Mohammad Motalab

Graphene and its elemental analogue: A molecular dynamics view of fracture phenomenon

Abstract Graphene and some graphene like two dimensional materials; hexagonal boron nitride (hBN) and silicene have unique mechanical properties which severely limit the suitability of conventional theories used for common brittle and ductile materials to predict the fracture response of these materials. This study revealed the fracture response of graphene, hBN and silicene nanosheets under different tiny crack lengths by molecular dynamics (MD) simulations using LAMMPS. The useful strength of these two dimensional materials are determined by their fracture toughness. Our study shows a comparative analysis of mechanical properties among the elemental analogues of graphene and suggested that hBN can be a good substitute for graphene in terms of mechanical properties. We have also found that the pre-cracked sheets fail in brittle manner and their failure is governed by the strength of the atomic bonds at the crack tip. The MD prediction of fracture toughness shows significant difference with the fracture toughness determined by Griffths theory of brittle failure which restricts the applicability of Griffiths criterion for these materials in case of nano-cracks. Moreover, the strengths measured in armchair and zigzag directions of nanosheets of these materials implied that the bonds in armchair direction have the stronger capability to resist crack propagation compared to zigzag direction.

Atomistic Representation of Anomalies in the Failure Behaviour of Nanocrystalline Silicene

Silicene, a 2D analogue of graphene, has spurred a tremendous research interest in the scientific community for its unique properties essential for next-generation electronic devices. In this work, for the first time, we present a molecular dynamics (MD) investigation to determine the fracture strength and toughness of nanocrystalline silicene (nc-silicene) sheet of varying grain sizes and pre-existing cracks at room temperature. Our results suggest a transition from an inverse pseudo Hall-Petch to a pseudo Hall-Petch behaviour in nc-silicene at a critical grain size of 17.32 nm. This phenomenon is also prevalent in nanocrystalline graphene. However, nc-silicene with pre-existing cracks exhibits anomalous crack propagation and fracture toughness behaviour. We observed two distinct types of failure mechanisms (crack sensitive and insensitive failure) and devised mechano-physical conditions under which they occur. The most striking outcome is: despite the presence of a pre-existing crack, the crack sensitivity of nc-silicene is found to be dependent on the grain size and their orientations. The calculated Fracture toughness from both Griffith’s theory and MD simulations indicate that the former over-predicts the fracture toughness of nc-silicene. Finally, this study is the first direct comparison of atomistic simulations to the continuum theories to predict the anomalous behaviour in deformation and failure mechanisms of nc-silicene.

Tuning the mechanical properties of silicene nanosheet by auxiliary cracks: a molecular dynamics study

Silicene has become a topic of interest nowadays due to its potential application in various electro-mechanical nanodevices. In our previous work on silicene, fracture stresses of single crystal and polycrystalline silicene have been investigated. Existence of defects in the form of cracks reduces the fracture strength of silicene nanosheets to a great extent. In this study, an engineering way has been proposed for improving the fracture stress of silicene nanosheets with a pre-existing crack by incorporating auxiliary cracks symmetrically in a direction perpendicular to the main crack. We call this mechanism the “Failure shielding mechanism”. An extensive molecular dynamics simulation based analysis has been performed to capture the atomic level auxiliary crack-main crack interactions. It is found that the main crack tip stress distribution is significantly changed with the presence of auxiliary cracks for loading along both armchair and zigzag directions. The effects of temperature and the crack propagation speed of silicene have also been studied. Interestingly, in the case of loading along the zigzag direction, SW defect formation is observed at the tip of main crack. This leads to a reduction of the tip stress resulting in a more prominent failure shielding in case of zigzag loading than in armchair loading. Moreover, the position and length of the cracks as well as the loading directions have significant impacts on the tip stress distribution. Finally, this study opens the possibilities of strain engineering for silicene by proposing an engineering way to tailor the fracture strength of silicene.

Temperature and size effect on the mechanical properties of indium phosphide nanowire: An atomistic study

The mechanical properties of Indium Phosphide (InP) nanowire is an emerging issue due to its application as optoelectronic material. In this paper, atomistic simulations are conducted to find thermo-mechanical properties of Indium Phosphide (InP) nanowire under uniaxial tension. Vashishta potential is employed to define the atomic interactions between the atoms. The effect of variation of temperatures (100K-500K) on the tensile response of the InP nanowires is investigated in this study. Also, size effect is investigated for the temperature of 300 K by varying the cross sectional area of the nanowire. Results suggest that increment of temperature results in the failure of InP nanowire at a lower value of stress (from 8.60 GPa at 100K to 6.50 GPa at 500K) along with the decrement of Young’s modulus. Results also suggest that size has little effect on the tensile properties of this nanowire. Finally, failure mechanisms of indium phosphide nanowire are also investigated from the atomic images obtained from the simulation results.The mechanical properties of Indium Phosphide (InP) nanowire is an emerging issue due to its application as optoelectronic material. In this paper, atomistic simulations are conducted to find thermo-mechanical properties of Indium Phosphide (InP) nanowire under uniaxial tension. Vashishta potential is employed to define the atomic interactions between the atoms. The effect of variation of temperatures (100K-500K) on the tensile response of the InP nanowires is investigated in this study. Also, size effect is investigated for the temperature of 300 K by varying the cross sectional area of the nanowire. Results suggest that increment of temperature results in the failure of InP nanowire at a lower value of stress (from 8.60 GPa at 100K to 6.50 GPa at 500K) along with the decrement of Young’s modulus. Results also suggest that size has little effect on the tensile properties of this nanowire. Finally, failure mechanisms of indium phosphide nanowire are also investigated from the atomic images obtained from t...

Shear based analysis of nickel nano-plate by molecular dynamics simulations

The determination of shear based properties of Nickel (Ni) has a great importance since it is more likely to fail by shear than tension due to its ductile nature. It also features a wide variety of applications in structure, thin film, tubes, and plates due to its unique thermal and electrical properties. Molecular Dynamics Simulations were performed on Ni nano-plate subjected to shear loading to study the effect of voids in the structure using embedded atom method (EAM) potential. The shear stress-strain behavior was observed for Ni nano-plate with voids of 1.0 nm, 1.5 nm, and 2.0 nm radius. Snapshots taken at different strains show the formation of slip planes, crack propagation, and dislocation activity. Simulation results show that the modulus of rupture decreases with the increase of void radius due to more dislocation activity for larger void. Lastly, the effect of different void size on the shear modulus of rigidity is also incorporated.The determination of shear based properties of Nickel (Ni) has a great importance since it is more likely to fail by shear than tension due to its ductile nature. It also features a wide variety of applications in structure, thin film, tubes, and plates due to its unique thermal and electrical properties. Molecular Dynamics Simulations were performed on Ni nano-plate subjected to shear loading to study the effect of voids in the structure using embedded atom method (EAM) potential. The shear stress-strain behavior was observed for Ni nano-plate with voids of 1.0 nm, 1.5 nm, and 2.0 nm radius. Snapshots taken at different strains show the formation of slip planes, crack propagation, and dislocation activity. Simulation results show that the modulus of rupture decreases with the increase of void radius due to more dislocation activity for larger void. Lastly, the effect of different void size on the shear modulus of rigidity is also incorporated.

Strain rate and curing condition effects on the stress–strain behaviour of epoxy adhesive materials

Abstract Adhesive materials evolve properties that change significantly with the preparation procedures and curing conditions. In this study the effects of curing conditions (curing time and temperature), and strain rate on the stress–strain behaviour of the commercially available Lapox epoxy adhesive materials have been evaluated experimentally. The rectangular test specimens have been prepared with different curing temperatures and times. After preparation, the specimens have been tested in small scale tensile testing machine to investigate the stress–strain behaviour at room temperature. It has been observed that as the curing time or curing temperature is increased, the ultimate tensile strength and the elastic modulus of the material also increase. A four parameter hyperbolic tangent model has been fitted to the experimental data and the model constants have been evaluated for different curing conditions and strain rates. Furthermore, for a fixed curing time and strain rate, empirical equations have been developed for modelling the dependence of curing temperature on the stress–strain curves. Finally, the developed equations have been implemented into the finite element analysis of a lap joint to investigate the stress and strain distributions of the adhesive layer for different curing conditions (curing time and temperature).

Influence of defects on thermal properties of stanene

Stanene is a two-dimensional, graphene-like honeycomb structure material, has been synthesized in a recent experimental study. Theoretically, it is expected to have a super conductive property near room temperature due to its spin orbital coupling effect. It is a potential material for the next generation nano-electronics application. Therefore, studying its thermal property is of particular interest. In this paper, we investigated the effect of different types of defects on the thermal conductivity of stanene nanosheets. Molecular Dynamics simulations are performed to calculate the thermal conductivity as a function of various types of defects. MEAM potential is used to describe the inter-atomic forces. It has been found that the presence of defects reduces the thermal conductivity significantly. Finally, vibrational density of states (DOS) are calculated to elucidate the underlying mechanisms of the reduction of thermal conductivity.