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Design of Aligned Carbon Nanotubes Structures Using Structural Mechanics Modeling<br/> Part 2: Aligned Carbon Nanotubes Structure Modeling
Get PDFThe aligned carbon nanotube (A-CNT) structure is composed of arrays of individual CNTs grown vertically on a flat substrate. The overall structure and properties of an A-CNTs are highly dependent upon the designs of various architectures and geometric parameters. In Part 2, we have presented the detailed designs and modeling of various aligned carbon nanotube structures. It is found the A-CNT structures generally have much lower modulus than an individual CNT. The reason is due to the high porosity and low density of the A-CNT structures, since the interstitial space between nanotubes is mostly occupied by air. Increasing the nanotube array density is seen to have significantly improved the modulus of ACNT structures. The mechanical property of the A-CNT structure can be affected by the individual nanotube atomic structure, but only at small wall thickness. As a material, the elastic modulus of the A-CNT is not affected by the size (height) of testing specimen
Structural Evolutions of the Clusters During the Melting and Coalescence Processes
Get PDFStudy on the behaviors of the melting and coalescence of clusters in atomic scale may create new structure at nanoscale, which is a very important research field. The structural evolutions of clusters Cu321, Co321, and Ni321 during their melting and coalescence processes were studied using molecular dynamics simulation with a general embedded atom method in this paper. It was found that the geometries of Cu321 and Co321 transformed to icosahedron from fcc near their melting points, which leads to the increase of their melting points. Concerning the coalescence, it was found that Cu atoms easily formed a coating layer on the surface of Co321 which inhibited the formation of icosahedron in the coalesced complex. The icosahedron was formed during the coalescence of Ni321Co321. These results indicated that the structures were tuned by changing the thermodynamic or coalescence processes
Numerical Simulation of Radiation-Induced Chemical Segregation and Phase Transformation in a Binary System
Get PDFWe present the development of a hybrid Monte Carlo-phase field model that is able to simulate radiation induced chemical segregation and the corresponding phase transformation and nano-structure evolution. Under irradiation by a lowenergy ion beam, defects (vacancies) are created and accumulate. In a binary crystalline material, AB, studied in this work, these defects are of the two types A and B and diffuse at different rates. These differential diffusivities are sufficient driving mechanisms for the formation of chemically distinct regions with accompany changes in phases and nano-structure. In this work, we present a model that can simulate these changes by treating the differential diffusion of the vacancies of the two components
Effect of Cartilage Endplate on Cell Based Disc Regeneration: A Finite Element Analysis
Get PDFThis study examines the effects of cartilage endplate (CEP) calcification
and the injection of intervertebral disc (IVD) cells on the nutrition distributions
inside the human IVD under physiological loading conditions using multiphasic
finite element modeling. The human disc was modeled as an inhomogeneous mixture
consisting of a charged elastic solid, water, ions (Na+ and Cl−), and nutrient solute(oxygen,glucose and lactate) phases. The effect of the endplate calcification was simulated by a reduction of the tissue porosity (i.e., water volume faction) from 0.60 to 0.48. The effect of cell injection was simulated by increasing the cell density in the nucleus pulposus (NP) region by 50%, 100%, and 150%. Strain-dependent transport properties(e.g., hydraulic permeability and solute diffusivities) were considered to couple the solute transport and the mechanical loading. The simulation results showed that nutrient solute distribution inside the discis maintained at a stable state during the day and night. The physiological diurnal cyclic loading does not change the nutrient environment in the human IVD. The cartilage endplate plays a significant role in the nutrient supply to human IVD. Calcification of the cartilage endplate significantly reduces the nutrient levels in human IVD. Therefore, in cell based therapy for IVD regeneration, theincreased nutrient demand as a result of cell injection needs to be addressed. Excessive numbers of injected cells may cause further deterioration of the nutrient environment in the degenerated disc. This study is important for understanding the pathology of IVD degeneration and providing new insights into cell based therapies for low back pain
An Analysis of the Bottomhole Assembly (BHA) in Directional Drilling, by Considering the Effects of the Axial Displacement
No full textThe modeling of the bottomhole assembly (BHA) is an essential problem in directional drilling. Some basic equations for predicting the performance of the BHA are presented in this paper. These equations take into account the effects of the axial displacement. The method of weighted residuals and the Newton-Raphson iterations are used to compute the nonlinear effects of the deformation of the BHA. A computer program is developed for the analysis of the BHA in order to quantitatively predict the performance of the BHA in directional drilling. In addition, a case study is presented to evaluate the effect of the axial displacement in the governing equation on the performance of the BHA. It is concluded that this effect is so small, that it can be ignored in actual calculations, and in the design and operations pertaining to directional drilling
A Three-Dimensional Constitutive Equation And Finite Element Method Implementation for Shape Memory Polymers
No full textIn order to describe the thermomechanical deformation and shape memory effect of shape memory polymers (SMPs), a three-dimensional thermomechanical constitutive model that considers elastic, viscoelastic strain and thermal expansion is proposed for isotropic SMPs. A three-dimensional finite element procedure is developed by implementing the proposed constitutive model into the user material subroutine (UMAT) in ABAQUS program. Numerical examples are used to compare it with existing experimental data in a one dimensional case and to demonstrate the thermomechanical behavior of SMPs with 3D deformation. It is shown that the present constitutive theory and the finite element method can effectively simulate the thermomechanical behavior and shape memory effect of SMPs under complicated deformation states
Fracture & Fatigue Analyses: SGBEM-FEM or XFEM? Part 2: 3D Solids
No full textThe SGBEM-FEM alternating method is compared with the recently popularized XFEM, for analyzing mixed-mode fracture and fatigue growth of 3D nonplanar cracks in complex solid and structural geometries. A large set of 3D examples with different degrees of complexity is analyzed by the SGBEM-FEM alternating method, and the numerical results are compared with those obtained by XFEM available in the open literature. It is clearly shown that: (a) SGBEM-FEM alternating method gives extremely high accuracy for the stress intensity factors; but the XFEM gives rather poor computational results, even for the most simple 3D cracks; (b) while SGBEM-FEM alternating method requires very coarse meshes, which are independent of each other, for both the uncracked solid as well as the non-planar crack-surface, XFEM requires, on the other hand, an extremely fine mesh for 3D solids, which can sometimes be un-usable on a normal PC; (c) the SGBEM-FEM alternating method requires very minimal computational as well as human-labor costs for modeling the non-planar fatigue growth of 3D cracks; on the other hand, fatigue analysis by XFEM requires intensive computational as well as human-labor costs even for the most simple problems; (d) because of the very poor accuracy for the stress intensity factors as computed by XFEM, the number fatigue cycles for crack-growth and failure as predicted by XFEM are meaningless for the most part, even for the most simple 3D problems computed even with extremely fine meshes; (e) with very low computational as well as human-labor costs, the SGBEM-FEM alternating method can very accurately model complex 3D cracked-solids easily, even for those cases which are too complex to be solved by XFEM . It is thus concluded that the SGBEM-FEM alternating method, among the many alternating methods developed in the past 20-30 years by Atluri and his many collaborators, are far more efficient, far more accurate, and far more reliable than XFEM for analyzing fracture and 3D non-planar fatigue crack propagation in complex structures. The implementation of the SGBEM-related method as presented in this study, as well as those presented in its companion Part 1 [Dong and Atluri (2013b)], in general-purpose off-the-shelf commercial software, is greatly valuable and is thus being pursued by the authors
Dam-break model with Characteristic-Based Operator-Splitting Finite Element Method
No full textA finite element method, which is the characteristic-based operatorsplitting (CBOS) algorithm, is adopted to solve unsteady incompressible Navier- Stokes (N-S) equations. In each time step, the equations are split into the diffusive part and the convective part. The convective part is discretized using the characteristic Galerkin method and solved explicitly. The moving interface is captured by the pseudo-concentration method, thus, a new dam-break model is established. Through the validation of a dam-break onto a downstream dry bed or wet bed, it is shown that the present model can accurately simulate the moving interface flows. We also study dam-break in a confined reservoir and the dam-break wave impact on a spur dike. The complexity of the interface shape occurring in the different stages, including the gradual formation of the air bubble in the case of dam-break in a confined reservoir, and the generation of dam-break wave, the overtopping the spur dike and the impacting on the downstream seabed in the case of the dam-break wave impact on a spur dike, can be obtained
Multiobjective Optimization for Ship Hull Form Design Using SBD Technique
No full textWith the rapid development of computer technology and the continuous improvement of optimization theory, optimization techniques have been introduced into the field of ship design. Optimization algorithms and advanced CFD techniques are successfully integrated together into what is known as Simulation- Based Design (SBD) techniques, which opens a new situation for hull-form optimization design and configuration innovation. In this paper, fundamental elements of the SBD techniques are described and crucial components are analyzed profoundly. Focus is on breaking through key technologies as hull geometry modification and reconstruction, global optimization algorithms, and codes integration. Combined with high-fidelity CFD codes (on RANS), an automatic hull-form design optimization framework is established. Based on that, an application of the framework application for a surface combatant hull multi-objective optimization is illustrated. The results show that the reduction of the total resistance is about 6% for the optimized hullform at the design speed (Fn=0.28). The given combatant design optimization example demonstrates the practicability and superiority of the developed SBD framework for the mid-high speed ship
Flexural wave dispersion in finitely pre-strained solid and hollow circular cylinders made of compressible materials
No full textFlexural wave dispersion in finitely pre-stretched (or pre-compressed) solid and hollow, circular cylinders is investigated with the use of the threedimensional linearized theory of elastic waves in initially stressed bodies. It is assumed that the initial strains in the cylinders are homogeneous and correspond to the uniaxial tension, or compression, along their central axes. The elasticity relations of the cylinders’ materials are described by the harmonic potential. The analytical solution of the corresponding field equations is presented and, using these solutions, the dispersion equations for the cases under consideration are obtained. The dispersion equations are solved numerically and based on these solutions, dispersion curves and dispersion diagrams are constructed for various values of the elongation parameter through which the magnitude of the initial strains is determined. The numerical results are obtained for the first and second lowest modes of the solid cylinder and for the first three lowest modes of the hollow cylinder. According to the analyses, in particular, it is established that the finite initial uniaxial stretching, as well the finite initial uniaxial compressing, change the dispersion of the flexural waves in the solid and hollow cylinders not only quantitatively, but also qualitatively