Mechanics Of Inclusions And Interfaces In Metallic And Biological Systems

This thesis deals with the mechanics of inclusions and interfaces in metallic andbiological systems. Based on the nature of the studied subjects, the continuum theory andmolecule dynamics method are employed for these two systems respectively, and a seriesof new models, solutions and observations is drawn.The inclusions, interfaces and dislocations in metallic materials are key factorsinfluencing their deformation behavior and failure mechanisms, and they have beenacknowledged as the main focus in the long term research of Solid Mechanics andMaterials Science. Based on the theory of Continuum Mechanics, this thesis hasaccomplished the following important contributions:(1) Based on the theory of Plasticityand Eshelby inclusion theory, an explicit expression is derived for motion velocity of anelliptical inclusion defect (including void defect) in isotropic matrix driven by interfacediffusion under gradient stress field, and it reveals the physical mechanisms ofexperimental observations, such as inclusion aggregation at crack tip and related failurephenomenon.(2) An approximate continuum theory is developed for the interactionbetween dislocations and inhomogeneity of any shape and properties. The proposedcontinuum theory is applicable to a variety of inhomogeneities, such as pore, gas bubble,shear band and plastically deformed zone. Compared to the existing theories which arelimited to the elastic inhomogeneities, the developed theory is one of general continuumtheory that can effectively handle the problems of elastic inhomogeneities as well asnon-elastic inhomogeneities.(3) Approximate analytical solutions are derived for creep rate and stress relaxation induced by interface diffusion in fiber-reinforced andparticle-reinforced metal matrix composites. The effects of the uniaxial and biaxial loadingconditions are compared. The results provide theoretical bases for the strength loss, stressstability and interface slip of Metal Matrix Composites under high temperature.(4) Ananalytical solution that interprets the effects of grain surface and interface diffusion as wellas grain boundary grooving on the creep rate in free-standing polycrystalline thin metalfilms is presented. The results reveal that films with coarse-grained structure, low surfacediffusivity and high surface free energy have high creep resistance and stress stability, andthey provide guidance for the microstructure design and surface characterization ofpolycrystalline thin metal films. And (5) A phase field model is established formorphological evolution of inclusion in interconnects under electric field. The effects ofinclusion shape, conductivity as well as anisotropic inclusion interface are illustrated.With the rise of nanotechnology and the advances in interdisciplinary research,nanomaterials have received intense global interest due to their potential biomedicalapplications. Taking advantage of the unique size-dependent properties over traditionaldyes and proteins, nanomaterials have shown great potential applications in the areas ofspecific targeting, drug delivery, and enhanced bioimaging. The investigation about theinteraction between nanomaterials and cell is critical to safe design and functionalization ofnanomaterial-enabled biomedical materials, which is the cutting-edge project in the currentresearch of biomechanics. However, such novel materials cannot be effectively addressedby the theory of continuum mechanics. This thesis thus investigates the penetration ofnanomaterials across a cell membrane using molecular dynamics simulations andcalculates the accompanied free energy evolution of the biological system bythermodynamic integration method. The following progresses are achieved:(1) Thedissipative particle dynamics simulations are performed to analyze the evolution of freeenergy as the ligand-coated nanoparticles (NPs) pierce through a lipid bilayer. Fourcharacteristic ligand patterns are considered, and their penetration modes are found to be strongly influenced by the ligand pattern on the nanoparticle surface. The results reveal thephysical mechanism behind an intriguing experimental phenomenon, and they provideuseful guidelines for the molecular design of patterned NPs for controllable cellpenetrability.(2) The interactions of graphene microsheets with cell membrane areinvestigated by combining coarse-grained molecular dynamics (MD), all-atom MD,analytical modeling, confocal fluorescence imaging, and electron microscopic imaging.The entry mode for graphene is proposed for the first time that the penetration initiates atcorners or asperities. Local piercing by these sharp protrusions initiates the membranepropagation along the extended graphene edge. For a small graphene flake, the Brownianmotion and entropic driving forces in the near-membrane region first position the flakeorthogonal to the bilayer plane, leading to spontaneous corner piercing.(3) Moleculardynamics simulations are performed to investigate the mechanical properties of hydrogenfunctionalized graphene allotropes (GAs) for H-coverage spanning the entire range(0-100%). Four allotropes with larger unit lattice size than graphene are considered. Themechanical properties of the hydrogenated GAs are found to deteriorate drastically withincreasing H-coverage within the sensitive threshold, beyond which the mechanicalproperties remain insensitive to the increase in H-coverage. The above research outcomesprovide insights for the potential application of graphene allotropes in the areas ofsuperconducting, electronics, energy, and photoelectrics.In summary, the outcomes of this thesis enrich and advance the continuum theory onthe mechanics of inclusions and interfaces. The interdisciplinary researches of the thesisadvance the framework of traditional continuum mechanics and promote the developmentof Solid Mechanics in the challenging research topics of Nano-Bio science.