Vikas Tomar

Role of the nanoscale interfacial arrangement in mechanical strength of tropocollagen–hydroxyapatite-based hard biomaterials

Nanoscale interfacial interactions between a polypeptide (e.g. tropocollagen (TC)) phase and a mineral (e.g. hydroxyapatite (HAP), aragonite) phase is a strong determinant of the strength of hard biological materials such as bone, dentin and nacre. This work presents a mechanistic understanding of such interfacial interactions by examining idealized TC and HAP interfacial systems. For this purpose, three-dimensional molecular dynamics analyses of tensile and compressive failure in two structurally distinct TC-HAP supercells with TC molecules arranged either along or perpendicular to a chosen HAP surface are performed. Analyses point out that the peak interfacial strength for failure results when the load is applied in the direction of TC molecules aligned along the HAP surface such that the contact area between the TC and HAP phases is at a maximum. Such an alignment also leads to the localization of peak stress over a larger length scale resulting in higher fracture strength. The addition of water is found to invariably cause an increase in the mechanical strength. Overall, analyses point out that the relative alignment of TC molecules with respect to the HAP mineral surface such that the contact area is maximal, the optimal direction of applied loading with respect to the TC-HAP orientation and the increase in strength in a hydrated environment can be important factors that contribute to making nanoscale staggered arrangement a preferred structural configuration in biomaterials.

Role of heat flow direction, monolayer film thickness, and periodicity in controlling thermal conductivity of a Si–Ge superlattice system

Superlattices are considered one of the most promising material systems for nanotechnological applications in fields such as high figure of merit (ZT) thermoelectrics, microelectronics, and optoelectronics owing to the possibility that these materials could be tailored to obtain desired thermal properties. Factors that could be adjusted for tailoring the thermal conductivity of the superlattices include the monolayer film thickness, periodicity, heat flow direction, straining, and temperature of operation. In the presented study, nonequilibrium molecular dynamics (NEMD) simulations are performed to obtain an understanding of the effect of such factors on the thermal conductivity of Si–Ge superlattices at three different temperatures (400, 600, and 800 K). The NEMD simulations are performed using Tersoff bond-order potential. The thermal conductivity is found to increase with an increase in the number of periods as well as with the increase in the period thickness. The dependence of thermal conductivity on...

Tension-compression strength asymmetry of nanocrystalline α-Fe2O3+fcc-Al ceramic-metal composites

The dependence on composition and loading direction of the strength of nanocrystalline α-Fe2O3+fcc-Al composites is analyzed using molecular dynamics simulations with a recently developed multicomponent interatomic potential. Compressive strength values are found to be higher than the tensile strength values at all volume fractions of the phases. Reverse Hall-Petch relations are observed for tension and forward Hall-Petch relations are observed for compression. The observed asymmetry in behavior and the direct or reverse nature of the Hall-Petch relations are found to reflect the different manners in which pairwise electrostatic forces influence grain boundary sliding which is the primary deformation mechanism.

Modeling of Dynamic Fracture and Damage in Two-Dimensional Trabecular Bone Microstructures Using the Cohesive Finite Element Method

Trabecular bone fracture is closely related to the trabecular architecture, microdamage accumulation, and bone tissue properties. Micro-finite-element models have been used to investigate the elastic and yield properties of trabecular bone but have only seen limited application in modeling the microstructure dependent fracture of trabecular bone. In this research, dynamic fracture in two-dimensional (2D) micrographs of ovine (sheep) trabecular bone is modeled using the cohesive finite element method. For this purpose, the bone tissue is modeled as an orthotropic material with the cohesive parameters calculated from the experimental fracture properties of the human cortical bone. Crack propagation analyses are carried out in two different 2D orthogonal sections cut from a three-dimensional 8 mm diameter cylindrical trabecular bone sample. The two sections differ in microstructural features such as area fraction (ratio of the 2D space occupied by bone tissue to the total 2D space), mean trabecula thickness, and connectivity. Analyses focus on understanding the effect of the rate of loading as well as on how the rate variation interacts with the microstructural features to cause anisotropy in microdamage accumulation and in the fracture resistance. Results are analyzed in terms of the dependence of fracture energy dissipation on the microstructural features as well as in terms of the changes in damage and stresses associated with the bone architecture variation. Besides the obvious dependence of the fracture behavior on the rate of loading, it is found that the microstructure strongly influences the fracture properties. The orthogonal section with lesser area fraction, low connectivity, and higher mean trabecula thickness is more resistant to fracture than the section with high area fraction, high connectivity, and lower mean trabecula thickness. In addition, it is found that the trabecular architecture leads to inhomogeneous distribution of damage, irrespective of the symmetry in the applied loading with the fracture of the entire bone section rapidly progressing to bone fragmentation once the accumulated damage in any trabeculae reaches a critical limit.

The effect of tensile and compressive loading on the hierarchical strength of idealized tropocollagen―hydroxyapatite biomaterials as a function of the chemical environment

Hard biomaterials such as bone, dentin and nacre have primarily a polypeptide phase (e.g.xa0tropocollagen (TC)) and a mineral phase (e.g.xa0hydroxyapatite (HAP) or aragonite) arranged in a staggered manner. It has been observed that the mechanical behaviour of such materials changes with the chemical environment and the direction of applied loading. In the presented investigation, explicit three-dimensional molecular dynamics (MD) simulations based analyses are performed on idealized TC-HAP composite biomaterial systems to understand the effects of tensile and compressive loadings in three different chemical environments: (1)xa0unsolvated, (2) solvated with water and (3) calcinated and solvated with water. The MD analyses are performed on two interfacial supercells corresponding to the lowest structural level (level n) of TC-HAP interactions and on two other supercells with HAP supercells arranged in a staggered manner (level n+1) in a TC matrix. The supercells at level n+1 are formed by arranging level n interfacial supercells in a staggered manner. Analyses show that at level n, the presence of water molecules results in greater stability of TC molecules and TC-HAP interfaces during mechanical deformation. In addition, water also acts as a lubricant between adjacent TC molecules. Under the application of shear stress dominated loading, water molecules act to strengthen the TC-HAP interfacial strength in a manner similar to the action of glue. An overall effect of the observed mechanisms is that, in a staggered arrangement, tensile strength increases in the presence of water and calcinated water environments. On the other hand, corresponding compressive strength decreases under similar circumstances. Fundamentally, supercells with primarily normal load transfer at the TC-HAP interfaces are stronger in tensile shear loading. On the other hand, supercells with primarily tangential or shear load transfer at the TC-HAP interfaces are stronger in compressive shear loading. A combination of changes in chemical environment from vacuum to calcinated water and changes in interfacial configurations in a staggered arrangement could be chosen to make the TC-HAP material stronger under applied deformation.

Accelerating the molecular time steps for nanomechanical simulations: Hybrid Monte Carlo method

A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at a time scale of the order of microseconds in a reasonable computing time. Additionally, MD simulations have to be carried out at very high loading rates (∼108u2009s−1) rarely observed during experiments. In this investigation, a modified hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain-rate-dependent) atomistic mechanical deformation of nanostructures at higher time scales than currently possible using MD is established for a Cu nanowire and for a nanocrystalline Ni sample. In this method, there is no restriction on the size of MD time step except that it must ensure a reasonable acceptance rate between consecutive Monte Carlo (MC) steps. In order to establish the method, HMC analyses of a Cu nanowire defo...

A Molecular Dynamics Simulation Framework for an Al+Fe2O3 Reactive Metal Powder Mixture

This work focuses on the development of a classical molecular dynamics (MD) framework that can be used to analyze the quasi-static strength, elastic-constants, and dynamic strength of nanostructured Fe 2 O 3 +Al reactive metal powder (NRMP) mixtures. An interatomic potential is developed for the Fe 2 O 3 +Al NRMP system, accounting for the behavior of Al, Fe, Fe-Al intermetallics, Fe 2 O 3 , and Al 2 O 3 . This potential can be regarded as a generalization and combination of several existing potentials for the individual components in the system. The potential incorporates electronegativity equalization to account for pressure-induced phase transformation. In addition, the parameter set of the potential is capable of predicting the elastic properties of components over a range of 0 - 900 K.

Multiobjective Composite Material Design using the Variable Fidelity Model Management Optimization Framework

Design tool development for multiple phase material design is key to understanding the effect of reinforcement particles on the behavior composites. The increasing computational requirements of advanced numerical tools for simulating material behavior can prohibit direct integration of these tools in a design optimization procedure where multiple iterations are required. One, therefore, requires a design approach that can incorporate multiple simulations (multi-physics with different input variables) of varying fidelity in an iterative model management framework that can significantly reduce design cycle times. In this research, a material design tool based on a variable fidelity model management framework is applied to obtain the optimal size of a second phase, consisting of silicon carbide (SiC) fibers, in a silicon-nitride (Si3N4) matrix to obtain continuous fiber SiC-Si3N4 ceramic composites (CFCCs) with optimal high temperature strength and high temperature creep resistance. To study the trade-offs between these conflicting design objectives and to explore design options, the optimization problem needs to be formulated with multiple objectives. In the variable fidelity material design tool, complex 3 dimensional (3-D) “high fidelity” FEM based analyses are performed only to guide the 2 dimensional (2-D) “low-fidelity” FEM model toward the optimal material design. Traditionally the variable fidelity approach has only been applicable to variable fidelity models with matching input variables and responses. This investigation shows how models with different input design variables can be handled and integrated efficiently by the trust region model management framework in application to the design of multiphase composite materials. Using the variable fidelity material design tool in application to a test problem, a reduction in design cycle times of between 50 and 70 percent is achieved as compared to using a conventional design optimization approach that exclusively calls the 3-D high fidelity model. The pareto optimal design obtained using the variable fidelity approach is the same as that obtained using the conventional procedure.

Fracture Mechanics of Architecture Dependent Fracture in Trabecular Bone Using the Cohesive Finite Element Method

Trabecular bone fracture is closely related to the trabecular architecture and microdamage accumulation. Micro-finite element models have been used to investigate the elastic and yield properties of trabecular bone but have only seen limited application in modeling the microstructure dependent fracture of trabecular bone, [1, 2]. In the presented research a cohesive finite element method (CFEM) based approach that can be used to model microstructure and loading rate dependent fracture in trabecular bone is developed for the first time. The emphasis is on understanding the effect of the rate of loading and its correlation with the bone microstructure on the microdamage accumulation and fracture behavior in the trabecular bone. Analyses focus on understanding the effect of the rate of loading, change in bone tissue properties with aging, and their correlation with the bone microstructure on the microdamage accumulation and the fracture behavior in the trabecular bone.Copyright

A Methodology for Multiscale Computational Design of Continuous Fiber SiC-Si 3 N 4 Ceramic Composites Based on the Variable Fidelity Model Management Framework

Research applications involving design tool development for multiple phase material design are at an early stage of development. The computational requirements of advanced numerical tools for simulating material behavior such as the finite element method (FEM) and the molecular dynamics method (MD) can prohibit direct integration of these tools in a design optimization procedure where multiple iterations are required. The complexity of multiphase material behavior at multiple scales restricts the development of a comprehensive meta-model that can be used to replace the multiscale analysis. One, therefore, requires a design approach that can incorporate multiple simulations (multiphysics) of varying fidelity such as FEM and MD in an iterative model management framework that can significantly reduce design cycle times. In this research a material design tool based on a variable fidelity model management framework is presented. In the variable fidelity material design tool, complex “high fidelity” FEM analyses are performed only to guide the analytic “low-fidelity” model toward the optimal material design. The tool is applied to obtain the optimal distribution of a second phase, consisting of silicon carbide (SiC) fibers, in a silicon-nitride (Si3N4) matrix to obtain continuous fiber SiC-Si3N4 ceramic composites (CFCCs) with optimal fracture toughness. Using the variable fidelity material design tool in application to one test problem, a reduction in design cycle time around 80 percent is achieved as compared to using a conventional design optimization approach that exclusively calls the high fidelity FEM. The optimal design obtained using the variable fidelity approach is the same as that obtained using the conventional procedure. The variable fidelity material design tool is extensible to multi-scale multi-phase material design by using MD based material performance analyses as the “high fidelity” analyses in order to guide “low-fidelity” continuum level numerical tools such as the FEM or FDM (finite difference method) with significant savings in the computational time.