ATOMISTIC/CONTINUUM SIMULATION OF INTERFACIAL FRACTURE PART Ⅱ:ATOMISTIC/DISLOCATION/CONTINUUM SIMULATION

Coupled atomistic/dislocation/continuum simulation of interfacial fracture is performed in this paper.The model consists of a nanoscopic core made by atomistic assembly and a surrounding elastic continuum with discrete dislocations. Atomistic dislocations nucleate from the crack tip and move to the continuum layer where they glide according to the dislocation dynamics curve.An atoms/continuum overlapping belt is devised to facilitate the transition between the two scales.The continuum constraint on the atomic assembly is imposed through the mechanics at- mosphere along the overlapping belt.Transmissions of mechanics parameters such as displacements,stresses,masses and momenta across the belt are realized.The present model allows us to explore interfacial fracture processes under different mode mixity.The effect of atomistic zigzag interface on the fracture process is revealed:it hinders dislocation emission from the crack tip,especially under high mode mixity.

ATOMISTIC/CONTINUUM SIMULATION OF INTERFACIAL FRACTURE——PART Ⅰ: ATOMISTIC SIMULATION

The phenomenon of interfacial fracture, as manifested by atom- istic cleavage, debonding and dislocation emission, provides a challenge for combined atomistic-continuum analysis. As a precursor for fully coupled atomistic-continuum simulation of interfacial fracture, we focus here on the atomistic behavior within a nanoscopic core surrounding the crack tip. The inter-atomic potential under Em- bedded Atom Method is recapitulated to form an essential framework of atomistic simulation. The calculations are performed for a side-cracked disc configuration un- der a remote K field loading. It is revealed that a critical loading rate defines the brittle-to-ductile transition of homogeneous materials. We further observe that the near tip mode mixity dictates the nanoscopic profile near an interfacial crack tip. A zigzag interface structure is simulated which plays a significant role in the dislocation emission from an interfacial crack tip, as will be explored in the second part of this investigation.

Propagation Properties of Shock Waves in Polyurethane Foam based on Atomistic Simulations

Porous materials are widely used in the field of protection because of their excellent energy absorption characteristics.In this work,a series of polyurethane microscopic models are established and the effect of porosity on the shock waves is studied with classical molecular dynamics simulations.Firstly,shock Hugoniot relations for different porosities are obtained,which compare well with the experimental data.The pores collapse and form local stress wave,which results in the complex multi-wave structure of the shock wave.The microstructure analysis shows that the local stress increases and the local velocity decreases gradually during the process of pore collapse to complete compaction.Finally,it leads to stress relaxation and velocity homogenization.The shock stress peaks can be fitted with two exponential functions,and the amplitude of attenuation coefficient decreases with the increase of density.Besides,the pore collapse under shock or non-shock are discussed by the entropy increase rate of the system.The energy is dissipated mainly through the multiple interactions of the waves under shock.The energy is dissipated mainly by the friction between atoms under non-shock.

MicroMagnetic.jl:A Julia package for micromagnetic and atomistic simulations with GPU support

MicroMagnetic.jl is an open-source Julia package for micromagnetic and atomistic simulations.Using the features of the Julia programming language,MicroMagnetic.jl supports CPU and various GPU platforms,including NVIDIA,AMD,Intel,and Apple GPUs.Moreover,MicroMagnetic.jl supports Monte Carlo simulations for atomistic models and implements the nudged-elastic-band method for energy barrier computations.With built-in support for double and single precision modes and a design allowing easy extensibility to add new features,MicroMagnetic.jl provides a versatile toolset for researchers in micromagnetics and atomistic simulations.

Atomistic evaluation of tension–compression asymmetry in nanoscale body-centered-cubic AlCrFeCoNi high-entropy alloy

The tension and compression of face-centered-cubic high-entropy alloy(HEA) nanowires are significantly asymmetric, but the tension–compression asymmetry in nanoscale body-centered-cubic(BCC) HEAs is still unclear. In this study,the tension–compression asymmetry of the BCC Al Cr Fe Co Ni HEA nanowire is investigated using molecular dynamics simulations. The results show a significant asymmetry in both the yield and flow stresses, with BCC HEA nanowire stronger under compression than under tension. The strength asymmetry originates from the completely different deformation mechanisms in tension and compression. In compression, atomic amorphization dominates plastic deformation and contributes to the strengthening, while in tension, deformation twinning prevails and weakens the HEA nanowire.The tension–compression asymmetry exhibits a clear trend of increasing with the increasing nanowire cross-sectional edge length and decreasing temperature. In particular, the compressive strengths along the [001] and [111] crystallographic orientations are stronger than the tensile counterparts, while the [110] crystallographic orientation shows the exactly opposite trend. The dependences of tension–compression asymmetry on the cross-sectional edge length, crystallographic orientation,and temperature are explained in terms of the deformation behavior of HEA nanowire as well as its variations caused by the change in these influential factors. These findings may deepen our understanding of the tension–compression asymmetry of the BCC HEA nanowires.

Adhesive contact:from atomistic model to continuum model

Two types of Lennard-Jones potential are widely used in modeling adhesive contacts. However, the relationships between the parameters of the two types of Lennard-Jones potential are not well defined. This paper employs a self- consistent method to derive the Lennard-Jones surface force law from the interatomic Lennard-Jones potential with emphasis on the relationships between the parameters. The ei^ect of using correct parameters in the adhesion models is demonstrated in single sphere-flat contact via continuum models and an atomistic model. Furthermore, the adhesion hysteresis behaviour is investigated, and the S-shaped force-distance relation is revealed by the atomistic model. It shows that the adhesion hysteresis loop is generated by the jump-to-contact and jump-off-contact, which are illustrated by the S-shaped force-distance curve.

Interface state-based bound states in continuum and below-continuum-resonance modes with high-Q factors in the rotational periodic system

We have introduced a new approach to calculate the orbital angular momentum(OAM)of bound states in continuum(BICs)and below-continuum-resonance(BCR)modes in the rotational periodic system nested inside and outside by transforming the Bloch wave number from the translational periodic system.We extensively classify and study these BICs and BCR modes,which exhibit high-quality(high-Q)factors,in different regions relative to the interface of the system.These BICs and BCR modes with a high-Q factor have been studied in detail based on distinctive structural parameters and scattering theory.The outcomes of this research break the periodic limitation of interface state-based BICs,and realize more and higher symmetry interface state-based BICs and BCR modes.Moreover,we can control the region where light is captured by adjusting the frequency,and show that the Q factor of BICs is more closely related to the ordinal number of rings and the rotational symmetry number of the system.

Nanoscale finite element models for vibrations of single-walled carbon nanotubes: atomistic versus continuum

By the atomistic and continuum finite element models, the free vibration behavior of single-walled carbon nanotubes （SWCNTs） is studied. In the atomistic finite element model, the bonds and atoms are modeled by the beam and point mass elements, respectively. The molecular mechanics is linked to structural mechanics to determine the elastic properties of the mentioned beam elements. In the continuum finite element approach, by neglecting the discrete nature of the atomic structure of the nanotubes, they are modeled with shell elements. By both models, the natural frequencies of SWCNTs are computed, and the effects of the geometrical parameters, the atomic structure, and the boundary conditions are investigated. The accuracy of the utilized methods is verified in comparison with molecular dynamic simulations. The molecular structural model leads to more reliable results, especially for lower aspect ratios. The present analysis provides valuable information about application of continuum models in the investigation of the mechanical behaviors of nanotubes.

Using fracture-based continuum modeling of coupled geomechanical-hydrological processes for numerical simulation of hydraulic fracturing

This paper describes numerical simulation of hydraulic fracturing using fracture-based continuum modeling(FBCM)of coupled geomechanical-hydrological processes to evaluate a technique for high-density fracturing and fracture caging.The simulations are innovative because of modeling discrete fractures explicitly in continuum analysis.A key advantage of FBCM is that fracture initiation and propagation are modeled explicitly without changing the domain grid(i.e.no re-meshing).Further,multiple realizations of a preexisting fracture distribution can be analyzed using the same domain grid.The simulated hydraulic fracturing technique consists of pressurizing multiple wells simultaneously:initially without permeating fluids into the rock,to seed fractures uniformly and at high density in the wall rock of the wells;followed by fluid injection to propagate the seeded fracture density hydraulically.FBCM combines the ease of continuum modeling with the potential accuracy of modeling discrete fractures and fracturing explicitly.Fractures are modeled as piecewise planar based on intersections with domain elements;fracture geometry stored as continuum properties is used to calculate parameters needed to model individual fractures;and rock behavior is modeled through tensorial aggregation of the behavior of discrete fractures and unfractured rock.Simulations are presented for previously unfractured rock and for rock with preexisting fractures of horizontal,shallow-dipping,steeply dipping,or vertical orientation.Simulations of a single-well model are used to determine the pattern and spacing for a multiple-well design.The results illustrate high-density fracturing and fracture caging through simultaneous fluid injection in multiple wells:for previously unfractured rock or rock with preexisting shallow-dipping or horizontal fractures,and in situ vertical compressive stress greater than horizontal.If preexisting fractures are steeply dipping or vertical,and considering the same in situ stress condition,well pressurization without fluid permeation appears to be the only practical way to induce new fractures and contain fracturing within the target domain.

Enhancing the Goos-Hänchen shift based on quasi-bound states in the continuum through material asymmetric dielectric compound gratings

Quasi-bound state in the continuum(QBIC)resonance is gradually attracting attention and being applied in Goos-Hänchen(GH)shift enhancement due to its high quality(Q)factor and superior optical confinement.Currently,symmetry-protected QBIC resonance is often achieved by breaking the geometric symmetry,but few cases are achieved by breaking the material symmetry.This paper proposes a dielectric compound grating to achieve a high Q factor and high-reflection symmetry-protectede QBIC resonance based on material asymmetry.Theoretical calculations show that the symmetry-protected QBIC resonance achieved by material asymmetry can significantly increase the GH shift up to-980 times the resonance wavelength,and the maximum GH shift is located at the reflection peak with unity reflectance.This paper provides a theoretical basis for designing and fabricating high-performance GH shift tunable metasurfaces/dielectric gratings in the future.

Static-to-kinematic modeling and experimental validation of tendon-driven quasi continuum manipulators with nonconstant subsegment stiffness

Continuum robots with high flexibility and compliance have the capability to operate in confined and cluttered environments. To enhance the load capacity while maintaining robot dexterity, we propose a novel non-constant subsegment stiffness structure for tendon-driven quasi continuum robots(TDQCRs) comprising rigid-flexible coupling subsegments.Aiming at real-time control applications, we present a novel static-to-kinematic modeling approach to gain a comprehensive understanding of the TDQCR model. The analytical subsegment-based kinematics for the multisection manipulator is derived based on screw theory and product of exponentials formula, and the static model considering gravity loading,actuation loading, and robot constitutive laws is established. Additionally, the effect of tension attenuation caused by routing channel friction is considered in the robot statics, resulting in improved model accuracy. The root-mean-square error between the outputs of the static model and the experimental system is less than 1.63% of the arm length(0.5 m). By employing the proposed static model, a mapping of bending angles between the configuration space and the subsegment space is established. Furthermore, motion control experiments are conducted on our TDQCR system, and the results demonstrate the effectiveness of the static-to-kinematic model.

Nonlinear wave dispersion in monoatomic chains with lumped and distributed masses:discrete and continuum models

The main objective of this paper is to investigate the influence of inertia of nonlinear springs on the dispersion behavior of discrete monoatomic chains with lumped and distributed masses.The developed model can represent the wave propagation problem in a non-homogeneous material consisting of heavy inclusions embedded in a matrix.The inclusions are idealized by lumped masses,and the matrix between adjacent inclusions is modeled by a nonlinear spring with distributed masses.Additionally,the model is capable of depicting the wave propagation in bi-material bars,wherein the first material is represented by a rigid particle and the second one is represented by a nonlinear spring with distributed masses.The discrete model of the nonlinear monoatomic chain with lumped and distributed masses is first considered,and a closed-form expression of the dispersion relation is obtained by the second-order Lindstedt-Poincare method(LPM).Next,a continuum model for the nonlinear monoatomic chain is derived directly from its discrete lattice model by a suitable continualization technique.The subsequent use of the second-order method of multiple scales(MMS)facilitates the derivation of the corresponding nonlinear dispersion relation in a closed form.The novelties of the present study consist of(i)considering the inertia of nonlinear springs on the dispersion behavior of the discrete mass-spring chains;(ii)developing the second-order LPM for the wave propagation in the discrete chains;and(iii)deriving a continuum model for the nonlinear monoatomic chains with lumped and distributed masses.Finally,a parametric study is conducted to examine the effects of the design parameters and the distributed spring mass on the nonlinear dispersion relations and phase velocities obtained from both the discrete and continuum models.These parameters include the ratio of the spring mass to the lumped mass,the nonlinear stiffness coefficient of the spring,and the wave amplitude.

Human immunodeficiency virus cascade–continuum of care stages and outcomes in a hospital in southern Brazil

BACKGROUND The human immunodeficiency virus(HIV)continuum of care cascade illustrates the 90-90-90 goals defined by the Joint United Nations Program on HIV/acquired immunodeficiency syndrome(UNAIDS).The care cascade includes the following five steps:Diagnosis,linkage to care,retention in care,adherence to antiretroviral therapy(ART),and viral suppression.AIM To elaborate the HIV cascade of patients diagnosed with HIV at the Nossa Senhora da Conceição Hospital(HNSC)and to determine possible local causes for the loss of patients between each step of the cascade.METHODS This retrospective cohort study included patients diagnosed with HIV infection from January 1,2015 to December 31,2016 and followed up until July 31,2019.The data were analyzed by IBM SPSS software version 25,and Poisson regression with simple robust variance was used to analyze variables in relation to each step of the cascade.Variables with P<0.20 were included in multivariable analysis,and P<0.05 was considered significant.Pearson’sχ^(2) test was used to compare the groups of patients followed up at the HNSC and those followed up at other sites.RESULTS The results were lower than those expected by the UNAIDS,with 94%of patients linked,91%retained,81%adhering to ART,and 84%in viral suppression.Age and site of follow-up were the variables with the highest statistical significance.A comparison showed that the cascade of patients from the HNSC had superior results than outpatients,with a significant difference in the last step of the cascade.CONCLUSION The specialized and continued care provided at the HNSC was associated with better results and was closer to the goals set by the UNAIDS.The development of the HIV cascade using local data allowed for the stratification and evaluation of risk factors associated with the losses occurring between each step of the cascade.

A Posteriori Error Estimate and Adaptive Mesh Refinement Algorithm for Atomistic/Continuum Coupling with Finite Range Interactions in Two Dimensions

In this paper,we develop the residual based a posteriori error estimates and the corresponding adaptive mesh refinement algorithm for atomistic/continuum(a/c)coupling with finite range interactions in two dimensions.We have systematically derived a new explicitly computable stress tensor formula for finite range in-teractions.In particular,we use the geometric reconstruction based consistent atomistic/continuum(GRAC)coupling scheme,which is quasi-optimal if the continuum model is discretized by P1 finite elements.The numerical results of the adaptive mesh refinement algorithm is consistent with the quasi-optimal a priori error estimates.

Atomistic simulation of thermal effects and defect structures during nanomachining of copper

Molecular dynamics （MD） simulations of monocrystalline copper （100） surface during nanomachining process were performed based on a new 3D simulation model. The material removal mechanism and system temperature distribution were discussed. The simulation results indicate that the system temperature distribution presents a roughly concentric shape, a steep temperature gradient is observed in diamond cutting tool, and the highest temperature is located in chip. Centrosymmetry parameter method was used to monitor defect structures. Dislocations and vacancies are the two principal types of defect structures. Residual defect structures impose a major change on the workpiece physical properties and machined surface quality. The defect structures in workpiece are temperature dependent. As the temperature increases, the dislocations are mainly mediated from the workpiece surface, while the others are dissociated into point defects. The relatively high cutting speed used in nanomachining results in less defect structures, beneficial to obtain highly machined surface quality.

Primary and secondary modes of deformation twinning in HCP Mg based on atomistic simulations

Deformation twinning, i.e., twin nucleation and twin growth （or twin boundary migration, TBM） activated by impinged basal slip at a symmetrical tilt grain boundary in HCP Mg, was examined with molecular dynamics （MD） simulations. The results show that the {1^-1^-21}-type twinning acts as the most preferential mode of twinning. Once such twins are formed, they are almost ready to grow. The TBM of such twins is led by pure atomic shuffling events. A secondary mode of twinning can also occur in our simulations. The {112^-2} twinning is observed at 10 K as the secondary twin. This secondary mode of twinning shows different energy barriers for nucleation as well as for growth compared with the {1^-1^-21}-type twining. In particular, TBMs in this case is triggered intrinsically by pyramidal slip at its twin boundary.

INVESTIGATION ON APPLICABILITY OF VARIOUS STRESS DEFINITIONS IN ATOMISTIC SIMULATION

How to correctly extract Cauchy stress from the atomistic simulations is a crucial issue in studying the mechanical behaviours of atomic systems, but is still in controversy. In this paper, three typical atomistic simulation examples are used to validate various existing stress definitions. It is found that the classical virial stress fails in predicting the stresses in these examples, because the velocity depends on the choice of the local average volume or the reference frame velocity and other factors. In contrast, the Lagrangian cross-section stress and Lagrangian virial stress are validated by these examples, and the instantaneous Lagrangian atomic stress definition is also proposed for dynamical problems.

Atomistic simulation of free transverse vibration of graphene,hexagonal SiC, and BN nanosheets

Free transverse vibration of monolayer graphene, boron nitride (BN), and silicon carbide (SiC) sheets is investigated by using molecular dynamics finite element method. Eigenfrequencies and eigenmodes of these three sheets in rectangular shape are studied with different aspect ratios with respect to various boundary conditions. It is found that aspect ratios and boundary conditions affect in a similar way on natural frequencies of graphene, BN, and SiC sheets. Natural frequencies in all modes decrease with an increase of the sheet’s size. Graphene exhibits the highest natural frequencies, and SiC sheet possesses the lowest ones. Missing atoms have minor effects on natural frequencies in this study.

ATOMISTIC SIMULATIONS FOR Ni/Al INTERMETALLICS

A large number of Embedded Atom Method (EAM) potentials have been developed for the Ni/Al system. These potentials are compared to a common data base. It is found that there is significant difference in quality in these potentials. One of the potentials has also been extended to represent the properties of hydrogen in Ni/Al intermetallics. This potential describes the solution and migration behavior of hydrogen in both Ni and Al.A number of calculations using the Ni/Al/H potential have been performed. It is found that hydrogen strongly prefers sites in Mi3AI that are surrounded by 6 Ni atoms. Calculations of trapping of hydrogen to a number of grain boundaries in Ni3Al have been performed as a function of hydrogen chemical potential at room temperature. The failure of these bicrystals under tensile stress has been examined and will be compared to the failure of pure Mi3AI boundaries.In order to investigate the potential embrittlement of γ/γ'alloys, trapping of hydrogen to a spherical Hi3Al precipitate in Ni as a function of chemical potential at room temperature has been calculated. It appears that the boundary is not a strong trap for hydrogen, hence embrittlement in these alloys is not primarily due to interactions of hydrogen with the γ/γ'interface. (Supported by the U. S. DOE through Contract DE-AC04-94AL85000.)

Atomistic calculations of surface and interfacial energies of Mg_(17)Al_(12)-Mg system

It is well known that precipitation hardening in magnesium(Mg)alloys is far less effective than in aluminum alloys.Thus,it is important to understand the surface and interfacial structure and energetics between precipitates and matrix.In upscale modeling of magnesium alloys,these energy data are of great significance.In this work,we calculated the surface and interfacial energies of Mg_(17)Al_(12)-Mg system by carefully selecting the surface or interface termination,using atomistic simulations.The results show that,the higher fraction of Mg atoms on the surface,the lower the surface energy of Mg_(17)Al_(12).The interfacial energy of Mg/Mg_(17)Al_(12)was calculated in which the Burgers orientation relationship(OR)was satisfied.It was found that the(011)P|(0002)Mg interface has the lowest interfacial energy(248 mJ/m 2).Because the Burgers OR breaks when{10¯12}twin occurs,which reorients the matrix,the interfacial energy for Mg_(17)Al_(12)and a{10¯12}twin was also calculated.The results show that after twinning,the lowest interfacial energy increases by 244 mJ/m^(2),and the interface becomes highly incoherent due to the change in orientation relationship between Mg_(17)Al_(12)and the matrix.