Investigation on the Mechanical Properties of Polycrystalline Mg Using Molecular Dynamics Simulation

Magnesium(Mg)and its composites have been widely used in different fields,but the mechanical properties and deformation mechanisms of polycrystalline Mg(polyMg)at the atomic scale are poorly understood.In this paper,the effects of grain size,temperature,and strain rate on the tensile properties of polyMg are explored and discussed by theMolecular dynamics(MD)simulation method.The calculated results showed that there exists a critical grain size of 10 nm for the mechanical properties of polyMg.The flow stress decreases with the increase of grain size if the average grain size is larger than 10 nm,which shows the Hall-Petch effect,and the deformation mechanism of large grain-sized polyMg is mainly dominated by the movement of dislocations.When the average grain size is less than 10 nm,it shows the reverse Hall-Petch effect that the flow stress decreases with the decrease of grain size,and the deformation mode of polyMg with small grain-size is the movement and deformation of atoms at the grain boundary.Due to the more active motion of atoms as the system temperature increases,the material can easily reach the plastic stage under tensile loading,and the mechanical properties of polyMg decrease at high temperatures.The strain rate has a hardening effect on the properties of composite.Based on our calculated results,it can provide theoretical guidance for the applications of Mg metal and Mg matrix composites.

Influence of Functionalization on the Structural and Mechanical Properties of Graphene

Molecular dynamics simulations were applied in order to calculate the Young’s modulus of graphene functionalized with carboxyl,hydroxyl,carbonyl,hydrogen,methyl,and ethyl groups.The influence of the grafting density with percentages of 3,5,7,and 10%and the type of distribution as a single cluster or several small clusters were also studied.The results show that the elastic modulus is dependent on the type of functional groups.The increasing coverage density also evidenced a decrease of the Young’s modulus,and the organization of functional groups as single cluster showed a lesser impact than for several small clusters.Furthermore,the bond length and angle distribution probability analyses reveal that lengths and angles are affected with increasing functionalization suggesting more out-of-plane displacements of the carbon atoms within the graphene structure.

Probing the compound effect of spatially varying intrinsic defects and doping on mechanical properties of hybrid graphene monolayers

Doping in pristine 2 D materials brings about the advantage of modulating wide range of mechanical properties simultaneously.However,intrinsic defects(such as Stone-Wales and nanopore) in such hybrid materials are inevitable due to complex manu facturing and synthesis processes.Besides that,de fects and irregularities can be intentionally induced in a pristine nanostructure for multi-synchronous modulation of various multi-functional properties.Whatever the case may be,in order to realistically analyse a doped graphene sheet,it is of utmost importance to investigate the compound effect of doping and defects in such 2 D monolayers.Here we present a molecular dynamics based investigation for probing mechanical properties(such as Young’s modulus,post-elastic behaviour,failure strength and strain)of doped graphene(C14 and Si) coupling the effect of inevitable defects.Spatial sensitivity of defect and doping are systematically analyzed considering different rational instances.The study reveals the effects of individual defects and doping along with their possible compounded influences on the failure stress,failure strain,Young’s modulus and constitutive relations beyond the elastic regime.Such detailed mechanical characterization under the practically relevant compound effects would allow us to access the viability of adopting doped graphene in various multifunctional nanoelectromechanical devices and systems in a realistic situation.

EFFECTS OF SI, N AND B DOPING ON THE MECHANICAL PROPERTIES OF GRAPHENE SHEETS

Molecular dynamics （MD） simulations were performed to stretch the rectangular graphene sheets doped with silicon, nitrogen or boron atoms. Young＇s modulus, ultimate stress （strain） and energy absorption were measured for the graphene sheets with the doping concentration （DC） ranging from 0 to 5%. The emphasis was placed on the distinct effects of each individual dopant on the fundamental mechanical properties of graphene. The results indicated that incorpo- rating the dopants into graphene led to an almost linear decrease in Young＇s modulus. Monotonic reductions in ultimate strength, ultimate strain and energy absorption were also observed. Such doping effects were found to be most significant for silicon, less pronounced for boron, and small or negligible for nitrogen. The outputs provide an important guidance for the development and optimization of novel nanoscale devices, and facilitate the development of graphene-based M/NEMS.

Molecular dynamics simulations of mechanical properties of epoxy-amine:Cross-linker type and degree of conversion effects

Molecular dynamics(MD)simulations are conducted to study the thermo-mechanical properties of a family of thermosetting epoxy-amines.The crosslinked epoxy resin EPON862 with a series of cross-linkers is built and simulated under the polymer consistent force field(PCFF).Three types of curing agents(rigidity1,3-phenylenediamine(1,3-P),4,4-diaminodiphenylmethane(DDM),and phenol-formaldehyde-ethylenediamine(PFE))with different numbers of active sites are selected in the simulations.We focus on the effects of the cross-linkers on thermo-mechanical properties such as density,glass transition temperature(T_(g)),elastic constants,and strength.Our simulations show a significant increase in the Tg,Young’s modulus and yield stress with the increase in the degree of conversion.The simulation results reveal that the mechanical properties of thermosetting polymers are strongly dependent on the molecular structures of the cross-linker and network topological properties,such as end-to-end distance,crosslinking density and degree of conversion.

Mechanical properties and thermal conductivity of pristine and functionalized carbon nanotube reinforced metallic glass composites:A molecular dynamics approach

This work uses the molecular dynamics approach to study the effects of functionalization of carbon nanotubes(CNTs)on the mechanical properties of Cu64Zr36 metallic glass(MG).Three types of functional groups,carboxylic,vinyl and ester were used.The effect of CNT volume fraction(Vf)and the number of functional groups attached to CNT,on the mechanical properties and thermal conductivity of CNT-MG composites was analysed using Biovia Materials Studio.At lower values of Vf(from 0 to 5%),the percentage increase in Young’s modulus was approximately 66%.As the value of Vf was increased further(from 5 to 12%),the rate of increase in Young’s modulus was reduced to 16%.The thermal conductivity was found to increase from 1.52 W/mK at Vf?0%to 5.88 W/mK at Vf?12%,thus giving an increase of approximately 286%.Functionalization of SWCNT reduced the thermal conductivity of the SWCNT-MG composites.

Molecular Dynamics Study of Collagen Fibrils: Relation between Mechanical Properties and Molecular Chirality

Collagen is a basic biopolymer usually found in animal bodies, but its mechanical property and behavior are not sufficiently understood so as to apply to effective regenerative medicine and so on. Since the collagen material is composed of many hierarchical structures from atomistic level to tissue or organ level, we need to well understand fundamental and atomistic mechanism of the collagen in mechanical response. First, we approach at exactly atomistic level by using all-atom modeling of tropocollagen (TC) molecule, which is a basic structural unit of the collagen. We perform molecular dynamics (MD) simulations concerning tensile loading of a single TC model. The main nature of elastic (often superelastic) behavior and the dependency on temperature and size are discussed. Then, to aim at coarse-graining of atomic configuration into some bundle structure of TC molecules (TC fibril), as a model of higher collagen structure, we construct a kind of mesoscopic model by adopting a simulation framework of beads-spring model which is ordinarily used in polymer simulation. Tensile or compression simulation to the fibril model reveals that the dependency of yield or buckling limit on the number of TCs in the model. Also, we compare the models with various molecular orientations in winding process of initial spiral of TC. The results are analyzed geometrically and it shows that characteristic orientational change of molecules increases or decreases depending on the direction and magnitude of longitudinal strain.

Molecular Dynamics Study on Mechanical Properties in the Structure of Self-Assembled Quantum Dot

Stress and strain in the structure of self-assembled quantum dots constructed in the Ge/Si(001) system is calculated by using molecular dynamics simulation. Pyramidal hut cluster composed of Ge crystal with {105} facets surfaces observed in the early growth stage are computationally modeled. We calculate atomic stress and strain in relaxed pyramidal structure. Atomic stress for triplet of atoms is approximately defined as an average value of pairwise (virial) quantity inside triplet, which is the product of vectors between each two atoms. Atomic strain by means of atomic strain measure (ASM) which is formulated on the Green’s definition of continuum strain. We find the stress (strain) relaxation in pyramidal structure and stress (strain) concentration in the edge of pyramidal structure. We discuss size dependency of stress and strain distribution in pyramidal structure. The relationship between hydrostatic stress and atomic volumetric strain is basically linear for all models, but for the surface of pyramidal structure and Ge-Si interface. This means that there is a reasonable correlation between atomic stress proposed in the present study and atomic strain measure, ASM.

Molecular Dynamics Simulation of Mechanical Properties for α-SiO₂Crystal

The mechanical properties of the α-SiO2 crystal are studied by molecular dynamics method with Tersoff potential function. The results show that the α-SiO2 crystal goes through elastic deformation, plastic deformation and fracture deformation in the process of uniaxial loading at room temperature. The α-SiO2 is from crystal phase transformation to amorphous phase in plastic deformation. And also by studying the influence of temperature on the tensile mechanical properties of α-SiO2, it finds that the yield strength and elastic modulus of α-SiO2 decrease gradually as the temperature increases. Moreover, the higher the temperature, the lower the fracture stress and fracture strain;the α-SiO2 crystal is easy to be broke under high temperature uniaxial loading. And it also finds that the crack is able to decrease the mechanical properties of α-SiO2 crystal.

Mechanical Properties of Ni-Coated Single Graphene Sheet and Their Embedded Aluminum Matrix Composites

The effects of Ni coating on the mechanical behaviors of single graphene sheet and their embedded Al matrix composites under axial tension are investigated using molecular dynamics （MD） simulation method. The results show that the Young＇s moduli and tensile strength of graphene obviously decrease after Ni coating. The results also show that the mechanical properties of Al matrix can be obviously increased by embedding a single graphene sheet. From the simulation, we also find that the Young＇s modulus and tensile strength of the Ni-coated graphene/Al composite is obviously larger than those of the uncoated graphene/Al composite. The increased magnitude of the Young＇s modulus and tensile strength of graphene/Al composite are 52.27% and 32.32% at 0.01 K, respectively, due to Ni coating. By exploring the effects of temperature on the mechanical properties of single graphene sheet and their embedded Al matrix composites, it is found that the higher temperature leads to the lower critical strain and tensile strength.

Molecular dynamics investigation of mechanical properties of single-layer phagraphene

Phagraphene is a very attractive two-dimensional (2D) full carbon allotrope with very interesting mechanical, electronic, optical, and thermal properties. The objective of this study is to investigate the mechanical properties of this new graphene like 2D material. In this work, mechanical properties of phagraphene have been studied not only in the defect-free form, but also with the critical defect of line cracks, using the classical molecular dynamics simulations. Our study shows that the pristine phagraphene in zigzag direction experience a ductile behavior under uniaxial tensile loading and the nanosheet in this direction are less sensitive to temperature changes as compared to the armchair direction. We studied different crack lengths to explore the in fluence of defects on the mechanical properties of phagraphene. We also investigated the temperature effect on the mechanical properties of pristine and defective phagraphene. Our classical atomistic simulation results confirm that larger cracks can reduce the strength of the phagraphene. Moreover, it was shown the temperature has a considerable weakening effect on the tensile strength of phagraphene. The results of this study may be useful for the design of nano-devices using the phagraphene.

Effects of Strain Rate,Temperature and Grain Size on the Mechanical Properties and Microstructure Evolutions of Polycrystalline Nickel Nanowires:A Molecular Dynamics Simulation

Through molecular dynamics（MD） simulation, the dependencies of temperature, grain size and strain rate on the mechanical properties were studied. The simulation results demonstrated that the strain rate from 0.05 to 2 ns–1 affected the Young＇s modulus of nickel nanowires slightly, whereas the yield stress increased. The Young＇s modulus decreased approximately linearly; however, the yield stress firstly increased and subsequently dropped as the temperature increased. The Young＇s modulus and yield stress increased as the mean grain size increased from 2.66 to 6.72 nm. Moreover, certain efforts have been made in the microstructure evolution with mechanical properties association under uniaxial tension. Certain phenomena such as the formation of twin structures, which were found in nanowires with larger grain size at higher strain rate and lower temperature, as well as the movement of grain boundaries and dislocation, were detected and discussed in detail. The results demonstrated that the plastic deformation was mainly accommodated by the motion of grain boundaries for smaller grain size. However, for larger grain size, the formations of stacking faults and twins were the main mechanisms of plastic deformation in the polycrystalline nickel nanowire.

Mechanical property and deformation mechanism of gold nanowire with non-uniform distribution of twinned boundaries:A molecular dynamics simulation study

The mechanical property and deformation mechanism of twinned gold nanowire with non-uniform distribution of twinned boundaries(TBs)are studied by the molecular dynamics(MD)method.It is found that the twin boundary spacing(TBS)has a great effect on the strength and plasticity of the nanowires with uniform distribution of TBs.And the strength enhances with the decrease of TBS,while its plasticity declines.For the nanowires with non-uniform distribution of TBs,the differences in distribution among different TBSs have little effect on the Young's modulus or strength,and the compromise in strength appears.But the differences have a remarkable effect on the plasticity of twinned gold nanowire.The twinned gold nanowire with higher local symmetry ratio has better plasticity.The initial dislocations always form in the largest TBS and the fracture always appears at or near the twin boundaries adjacent to the smallest TBS.Some simulation results are consistent with the experimental results.

Structure and Mechanical Behavior of Cellulose Nanofiber and Micro-Fibrils by Molecular Dynamics Simulation

Cellulose nanofiber (CNF) and CNF micro-fibrils (CNF-MFs) are computationally modeled by molecular dynamics with united atom (UA) methodology of polymers. Structural stability and mechanical properties of these materials are focused on. Diffusion coefficient decreases with increase of the number of shells in CNF-MF. The structure of CNF-MFs with crystalline alignment is totally stabilized with twist which is an accumulation of torsion angles at Glycosidic bonds between monomers inside CNFs. Unique fiber drawing simulation, where a single CNF fiber is taken out of CNF-MF structure, is first conducted. The CNF fiber which is drawn out stretches up to relatively large strain, with linear increase of tensile stress. The computation results show that, the larger the number of shell structure of CNF-MF is, the larger the stretch and the stress of drawn fibers are.

A Modifi ed Molecular Structure Mechanics Method for Analysis of Graphene

Based on molecular mechanics and the deformation characteristics of the atomic lattice structure of graphene, a modifi ed molecular structure mechanics method was developed to improve the original one, that is, the semi-rigid connections were used to model the bond angle variations between the C-Cbonds in graphene. The simulated results show that the equivalent space frame model with semi-rigid connections for graphene proposed in this article is a simple, efficient, and accurate model to evaluate the equivalent elastic properties of graphene. Though the present computational model of the semi-rigid connected space frame is only applied to characterize the mechanical behaviors of the space lattices of graphene, it has more potential applications in the static and dynamic analyses of graphene and other nanomaterials.

POLYMER NETWORKS BY MOLECULAR DYNAMICS SIMULATION: FORMATION, THERMAL, STRUCTURAL AND MECHANICAL PROPERTIES

A molecular dynamics simulation method is presented and used in the study of the formation of polymer networks. We study the formation of networks representing the methylene repeating units as united atoms. The network formation is accomplished by cross-linking polymer chains with dedicated functional end groups. The simulations reveal that during the cross-linking process, initially branched molecules are formed before the gel point; approaching the gel point, larger branched entities are formed through integration of smaller branched molecules, and at the gel point a network spanning the simulation box is obtained; beyond the gel point the network continues to grow through the addition of the remaining molecules of the sol phase onto the gel （the network）; the final completion of the reaction occurs by intra-network connection of dangling ends onto unsaturated cross-linkers. The conformational properties of the strands in the undeformed network are found to be very similar with the conformational properties of the chains before cross-linking. The uniaxial deformation of the formed networks is investigated and the modulus determined from the stress-strain curves shows reciprocal scaling with the precursor chain length for networks formed from sufficiently large precursor chains （N≥ 20）.

Effect of Vacancy Defects on the Young＇s Modulus and Fracture Strength of Graphene： A Molecular Dynamics Study

The Young＇s modulus of graphene with various rectangular and circular vacancy defects is investigated by molecular dynamics simulation. By comparing with the results calculated from an effective spring model, it is demonstrated that the Young＇s modulus of graphene is largely correlated to the size of vacancy defects perpendicular to the stretching direction. And a linear reduction of Young＇s modulus with the increasing concentration of monoatomic-vacancy defects （Le., the slope of =0.03） is also observed. The fracture behavior of graphene, including the fracture strength, crack initiation and propagation are then studied by the molecular dynamics simulation, the effective spring model, and the quantized fracture mechanics. The blunting effect of vacancy edges is demonstrated, and the characterized crack tip radius of 4.44 A is observed.

Simulation on microstructure evolution and mechanical properties of Mg-Y alloys: Effect of trace Y

The influence of trace Y on the microstructure evolution and mechanical properties of Mg_(100−x)Y_(x)(x=0.25,_(0.75),1.5,3,4,5,at.%)alloys during solidification process was investigated via molecular dynamics(MD)simulations.The results show that the Mg_(100−x)Y_(x) alloys are mainly characterized by a face-centered cubic(FCC)crystal structure;this is different from pure metal Mg,which exhibits a hexagonal close packed(HCP)structure at room temperature.Among these alloys,Mg_(99.25)Y_(0.75) has a larger proportion of FCC cluster structures,with the highest fraction reaching 56.65%.As the content of the Y increases up to 5 at.%(Mg95Y5 alloy),the amount of amorphous structures increases.The mechanical properties of the Mg_(100−x)Y_(x) alloys are closely related to their microstructures.The Mg_(99.25)Y_(0.75) and Mg_(97)Y_(3) alloys exhibit the highest yield strengths of 1.86 and 1.90 GPa,respectively.The deformation mechanism of the Mg−Y alloys is described at the atomic level,and it is found that a difference in the FCC proportion caused by different Y contents leads to distinct deformation mechanisms.

Effects of tilt interface boundary on mechanical properties of Cu/Ni nanoscale metallic multilayer composites

The effect of tilt interfaces and layer thickness of Cu/Ni multilayer nanowires on the deformation mechanism are investigated by molecular dynamics simulations. The results indicate that the plasticity of the sample with a 45° tilt angle is much better than the others. The yield stress is found to decrease with increasing the tilt angle and it reaches its lowest value at 33°. Then as the tilt angle continues to increase, the yield strength increases. Furthermore, the studies show that with the decrease of layer thickness, the yield strength gradually decreases. The study also reveals that these different deformation behaviors are associated with the glide of dislocation.