clay-organic composite systems by means of reactive molecular simulations. In our models, two possible locations of failure initiation are present: one is the interlayer space of the clay platelet, and the other one is the clay-organic interface. We systematically modify the cohesiveness of the interface and assess how the failure mechanism changes when the different model composites are subjected to a tensile test. Besides a change in the failure mechanism, an increase in the released energy at the interface (meaning an increased overall toughness) are observed upon weakening the interface by bond removal. We propose a theoretical analysis of these results by considering a cohesive law that captures the effect of the interface on the composite mechanics. We suggest an atomistic interpretation of this cohesive law, in particular, how it relates to the degree of bonding at the interface. In a broader perspective, this work sheds light on the importance of the orthogonal behavior of interfaces to nanocomposites.MIT Energy InitiativeSchlumberger LimitedShell Oil CompanyFrench Research National Agency (ANR-11-LABX-0053)French Research National Agency (ANR-11-IDEX-0001-02

Fracture Mechanisms in Organic-Rich Shales: Role of Kerogen

In this work we study role of kerogen in the fracture properties of organic-rich shales and, in particular, in the ductility of shales. The presence of kerogen and clays in shale is known to increase the ductility. We propose here a multiscale approach to develop a fine understanding of shale ductility from the molecular scale. We develop and validate a methodology at the molecular scale that can capture the toughness and ductility of a material. We apply this methodology successfully to a silica polymorph and to a kerogen analog, and we confirm the significant ductility of kerogen. Interestingly the silica-kerogen interface exhibits a similar ductility, which is central for the properties of the heterogeneous shale. Finally, we consider a tentative upscaling considering the pull out phenomenon as a likely mechanism of fracture of the shale

Surface Chemistry and Atomic-Scale Reconstruction of Kerogen–Silica Composites

Interest in gas shale, a novel source rock of natural gas, has increased tremendously in recent years. Better understanding of the kerogen-rock interaction is of crucial importance for efficient gas extraction and, hence, asset management. In this study, we explore the possible chemical bonds between kerogen and silica, one of the most predominant mineral constituents of gas shale, by means of quantum chemistry. Energetically favorable bond formation reactions are found between alcoholic hydroxyl, carboxylate, and aldehyde groups, as well as aliphatic double bonds of kerogen and the silica surface. The performance of a reactive force field was also assessed in a representative set of chemical reactions and found to be satisfactory. The potential impact of bond formation reactions between the two phases on the actual kerogen-silica interface is discussed as a function of the kerogen type, maturity, and density. Finally, a methodology aiming to reconstruct realistic kerogen-silica interfaces is presented