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Abstract In this article, we investigate the creep mechanism of clay at the nanoscale. We conduct the molecular dynamics (MD) modeling of clay samples consisting of hexagonal particles under compression and shear. The MD simulations include oedometer creep, shear creep, direct shear tests, and stress relaxation. The numerical results show that the nanoscale creep mechanism of clay is related to particle rotation, translation, and stacking under different loading conditions. The clay sample under creep shows two types of particle arrangements, that is, the shifted face‐to‐face configuration and the face‐to‐edge configuration. The orientation angle of clay particles is computed to track the rotation of individual particles due to creep. The fabric variation of the clay under creep is characterized by the dihedral angle between the basal particle plane and the x‐y plane and the order parameter. It is found that the factors affecting the microstructure variation of the clay aggregate include stress levels, loading rates, and particle sizes. In the nanoscale shear creep test, the creep process comprises three stages, that is, primary, secondary, and tertiary. The microstructure change during creep depends on the initial alignment of clay particles. The clay creep under a more significant stress level results in a more considerable order parameter and a more orientated clay structure. 

Abstract The atomic‐scale cracking mechanism in clay is vital in discovering the cracking mechanism of clay at the continuum scale in that clay is a nanomaterial. In this article, we investigate mechanisms of modes I and II crack propagations in pyrophyllite and Ca‐montmorillonite with water adsorption through reactive molecular dynamics (MD) with a bond‐order force field. Clay water adsorption is considered by adding water molecules to the clay surface. During the equilibration stage, water adsorption could cause bending deformation of the predefined edge crack region. The relatively small orientating angle of water molecules indicates the formation of hydrogen bonds in the crack propagation process. The peak number density of adsorbed water decreases with the increasing strains. The atomistic structure evolution of the crack tip under loading is analyzed to interpret the nanoscale crack propagation mechanism. The numerical results show that the crack tip first gets blunted with a significant increase in the radius of the curvature of the crack tip and a slight change in crack length. The crack tip blunting is studied by tracking the crack tip opening distance and O–Si–O angle in the tetrahedral Si–O cell in modes I and II cracks. We compare bond‐breaking behaviors between Al–O and Si–O. It is found that Si–O bond breaking is primarily responsible for crack propagation. The critical stress intensity factor and critical energy release rate are determined from MD simulation results. 

Abstract The engineering problems involving clay under non‐isothermal conditions (e.g., geothermal energy harvest, landfill cover system, and nuclear waste disposal) are multiscale and multiphysics by nature. The nanoscale hydrodynamics of clay at elevated temperature is essential in developing a physics‐based multiscale model for clay under non‐isothermal conditions. The nonequilibrium molecular dynamics (NEMD) is a useful tool to study the nanoscale hydrodyndamics of clay. This article presents an NEMD modeling of hydrodynamics of clay nanopores at elevated temperatures. Water flow confined in pyrophyllite and montmorillonite clay nanopores is investigated. The nonequilibrium state is maintained by uniformly exerting an external force on each water molecule. The NEMD simulations have provided a molecular‐scale perspective of temperature effect on clay‐water density, water flow velocity, shear viscosity, clay‐water slip length, hydraulic conductivity, and clay‐water friction coefficient. The numerical results have shown a strong temperature dependence of fluid flow velocity, shear viscosity, clay‐water slip length, and hydraulic conductivity at the nanoscale. We have validated the applicability of cubic law in determining hydraulic conductivity at the nanopore scale at elevated temperatures. It is found from our numerical results that slip clay‐water boundary condition is an essential factor in properly determining nanoscale fluid flow velocity. By numerical examples, we also study the impact of nanopore size and clay layer thickness on the hydrodynamics of the clay‐water system. 

Abstract In this article we formulate a stable computational nonlocal poromechanics model for dynamic analysis of saturated porous media. As a novelty, the stabilization formulation eliminates zero‐energy modes associated with the original multiphase correspondence constitutive models in the coupled nonlocal poromechanics model. The two‐phase stabilization scheme is formulated based on an energy method that incorporates inhomogeneous solid deformation and fluid flow. In this method, the nonlocal formulations of skeleton strain energy and fluid flow dissipation energy equate to their local formulations. The stable coupled nonlocal poromechanics model is solved for dynamic analysis by an implicit time integration scheme. As a new contribution, we validate the coupled stabilization formulation by comparing numerical results with analytical and finite element solutions for one‐dimensional and two‐dimensional dynamic problems in saturated porous media. Numerical examples of dynamic strain localization in saturated porous media are presented to demonstrate the efficacy of the stable coupled poromechanics framework for localized failure under dynamic loads. 

Summary The mechanical and hydraulic properties of unsaturated clay under nonisothermal conditions have practical implications in geotechnical engineering applications such as geothermal energy harvest, landfill cover design, and nuclear waste disposal facilities. The water menisci among clay particles impact the mechanical and hydraulic properties of unsaturated clay. Molecular dynamics (MD) modeling has been proven to be an effective method in investigating clay structures and their hydromechanical behavior at the atomic scale. In this study, we examine the impact of temperature increase on the capillary force and capillary pressure of the partially saturated clay‐water system through high‐performance computing. The water meniscus formed between two parallel clay particles is studied via a full‐scale MD modeling at different elevated temperatures. The numerical results have shown that the temperature increase impacts the capillary force, capillary pressure, and contact angle at the atomic scale. The capillary force on the clay particle obtained from MD simulations is also compared with the results from the macroscopic theory. The full‐scale MD simulation of the partially saturated clay‐water system can not only provide a fundamental understanding of the impact of temperature on the interface physics of such system at the atomic scale, but also has practical implication in formulating physics‐based multiscale models for unsaturated soils by providing interface physical properties of such materials directly through high‐performance computing. 

Summary Geomaterials such as sand and clay are highly heterogeneous multiphase materials. Nonlocality (or a characteristic length scale) in modeling geomaterials based on the continuum theory can be associated with several factors, for instance, the physical interactions of material points within finite distance, the homogenization or smoothing process of material heterogeneity, and the particle or problem size‐dependent mechanical behavior (eg, the thickness of shear bands) of geomaterials. In this article, we formulate a nonlocal elastoplastic constitutive model for geomaterials by adapting a local elastoplastic model for geomaterials at a constant suction through the constitutive correspondence principle of the state‐based peridynamics theory. We numerically implement this nonlocal constitutive model via the classical return‐mapping algorithm of computational plasticity. We first conduct a one‐dimensional compression test of a soil sample at a constant suction through the numerical model with three different values of the nonlocal variable (horizon)δ. We then present a strain localization analysis of a soil sample under the constant suction and plane strain conditions with different nonlocal variables. The numerical results show that the proposed nonlocal model can be used to simulate the inception and propagation of shear banding as well as to capture the thickness of shear bands in geomaterials at a constant suction. 

This paper characterizes nanoscale soil-water retention mechanism of unsaturated clay through molecular dynamics simulation. Series of molecular dynamics simulations of clay at low degrees of saturation were conducted. Soil water was represented by a point cloud through the centre-of-massmethod. Water-air interface area was measured numerically by the alpha shape method. Spatial variation of water number density is characterized and used to determine the adsorbed water layer. The soil-water retention mechanism at the nanoscale was analysed by distinguishing adsorptive pressure and capillary pressure at different mass water contents and considering apparent interface area (water-air interface area per unit water volume).