Molecular Dynamics Simulation Of Deformation Research Articles

The effect of ultrasonic impact treatment on deformation and fracture of electron beam additive manufactured Ti-6Al-4V under uniaxial tension

The results of experimental investigations and molecular dynamics simulation of deformation and fracture of as-built wire-feed EBAM Ti-6Al-4V samples and the samples subjected to ultrasonic impact treatment are presented. Dislocation deformation of the samples under uniaxial tension is shown to occur through propagation of fine and coarse slip bands and shear bands. The ultrasonic impact treatment results in the formation of a 10 μm thick top layer consisting of the nanocrystalline α-Ti, β-Ti and TiO2 phases and an underlying 30–50 μm thick layer, which microstructure is characterized by curved and fragmented α-phase lamellae. As a consequence, dislocation sliding in the ultrasonically treated surface layer of the wire-feed EBAM Ti-6Al-4V samples is hindered. Molecular dynamics simulation has revealed that the grain boundary sliding and twinning are the main deformation mechanisms of the fragmented α-phase lamellae. The migration of the twin boundaries was happened by means of the development of reversible HCP→BCC→HCP transformations. At the prefracture stage the non-crystallographic shear bands nucleating in the underlying layers propagate in the ultrasonically treated surface layer. Intensive deformation inside the shear bands manifests itself in the formation of regions with ductile microrelief on the brittle fracture surface.

Deformation Model of Chains and Networks with Extendable Bonds

The nonlinear stress–strain response of synthetic and biological networks and gels manifested in a monotonic increase of an instantaneous modulus is a result of the nonlinear deformation of individual network strands. To describe this behavior, we develop a nonlinear network deformation model, which relates macroscopic stress–strain response with force-elongation behavior of polymer chains with bending rigidity and extendable bonds. In particular, network mechanical properties such as structural shear modulus, extensibility ratio, and bond deformation modulus are expressed in terms of chain bending constant, Kuhn length, bond elastic constant, and chain sizes in undeformed and fully extended states. The chain deformation model is tested by molecular dynamics simulations of deformation of coarse-grained chains with harmonic and finitely extensible nonlinear elastic (FENE) bonds and is applied to analyze the DNA deformation data. The network model predictions are confirmed by molecular dynamics simulations of diamond networks made of coarse-grained chains and are used to describe experimental data for biological networks of collagen, fibrin, and neurofilaments.

Coarse-grained molecular dynamics simulation of deformation and fracture in polycarbonate: Effect of molar mass and entanglement

Polycarbonate is a promising candidate material for applications as structural element owing to its prominent properties including lightness, strength and toughness. To sustain complex mechanical loadings and resist fracture, it is essential to obtain reliable constitutive relations of deformation. Such constitutive laws that are used for the design and optimization of polymer-based engineering products can be directly obtained from computer simulations. In this study we performed coarse-grained molecular dynamics (CGMD) simulations of polycarbonate to gain a better understanding of the deformation and fracture mechanisms at the molecular level and to clarify its dependency on the molecular structure. In particular, we investigated how the state of entanglement influences the deformation behavior for simulation models with various molar masses. We found a significant effect of the molar mass on the stress-strain curves; i.e. the larger the molar mass is, the larger stress can be reached after yielding, suggesting that molar mass plays a major role in the brittle-ductile transition of polycarbonate.

Coarse-grained molecular dynamics simulation of deformation and fracture in polycarbonate:

Coarse-grained molecular dynamics simulation of deformation and fracture in polycarbonate:

Molecular dynamics simulation of deformation and failure mechanism of kaolinite at different temperatures

Molecular dynamics simulation of deformation and failure mechanism of kaolinite at different temperatures

Coarse-grained molecular dynamics simulation of deformation and fracture in polycarbonate: Effect of loading mode, strain rate, temperature and molar mass

Polycarbonate is one of the most promising polymers having potential application as structural materials owing to its prominent properties including lightness, strength and toughness. While computer-aided design of polymer-based engineering products is demanded, obtaining reliable constitutive relations of deformation and fracture has been a challenge. In this study we performed coarse-grained molecular dynamics (CGMD) simulations of polycarbonate to acquire understanding of deformation and fracture mechanism at the molecular level and estimate yield stress under various loading modes. Inter-particle interactions for the CGMD was constructed based on all-atom MD using the COMPASS model, where molecular structures under strain were included. By tensile simulations of four types of loading, namely uniaxial stress, biaxial stress, uniaxial strain and isotropic stress, we calculated yield stress as a function of strain rate. We obtained two master curves of yield stress-strain rate relation; one for the former two loading types and the other for the latter two. The curves can be used for various temperatures by proper shift factors. We also found the effect of molar mass on stress-strain curves; i.e. the larger the molar mass is the larger stress can be attained after yielding, suggesting brittle-ductile transition with increasing molar mass.

Molecular dynamics simulation of deformation in plates on their oblique impact

The paper presents numerical results of molecular dynamics simulation of deformation in aluminum plates on their oblique impact. Symmetric and asymmetric impacts of the plates are considered. On symmetric impact, the shock waves are symmetric, and on asymmetric impact, the shock waves are asymmetric and their crests assume a vortex form. It is shown that on asymmetric impact, the wavelength is one and a half times larger than that on symmetric impact. The deformation and the atomic structure of the plates in the vicinity of their contact interface after welding are studied. It is shown that the material is melted in a thin layer near the contact interface. Away from the contact interface, single crystal-to-polycrystal transformation takes place.

Molecular dynamics simulation of deformation and fracture of graphene under uniaxial tension

The paper reports on molecular dynamics simulation of deformation and fracture of graphene under uniaxial tension. Dependences of Young’s modulus, critical force and fracture strain on the strain rate, temperature and angle between the tension direction and the graphene lattice are derived. The effect of defects on fracture of graphene is studied.

Molecular dynamics simulation of deformation and fracture of a “copper - molybdenum” nanocomposite plate under uniaxial tension

The paper presents the results of molecular dynamics simulation of deformation and fracture of a “copper - molybdenum” nanocomposite plate under uniaxial tension. It is shown that plastic deformation in shear bands in the copper culminates in the pore formation at the Cu-Mo contact boundary. In the molybdenum, deformation develops through martensite transformation and formation of a new phase.

Molecular dynamics simulation of deformation in SiO2 and Na2O-SiO2 glasses

The atomic scale deformation behavior of SiO 2 and Na 2 O-SiO 2 glasses is studied by Molecular Dynamics (MD) simulation methods. First, the pressure exerted on the simulation box is varied and the effects of isotropic compression and expansion imposed on both glasses are investigated. The SiO 2 glass shows the well-known densification under a compression pressure of 7 GPa or above. In contrast, no densification is observed for the Na 2 O-SiO 2 glass. Next, the shape of the simulation box is varied and the effects of shear deformation exerted on both glasses are investigated. In the case of the SiO 2 glass, the shear deformation brings about an increase in shear stress up to over 4 GPa at first; however, further shearing of the simulation cell results in a decrease in shear stress, accompanied with densification. On the other hand, in the case of Na 2 O-SiO 2 glass, shear stress rises up to about 1.5 GPa at first and the further tilting keeps almost the same value of shear stress, without densification. These calculated results suggest that two different behaviors of atomic scale deformation exist. The exchange of Si-O bonds is the main event occurring in SiO 2 glass during deformation. This phenomenon leads to the densification. In contrast, in the case of Na 2 O-SiO 2 glass, mobile sodium ions can accommodate their positions to relieve stress so that the breaking of Si-O bonds is considerably reduced. This phenomenon displays a behavior similar to plastic flow before fracture appears.

Molecular dynamics simulations of deformation in nanocrystalline Al–Pb alloys

A modified embedded-atom method (MEAM) potential was developed and used for molecular dynamics (MD) straining simulations of Al–Pb alloys with a grain size of 10 nm and Pb content up to 3 at.%. Monte Carlo (MC) simulations done at 300 K indicated that all the Pb is segregated to the grain boundaries in these alloys. As the Pb content increases, partial dislocation nucleation at grain boundaries is suppressed, and the plastic strain is accommodated by mechanisms other than dislocation slip. The increasing Pb content was accompanied by a reduction in the yield and peak stress values.

Molecular dynamics simulation of deformation and failure of nanocrystals of bcc metals

Simulations of uniaxial and hydrostatic tension of Fe and Mo nanocrystal are made by molecular dynamics method. Stress versus strain are obtained while regularities of lattice rearrangement during nanocrystal plastic deformation are considered. Local instability of nanocrystal lattice, which is the cause for transition from elastic to plastic deformation of nanocrystal, is found. It is shown that local shear stresses is a driving force of nanocrystal lattice rearrangements under the conditions of both uniaxial and hydrostatic tension, so, local instability of nanocrystal of bcc metals should be considered as shear instability. Realization of “orthorhombic” path of deformation at 〈1 0 0〉 tension of Mo nanocrystal is specific case of above effect. It is demonstrated that unlike covalent nanocrystal, metallic nanocrystals display “heterogeneous” mechanism of crack nucleation, which essence is that cracks nucleate not in homogeneous elastically deformed lattice but in shear bands or near their boundaries, i.e., after non-homogeneous plastic deformation of nanocrystal.

Cracks and crazes: on calculating the macroscopic fracture energy of glassy polymers from molecular simulations.

We combine molecular dynamics simulations of deformation at the submicron scale with a simple continuum fracture mechanics model for the onset of crack propagation to calculate the macroscopic fracture energy of amorphous glassy polymers. Key ingredients in this multiscale approach are the elastic properties of polymer crazes and the stress at which craze fibrils fail through chain pullout or scission. Our results are in quantitative agreement with dimensionless ratios that describe experimental polymers and their variation with temperature, polymer length, and polymer rigidity.

Yield conditions for deformation of amorphous polymer glasses.

Shear yielding of glassy polymers is usually described in terms of the pressure-dependent Tresca or von Mises yield criteria. We test these criteria against molecular dynamics simulations of deformation in amorphous polymer glasses under triaxial loading conditions that are difficult to realize in experiments. Difficulties and ambiguities in extending several standard definitions of the yield point to triaxial loads are described. Two definitions, the maximum and offset octahedral stresses, are then used to evaluate the yield stress for a wide range of model parameters. In all cases, the onset of shear is consistent with the pressure-modified von Mises criterion, and the pressure coefficient is nearly independent of many parameters. Under triaxial tensile loading, the mode of failure changes to cavitation, and the von Mises criterion no longer applies.