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The development of stress and fracture in Si-based anodes for lithium-ion batteries is strongly affected by lithiation-induced plasticity. Recent experiments indicate that the nature of plasticity of lithiated silicon is rate-dependent. We establish a theoretical model to capture the viscoplastic mechanical behavior of Si anodes during two-phase lithiation. It is demonstrated that the lithiation-induced stress field is determined by the migration speed of the Li-Li3.75Si interface and the characteristic size of the Si anodes. If experimentally measured interface velocity data in Si nanoparticle are available, the mechanistic model can directly predict the rate-sensitive spatiotemporal stress profile, which is hardly measured in experiments. [ABSTRACT FROM AUTHOR]

The anisotropic shear relaxationfunctions of confined thin filmsof unentangled polymer melts are measured via nonequilibrium step–strainsimulations of in-plane and out-of-plane shear using the finitelyextensible, nonlinear-elastic (FENE) model. We show that the classicalRouse model unsurprisingly fails to predict the thin-film relaxationfunctions in response to out-of-plane shear, due in part to non-Gaussianconformation statistics in the dimension perpendicular to the sub/superstrate.Using an alternate empirical model for the out-of-plane response,we quantify decreases in the plateau modulus G⊥P, relaxation time λ⊥, and viscosityη⊥and an increase in the logarithmic relaxationrate r⊥as functions of film thickness,and we discuss these anisotropic changes in stress-relaxation propertiesin terms of structural/conformation changes on the microscopic level,namely the relative contraction and non-Gaussian quality of polymerconformations in the dimension normal to the substrate and the resultingphenomenon of cooperative relaxation. We then incorporate these intoa semiempirical extension to the Rouse model which closely predictsour computational results and which will be useful for further studyof polymer thin films. [ABSTRACT FROM AUTHOR]

In this paper, we propose an immersed solid system (ISS) method to efficiently treat the fluid–structure interaction (FSI) problems. Augmenting a fluid in the moving solid domain, we introduce a volumetric force to obtain the correct dynamics for both the fluid and the structure. We further define an Euler–Lagrange mapping to describe the motion of the immersed solid. A weak formulation (WF) is then constructed and shown to be equivalent to both the FSI and the ISS under certain regularity assumptions. The weak formulation (WF) may be computed numerically by an implicit algorithm with the finite element method, and the streamline upwind/Petrov–Galerkin method. Compared with the successful immersed boundary method (IBM) by Peskin and co-workers (J Comput Phys 160:705–719, 2000; Acta Numerica 11:479–517, 2002; SIAM J Sci Stat Comput 13(6):1361–1376, 1992) the ISS method applies to more general geometries with non-uniform grids and avoids the inaccuracy in approximating the Dirac delta function [ABSTRACT FROM AUTHOR]

The discrete Laplacian operator is considered in the sense of the meshfree point collocation method which will be called the strong meshfree Laplacian operator. To define the strong meshfree Laplacian operator, we use the fast version of the generalized moving least square approximation, which can calculate the approximated derivatives of shape functions. Some types of the locally layered node distribution are defined in this paper, and two specific domains are constructed onto which we can distribute locally layered nodes. At such nodes, the discrete maximum principle can be shown to hold through the representation formula for the strong meshfree Laplacian operator. The discrete maximum principle, together with the reproducing property of the meshfree approximations, results in an a priori estimate for the strong meshfree Laplacian operator in the nodal solution space. Furthermore, the a priori estimate we have obtained guarantees the existence and the uniqueness of the numerical solution and plays a central role in achieving converged results for the Poisson problem with Dirichlet boundary conditions in the nodal solution space. The order of convergence of the nodal solutions can be raised up to O(h²) at the proposed type of nodes in specific domains. For generally shaped domains immersed in the previously mentioned domains, we can obtain the first order convergence result of O(h). [ABSTRACT FROM AUTHOR]

A class of cohesive solutions of moving glide dislocations with intersonic speeds has been derived on the basis of the fundamental equation of a moving dislocation introduced by Weertman in conjunction with a proposed generalized Bilby-Cottrell-Swinden-Dugdale model. In this model we assume a straight weak path within an infinite elastic plate. Two length scales, namely the width (thickness) of the weak path and the material intrinsic length, which scales strain-gradient-induced hardening and energy dissipation, are taken into account by applying the traction-separation law for the decohesion of the weak path. Dislocations propagate along this weak path with a speed higher than the shear wave speed. The accumulation of these moving dislocations forms a macroscale crack growth with a cohesive zone ahead of the crack tip. Similar to the Bilby-Cottrell-Swinden-Dugdale model, the remote enforced stress and/or stress-rate boundary conditions are represented as an equivalent crack surface traction associated with the dislocation distribution. The involved Cauchy integral and corresponding eigenvalue problem are solved using the algorithms introduced by Muskhelishvili and by Weertman. The problems associated with three types of decohesion law are constant traction, traction linearly dependent on separation, and separation- and separation-rate-dependent traction. These problems are solved using three different solution strategies: the direct-integration method, the iteration method and the Jacobi polynomial expansion respectively. The derived solutions provide explicit relations between the remote load propagation speed, the material intrinsic length, the weak path thickness and the strain-rate-hardening parameter. The solutions demonstrate that the intersonic speed region can be divided into two subdomains; steady-state propagation occurs within the subdomain where the propagation speeds are equal to or greater than the Eshelby speed ( c s × 2 1/2 , where c s is the shear wave speed). For a weak path with a finite width and the corresponding decohesion law scaled by material intrinsic length, an intersonic crack propagation will not take place if only a constant remote stress is imposed. A 'steady-state' crack surface load and/or remote stress-rate boundary condition, which can be considered as a point force or a distributed force with a constant distance to the moving crack tip, is required to maintain steady-state intersonic crack propagation. [ABSTRACT FROM AUTHOR]

Investigates the effect of material heterogeneity on damage evolution and subsequent crack propagation in bimaterial systems. Strain gradient theory analysis indicating that higher stress triaxiality always occur on the softer material side; Numerical procedures and crack growth simulation; Application of gradient plasticity to interface mechanics.

It has been experimentally observed that wrinkles formed on the suiface of electrospun polymer nanofibers when they are under uniaxial tension (Appl. Phys. Lett., 91, p. 151901 (2007)). Molecular dynamics (MD) simulations, finite element analyses (FEA), and continuum theory calculations have been performed to understand this interesting phenomenon. The suiface wrinkles are found to be induced by the cylindrical core-shell microstructure of polymer nanofibers, especially the mismatch of Poisson's ratio between the core and shell layers. Through the MD simulations, the polymer nanofiber is found to be composed of a glassy core embedded into a rubbery shell. The Poisson's ratios of the core and shell layers are close to that of the compressible (0.2) and incompressible (0.5) polymers, respectively. The core is twice stiffer than the shell, due to its highly packed polymer chains and large entanglement density. Based on this observation, a FEA model has been built to study suiface instability of the cylindrical core-shell soft solids under uniaxial tension. The "polarization" mechanism at the interphase between the core and shell layers, induced by the mismatch of their Poisson's ratios, is identified as the key element to drive the suiface wrinkles during the instability analysis. Through postbuckling analysis, the plastic deformation is also found to play an important role in this process. Without the plastic deformation, the initial imperfection cannot lead to suiface wrinkles. The FEA model shows that the yielding stress (or strain rate) can greatly affect the onset and modes of suiface wrinkles, which are in good agreement with experimental observations on electrospun polymer nanofibers. The deformation mechanism and critical condition for the suiface wrinkles are further clarified through a simplified continuum theory. This study provides a new way to understand and control the suiface morphology of cylindrical core-shell materials. [ABSTRACT FROM AUTHOR]

Nanoparticles (NPs) are in use to efficiently deliver drug molecules into diseased cells. The surfaces of NPs are usually grafted with polyethylene glycol (PEG) polymers, during so-called PEGylation, to improve water solubility, avoid aggregation, and prevent opsonization during blood circulation. The interplay between grafting density s p and grafted PEG polymerization degree N makes cellular uptake of PEGylated NPs distinct from that of bare NPs. To understand the role played by grafted PEG polymers, we study the endocytosis of 8 nm sized PEGylated NPs with different s p and N through large scale dissipative particle dynamics (DPD) simulations. The free energy change F polymer of grafted PEG polymers, before and after endocytosis, is identified to have an effect which is comparable to, or even larger than, the bending energy of the membrane during endocytosis. Based on self-consistent field theory F polymer is found to be dependent on both s p and N . By incorporating F polymer , the critical ligand-receptor binding strength for PEGylated NPs to be internalized can be correctly predicted by a simple analytical equation. Without considering F polymer , it turns out impossible to predict whether the PEGylated NPs will be delivered into the diseased cells. These simulation results and theoretical analysis not only provide new insights into the endocytosis process of PEGylated NPs, but also shed light on the underlying physical mechanisms, which can be utilized for designing efficient PEGylated NP-based therapeutic carriers with improved cellular targeting and uptake. [ABSTRACT FROM AUTHOR]

We explore the dynamics of entangled polymer chains embedded into nanocomposites. From primitive path analysis, highly entangled polymer chains are found to be significantly disentangled during incre-ment of the volume fraction of spherical nonattractive nanoparticles (NPs) from 0 to 42%. A critical volume fraction, &phgr;c = 31%, is found to control the crossover from polymer chain entanglements to "NP entanglements." While below &phgr;c, the polymer chain relaxation accelerates upon filling, above &phgr;c, the situation reverses: polymer dynamics becomes geometrically constrained upon adding NPs. Our findings provide a microscopic understanding of the dynamics of entangled polymer chains inside their compo-sites, and offer an explanation for the unusual rheological properties of polymer composites. [ABSTRACT FROM AUTHOR]

Nanowire (NW) assembly is currently of great interest, partly because NWs are considered as a fundamental component in the fabrication of a variety of devices. A powerful method has been developed to model the assembly of NWs. The three-dimensional dielectrophoretic (DEP) assembly of NWs across opposing electrodes is, for the first time, comprehensively studied using this new method. It is found that the DEP force reaches a maximum when the ratio of gap size to NW length is in the range 0.85−1.0. Both the magnitude and sign of the DEP torque on each NW varies with this ratio, and also with the orientation angle and the geometry and configuration of the electrode. The simulation of the dynamic assembly of individual and bundled NWs agrees with experiment. This method is of sufficient power that it will be of direct use in modeling DEP-based assembly and thus the manufacturing of nanoelectronic devices. [ABSTRACT FROM AUTHOR]

In this paper, we present all necessary generalisations to extend the bridging scale, a finite-temperature multiple scale method which couples molecular dynamics (MD) and finite element (FE) simulations, to two dimensions. The crucial development is a numerical treatment of the boundary condition acting upon the reduced atomistic system, as such boundary conditions are analytically intractable beyond simple one-dimension systems. The approach presented in this paper offers distinct advantages compared to previous works, specifically the compact size of the resulting time history kernel, and the fact that the time history kernel can be calculated using an automated numerical procedure for arbitrary multi-dimensional lattice structures and interatomic potentials. We demonstrate the truly two-way nature of the coupled FE and reduced MD equations of motion via two example problems, wave propagation and dynamic crack propagation. Finally, we compare both problems to benchmark full MD simulations to validate the accuracy and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

Through finite element analysis and a 3D printing assisted experimental study, we demonstrate a design of mechanical metamaterials for simultaneous mechanical wave filtering and energy harvesting. The mechanical metamaterials compromise a square array of free-standing cantilevers featuring piezoelectric properties being attached to a primary structural frame. A complete bandgap has thus been created via the strong coupling of the bulk elastic wave propagating along the structural frame and the distributed local resonance associated with the square array of piezoelectrically active cantilevers. Operating within the stop-band, external vibration energy has been trapped and transferred into the kinetic energy of the cantilevers, which is further converted into electric energy through mechano-electrical conversion of its integrated piezoelectric elements. Therefore, two distinct functions, vibration isolation and energy harvesting, are achieved simultaneously through the designed mechanical metamaterials. [ABSTRACT FROM AUTHOR]

Modern Li–ion batteries with LiFePO4 cathodes have been shown to be low cost, non-toxic, have a high theoretical capacity, and high (dis)charging rates. Although LiFePO4 has advantageous properties for electrical energy storage, it can lose some of its charging capacity when cycled. Researchers have found cracks that develop in LiFePO4 cathode particles during cycling, and it has been suggested that this is the main cause of the capacity loss. The work presented here develops a multi-physics computational model to investigate the possible causes of fracture in single LiFePO4 particles. The model combines the recently developed reaction-limited phase-field model for Li–ion intercalation with the phase-field model for brittle fracture. We use our numerical model to simulate single LiFePO4 cathode particles during galvanostatic discharging as well as under no charging. It was found that because of the phase transformation and two-phase coexistence of LiFePO4, cracks were able to grow due to large stresses at coherent phase boundaries. Phase nucleation at particle side facets was also examined and we show that pre-cracks grow that follow the high stresses at the coherent interface during charging. [ABSTRACT FROM AUTHOR]

Although most nanofabrication techniques can control nano/micro particle (NMP) size over a wide range, the majority of NMPs for biomedical applications exhibits a diameter of ~100 nm. Here, the vascular distribution of spherical particles, from 10 to 1,000 nm in diameter, is studied using intravital microscopy and computational modeling. Small NMPs (⩽100 nm) are observed to move with Red Blood Cells (RBCs), presenting an uniform radial distribution and limited near-wall accumulation. Larger NMPs tend to preferentially accumulate next to the vessel walls, in a size-dependent manner (~70% for 1,000 nm NMPs). RBC-NMP geometrical interference only is responsible for this behavior. In a capillary flow, the effective radial dispersion coefficient of 1,000 nm particles is ~3-fold larger than Brownian diffusion. This suggests that sub-micron particles could deposit within diseased vascular districts more efficiently than conventional nanoparticles. [ABSTRACT FROM AUTHOR]

Various nanowire or nanotube-based devices have been demonstrated to fulfill the anticipated future demands on sensors. To fabricate such devices, electric field-based methods have demonstrated a great potential to integrate one-dimensional nanostructures into various forms. This review paper discusses theoretical and experimental aspects of the working principles, the assembled structures, and the unique functions associated with electric field-based assembly. The challenges and opportunities of the assembly methods are addressed in conjunction with future directions toward high performance sensors [ABSTRACT FROM AUTHOR]

Ever since its inception, a popular DNA motif called the cross tile has been recognized to self-assemble into addressable 2D templates consisting of periodic square cavities. Although this may be conceptually correct, in reality certain types of cross tiles can only form planar lattices if adjacent tiles are designed to bind in a corrugated manner, in the absence of which they roll up to form 3D nanotube structures. Here we present a theoretical study on why uncorrugated cross tiles self-assemble into counterintuitive 3D nanotube structures and not planar 2D lattices. Coarse-grained normal mode analysis of single and multiple cross tiles within the elastic network model was carried out to expound the vibration modes of the systems. While both single and multiple cross tile simulations produce results conducive to tube formations, the dominant modes of a unit of four cross tiles (one square cavity), termed a quadruplet, fully reflect the symmetries of the actual nanotubes found in experiments and firmly endorse circularization of an array of cross tiles. [ABSTRACT FROM AUTHOR]

The dielectrophoretic assembly of silicon carbide (SiC) nanowires in a microfluidic flow is shown to enhance the orientation and deposition yield of nanowires. The fluid flow delivers and orients the nanowires in the vicinity of a gap, and they are attracted and deposited by a dielectrophoretic force. Depending upon their lengths, the nanowires are selectively attracted to the gap because the dielectrophoretic force is largest when the lengths are comparable to the gap size. Precise control over the fluid flow and dielectrophoresis shows various interesting phenomena such as landing, shifting, and uniform spacing of nanowires during the assembly process. As a result, the precise control enables the selective positioning of nanowires only at the gap where the fluid direction is consistent with the electric field orientation. [ABSTRACT FROM AUTHOR]

Studies the prediction of springback angle in the straight flanging operation of sheet metal. Use of an analytical model, finite element method and reproducing kernel particle method (RKPM); Development of contact algorithm and computation of springback angle; Material property parameters used in flanging simulation; Evidence that kinematic hardening law has a better prediction than the isotropic hardening law.

An integrated design framework that employs multiscale analysis to facilitate concurrent product, material, and manufacturing process design is presented in this work. To account for uncertainties associated with material structures and their impact on product performance across multiple scales, efficient computational techniques are developed for propagating material uncertainty with random field representation. Random field is employed to realistically model the uncertainty existing in material microstructure, which spatially varies in a product inherited from the manufacturing process. To reduce the dimensionality of random field representation, a reduced order Karhunen–Loeve expansion is used with a discretization scheme applied to finite-element meshes. The univariate dimension reduction method and the Gaussian quadrature formula are used to efficiently quantify the uncertainties in product performance in terms of its statistical moments, which are critical information for design under uncertainty. A control arm example is used to demonstrate the proposed approach. The impact of the initial microscale porosity random field produced during a casting process on the product damage is studied and a reliability-based design of the control arm is performed. [ABSTRACT FROM AUTHOR]