Abstract: Doping as one of the popular methods to manipulate the properties of nanomaterials has received extensive application in deriving different types of graphene derivates, while the understanding of the resonance properties of dopant graphene is still lacking in literature. Based on the large-scale molecular dynamics simulation, reactive empirical bond order potential, as well as the tersoff potential, the resonance properties of N-doped graphene were studied. The studied samples were established according to previous experiments with the N atom’s percentage ranging from 0.38%-2.93%, including three types of N dopant locations, i.e., graphitic N, pyrrolic N and pyridinic N. It is found that different percentages of N-dopant exert different influence to the resonance properties of the graphene, while the amount of N-dopant is not the only factor that determines its impact. For all the considered cases, a relative large percentage of N-dopant (2.65% graphitic N-dopant) is observed to introduce significant influence to the profile of the external energy, and thus lead to an extremely low Q-factor comparing with that of the pristine graphene. The most striking finding is that the natural frequency of the defective graphene with N-dopant’s percentage higher than 0.89% appears larger than its pristine counterpart. For the perfect graphene, the N-dopant shows larger influence to its natural frequency. This study will enrich the current understanding of the influence of dopants on graphene, which will eventually shed lights on the design of different molecules-doped graphene sheet.
Abstract: Nanocomposite coating films have been increasingly used in industrial applications because of their unique mechanical and physical properties. Residual stresses generated during the growth of sputter-deposited thin films due to a strain mismatch between the film and the substrate may lead to significant failure problems. Large residual stresses may generate buckling, delamination and film fracture. Although buckles with cracks in thin films have been experimentally observed, their origins are still not well understood.
Abstract: Zero-thickness interface elements can be used to simulate delamination processes or more in general to model discrete cracks or debonding effects in heterogeneous materials. These elements are inserted amongst the mesh lines in correspondence with the bi-material interface. The zero-thickness interfaces behave as non-linear elements that dissipate energy only after certain cracking conditions have been reached. To keep the interface closed while the cracking conditions are not yet satisfied, a fictitious elastic behaviour is imposed through penalty stiffness parameters. These are elastic stiffness parameters set as high as possible to guarantee that no penetration or separation take place while the cracking nucleation conditions are not satisfied, but not so high to avoid ill-posed problems. Based on a recent formulation, this paper investigates the use of optimal fictitious elastic parameters that guarantee the recovery of accurate pre-cracking inter-element forces and stresses at the bi-material interfaces.
Abstract: A texture-based representative volume element (TBRVE) model is developed for the three-dimensional crystal plasticity (CP) finite element simulations of the Bauschinger effect (BE) of polycrystalline aluminium alloy 7075 (AA7075). In the simulations, the grain morphology is created using the Voronoi tessellation method with the material texture systematically discretised from experiment. A modified CP constitutive model, which takes into account the backstress, is used to simulate the BE during cyclic loading. The model parameters are calibrated using the first cycle stress-strain curve and used to predict the mechanical response to the cyclic saturation of AA7075. The results indicate that the proposed TBRVE CP finite element model can effectively capture the BE at the grain level.
Abstract: Plastic deformation in face-centred cubic (or ‘FCC’) metals involves multi-scale phenomena which are initiated at atomic length and time scales (on order of 1.0e-15 seconds). Understanding the fundamental thresholds for plasticity at atomic and nano/meso scales requires rigorous testing, which cannot be feasibly achieved with current experimental methods. Hence, computer simulation-based investigations are extremely valuable. However, meso-scale simulations cannot yet accommodate atomically-informed grain boundary (or ‘GB’) effects and dislocation interactions. This study will provide a stress - strain analysis based on molecular dynamics simulations of a series of metastable grain boundaries with identical crystal orientations but unique grain boundary characteristics. Relationships between dislocation slip systems, resolved shear stresses and additional thermo-mechanical conditions of the system will be considered in the analysis of dislocation-grain boundary interactions, including GB penetration. This study will form the basis of new phenomenological relationships in an effort to enable accommodation of grain boundaries into meso scale dislocation dynamic simulations.
Abstract: Molecular dynamics simulation was used to study the effect of pressure on self-assembled monolayers (SAM) of n-alkanethiols [(CH3(CH2) n-1, n=14, 15] on Au (111) for dry systems and in the presence of water. The stress-strain behavior and effects of compression on structural characteristics under various normal pressures have been investigated. We found the effect of compression on tilt and tilt orientation angles for dry and hydrated SAM system. Furthermore, a comparison of the results obtained for tilt and tilt orientation angles for hydrated C14 and C15 indicates a more stable structure for C15 (an odd system) under high pressures. We also found excellent elastic recovery of SAM monolayers with and without water is evidence of their exceptional potential to be used under compression in various conditions. Young’s moduli are calculated for various systems under uniaxial compression.
Abstract: The development of new composite materials requires analysis and experimentation spanning scales from nanometres to metres, from “atoms to assemblies”. In this paper, concerned primarily with fibre reinforced epoxy composites, a methodology is presented which allows continuum level structural simulation to account for nanoand micro-scale size effects in composites. The novelty of this approach is the modular hierarchical nature of the simulation which ensures computational tractability, regardless of the length scales considered. Linking the nanoscale to the macroscopic scale in a single simulation allows for holistic materials development, including the addition of nanoadditives to polymer resin systems.
Abstract: Al and Mg alloys are widely used in industry as main lightweight alloys. They have excellent properties, such as low density, high ductility, and high specific strength, and so on. Generally speaking, Mg alloys are better than Al alloys. However the corrosion of Mg alloys is much more difficult to control compared Al alloys. Therefore to combine these two lightweight alloys, a composite-like structure is an ideal solution since Al alloys can be used as protective coatings for Mg alloys. Compound casting is a realistic technique to get this coating system. In the current study, we numerically study the compound casting using finite element method (FEM) to make these two alloys, a composite-like structure, satisfy requirements to resist corrosion required from industry, in which the aluminum layer is acting as a protective coating for the magnesium substrate. Several finite element models have been developed by using the birth and death element technique and we focus on compound casting-induced residual stresses in the compounded structure. The numerical results obtained from the proposed finite element models show the distribution profiles of thermal residual stresses. We found the major factors influencing the residual stresses are the temperature to pre-heating the Al substrate and the thickness of Mg deposits.
Abstract: In recent years, metal foams are becoming more and more popular due to their high energy absorption ability and low density, which are being widely used in automotive engineering and aerospace engineering. As a design guide, foams can be characterised by several main geometric parameters, such as pore size, pore shape, spatial distribution and arrangement and so on. Considering most foam materials have random distributions of cell size and cell shape, the digital material representation and modelling of such materials become more complex. Cell size and shape effects on mechanical behaviours of metal foams have been found and investigated numerically and experimentally in authors' previous studies in which the authors have developed a digital framework for the representation, modelling and evaluation of multi-phase materials including metal foams. In this study, 2-/3-D finite element models are both developed to represent metal foams with random cell distributions and then a series of digital testing are simulated to investigate the mechanical behaviours of such foams. For validation and verification purpose, the results obtained from 2-/3-D models have been compared and good agreement has been found which demonstrated the effectiveness of the digital framework developed for metal forms.