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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]

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]

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]