Snap transitions in adhesion

Equilibrium adhesion states are analyzed for nonlinear spherical caps adhered to a rigid substrate under the influence of adhesive tractions that depend on the local separation between the shell and substrate. Transitions between bistable snapped-in and snapped-out configurations are predicted as a function of four nondimensional parameters representing the adhesive energy, the undeformed shell curvature, the range of the adhesive interactions, and the magnitude of an externally applied load. Nonuniform energy and traction fields associated with free-edge boundary conditions are calculated to better understand localized phenomena such as the diffusion of impurities into a bonded interface and the diffusion of receptors in the cell membrane. The linear Griffith approximations commonly used in the literature are shown to be limited to shells with a small height to thickness ratio and short-range adhesive interactions. External loading is found to alter the adhered configurations and the spatial distributions of both adhesive and elastic energies. An important implication of the latter analysis is the theoretical prediction of the pull-off force, which is shown to depend not only on the interface properties, but also on the geometric and material parameters of the shell and on both the magnitude and type of external loading.     

The energy of a growing elastic surface

The potential energy of the elastic surface of an elastic body which is growing by the coherent addition of material is derived. Several equivalent expressions are presented for the energy required to add a single atom, also known as the chemical potential. The simplest involves the Eshelby stress tensors for the bulk medium and for the surface. Dual Lagrangian/Eulerian expressions are obtained which are formally similar to each other. The analysis employs two distinct types of variations to derive the governing bulk and surface equations for an accreting elastic solid. The total energy of the system is assumed to comprise bulk and surface energies, while the presence of an external medium can be taken into account through an applied surface forcing. A detailed account is given of the various formulations possible in material and current coordinates, using four types of bulk and surface stresses: the Piola-Kirchhoff stress, the Cauchy stress, the Eshelby stress and a fourth, called the nominal energy-momentum stress. It is shown that inhomogeneity surface forces arise naturally if the surface energy density is allowed to be position dependent.     

On using many-particle interatomic potentials to compute elastic properties of graphene and diamond

The elastic properties of diatomic crystals are considered. An approach is proposed that permits calculating the elastic characteristics of crystals by using the interatomic interaction parameters specified as many-particle potentials, i.e., potentials that take into account the effect of the environment on the diatomic interaction. The many-particle interaction is given in the general form obtained in the framework of linear elastic deformation. It is shown that, by expanding in series in small deformation parameters, a group of nonlinear potentials frequently used to model covalent structures can be reduced to this general form. An example of graphene and diamond lattices is used to determine how adequately these potentials describe the elastic characteristics of crystals.     

Energetics of misfit dislocation dipoles in anisotropic epitaxial films with nanoscale compositional modulation

The elastic strain and stress fields associated with nanoscale compositional modulation in an anisotropic epitaxial film on an anisotropic substrate are obtained by using Stroh formalism and the Eshelby-type inclusion method. The composition of the epitaxial film is considered to periodically fluctuate in a surface soft mode, with the amplitude of the composition modulation maximal near the growing surface and decreasing exponentially into the film. It has been experimentally observed that the composition modulation affects the formation of a new type of crystal defects, i.e., misfit dislocation dipoles, in III–V compound semiconductor materials. The formation energy of a misfit dislocation dipole under the elastic fields due to the composition modulation is calculated in this study. It is composed of the core and self energies of two dislocations, the interaction energy between two dislocations, and the interaction energies between the composition modulation and two dislocations. Numerical calculations are performed for a dislocation dipole in a lattice-matched Ga_0.5In_0.5P film on a GaAs substrate.     

Reflection and transmission of elastic waves by a periodic array of cracks: Oblique incidence

The interaction of elastic waves with a planar array of periodically spaced cracks of equal length is investigated. The angle of incidence is arbitrary. By the use of Fourier series techniques, the mixed-boundary value problem for a typical strip is reduced to a singular integral equation, which is solved numerically. It is shown that the reflected and transmitted displacement fields are the superposition of an infinite number of homogeneous and inhomogeneous plane body-wave modes. The reflection and transmission coefficients, which correspond to the modes of order zero, are plotted versus the frequency for three angles of incidence. Sharp resonance and antiresonance effects are observed. A check of the accuracy of the computations is provided by the balance of rates of energies.     

Rheology of columnar-grained ice

Creep of columnar-grained ice, under uniaxial compressive force normal to the columns, is shown to be composed of an instantaneous elastic response followed by a delayed elastic and viscous deformation. Both the delayed elastic and viscous strains are shown to have equal activation energies. Thus, this ice can be considered as a thermorheologically simple material with a nonlinear stress dependence. A simple phenomenological relationship has been developed that can be used for further analysis of the creep compliance function presented in a normalized form.     

Magneto-elastic modeling of composites containing chain-structured magnetostrictive particles

Magneto-elastic behavior is investigated for two-phase composites containing chain-structured magnetostrictive particles under both magnetic and mechanical loading. To derive the local magnetic and elastic fields, three modified Green's functions are derived and explicitly integrated for the infinite domain containing a spherical inclusion with a prescribed magnetization, body force, and eigenstrain. A representative volume element containing a chain of infinite particles is introduced to solve averaged magnetic and elastic fields in the particles and the matrix. Effective magnetostriction of composites is derived by considering the particle's magnetostriction and the magnetic interaction force. It is shown that there exists an optimal choice of the Young's modulus of the matrix and the volume fraction of the particles to achieve the maximum effective magnetostriction. A transversely isotropic effective elasticity is derived at the infinitesimal deformation. Disregarding the interaction term, this model provides the same effective elasticity as Mori-Tanaka's model. Comparisons of model results with the experimental data and other models show the efficacy of the model and suggest that the particle interactions have a considerable effect on the effective magneto-elastic properties of composites even for a low particle volume fraction.     

Uniform strain fields and exact results in an elastoplastic fibre-reinforced composite

This work is concerned with a two-phase material consisting of an elastoplastic matrix reinforced by linearly elastic fibres. It is first shown that uniform strain fields can be generated in this heterogeneous material. A return-mapping based algorithm is then proposed and used to find uniform strain loading paths. With the help of uniform strain fields, exact results, independent of the transverse geometry and arrangement of the fibres, are derived for the effective elastic properties and for the effective initial and current yield surfaces. To cite this article: Q.-C. He, H. Le Quang, C. R. Mecanique 332 (2004).     

Elastic Energies in Circular Inhomogeneities: Imperfect Versus Perfect Interfaces

The change in the total potential energy in a stressed elastic plane system, consisting of an unbounded matrix containing a cylindrical inhomogeneity of circular cross-section, is studied, when an imperfect bonding is formed across the interface. The imperfect bonding is simulated by linearly elastic springs distributed over the interface. Two loading cases are examined: an equilibrium system of fixed uniform tractions acting in the remote boundary of the matrix, and a phase transformation in the inhomogeneity prescribed by stress free uniform eigenstrains distributed in the inhomogeneity region. For both loadings, the fully elastic fields in explicit forms are derived involving the spring compliances and three new two-phase parameters depending on the elastic properties of the two materials. The elastic energies stored in the whole system and in its constituents are determined in simple and compact forms. It is shown that, in both loading cases, the total potential energy of the system is reduced. It is found that, in nanoscale, the ratio of the elastic energy stored in interface to the elastic energy stored in inhomogeneity increases rapidly for small values of the circular radius and tends to zero for large values. Also, this ratio increases as the matrix becomes softer compared to the inhomogeneity.     

Sequential linearization method for viscous/elastic heterogeneous materials

The paper addresses the problem of suitable approximation of the interaction between phases in heterogeneous materials that exhibit both viscous and elastic properties. A novel approach is proposed in which linearized subproblems for an inhomogeneity–matrix system with viscous or elastic interaction rules are solved sequentially within one incremental step. It is demonstrated that in the case of a self-consistent averaging scheme, an additional accommodation subproblem, besides purely viscous and elastic subproblems, is to be solved in order to estimate the material response satisfactorily. By examples of an isotropic two-phase material it is shown that the proposed approach provides acceptable predictions in comparison with the existing models.