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An equation of screw dislocations in a thin film is derived for arbitrary boundary conditions. The boundary conditions can be the free surface, the fixed surface or the gradient loading imposed on the surface. The new equation makes it possible to study changes in the dislocation structure under various gradient stress applied to the surface. The rigidity and mobility of screw dislocations in a thin film are explored by using the equation. It is found that the screw dislocation core in a thin film is like a Hookean body with a specific shear stress applied to the surface. Free-surface effects on the Peierls stress are investigated and compared with previous studies. An abnormal behavior of the Peierls stress of screw dislocations in a soft-inclusion film between two rigid films is predicted theoretically. [ABSTRACT FROM AUTHOR]

In this work, the authors apply the “top-down” nanoindentation testing method to assess the mechanical property changes in ion-irradiated metallic alloys for three different ion energies in order to understand the relationship between ion energy, damage peak depth and hardness peak depth. The samples were irradiated with He +2 ions having 1, 2 and 3 MeV beam energies respectively. The curves for ΔH (radiation induced hardness) have been obtained by calculating the difference of the irradiated and unirradiated hardness curves after these were corrected for indentation size effect. Three-dimensional analytical and numerical models have been developed to obtain greater insight into the mechanisms involved in the nanoindentation processes, the nature of the plastic zone, and how these affect the hardness results, including the full hardness profiles with respect to depth. This is particularly valuable in situations where the damage profile is non-uniform, as in the present case, and provides the means to predict expected hardness peak positions and values for a given irradiation dose. [ABSTRACT FROM AUTHOR]

A dislocation at an interface especially at a weak shear interface spreads its core associated with shearing the interface. Such core-spreading significantly reduces stress/strain concentration of the dislocation at the interface and thus traps the dislocation in the interface, correspondingly strengthening materials. Employing the Green's function for a single dislocation, we derived analytical expressions for the elastic fields associated with a core-spreading dislocation in anisotropic bimaterials. We proposed three fractional dislocation models to mimic the spreading core of a dislocation at an interface, i.e. uniform distribution (UD), linear distribution (LD) and cosine distribution (CD). The accuracy and efficiency of the three fractional models are validated by the continuity of both traction and displacement across the interface. Numerical results of the stress and displacement fields of the dislocation in the Cu/Nb bimaterial show that: (1) such core-spreading greatly reduces the stress intensity near the dislocation compared with the dislocation with a condensed core; (2) the distribution of the Burgers vector associated with the core spreading determines the magnitude and patterns of the elastic fields; (3) The influence of the core-spreading on the elastic fields can be negligible when the distance of a field point from the center of the dislocation core is greater than 2.17 times the width of the spreading core; (4) The LD model is simple while it is able to capture the interaction force acting on an incoming dislocation. The findings offer insights into understanding interface roles in strengthening materials and designing interfaces-dominated composites. [ABSTRACT FROM AUTHOR]

The current work deals with periodic thermomechanical composite media, in which the material constituents are considered to obey the generalized standard materials laws. The aim is to provide a proper homogenization framework that takes into account both the equilibrium and the thermodynamics laws in microscale and macroscale levels. The study is based on the asymptotic expansion homogenization technique, which permits to deduce useful results about the general structure of microscale and macroscale energy potentials and constitutive laws. The paper also proposes an incremental, linearized formulation that allows to identify suitable thermomechanical tangent moduli for the macroscale problem. The capabilities of this framework are illustrated with numerical examples on multilayered composites. [ABSTRACT FROM AUTHOR]

Bulk amorphous alloys are in general strong, but brittle. Properly introduced interfaces can effectively improve their ductility. In this paper, the tensile behavior of amorphous/amorphous (A/A) nanolaminates is investigated by atomistic simulations. Upon loading, multiple shear bands transmit through the A/A interfaces and interact with each other. As a result, plastic plateaus are presented in the simulated tensile stress–strain curves of the nanolaminates with small layer thickness. Moreover, the strength of the A/A nanolaminates increases with the layer thickness decreasing, due to the interface obstruction to the shear band motion. It is also found that the plastic deformation mechanisms transit from localized shearing in the soft amorphous layers to multiple shear band interactions in the whole sample at a critical size. An analytical model considering shear band motion is proposed to predict the critical layer thickness for the localized to homogenous deformation transition in A/A nanolaminates. [ABSTRACT FROM AUTHOR]

This paper reports the derivation of the explicit integral kernels for the elastic fields due to eigenstrains in two joined and perfectly bonded half-space solids or bimaterials. The domain integrations of these kernels result in the analytical solutions to displacements and stresses. When the eigenstrains are all in solid I , the kernel for the elastic fields has four groups in this solid and two groups in the other joined solid. Explicit closed-form solutions for a cuboidal inclusion with uniform eigenstrains are derived as the basic solutions, i.e., the influence coefficients. When the computational domain is meshed into cuboidal elements of the same sizes, the total elastic fields can be quickly obtained by implementing three-dimensional fast Fourier transform-based algorithms. The results for the fields due to a cuboidal, a spherical, and a cylindrical inclusion, as well as multiple cuboidal inclusions are presented and discussed. Residual stresses in an elasto-plastic contact involved a coated substrate is further analyzed with the new solution approach. [ABSTRACT FROM AUTHOR]

Thin multilayered structures with glass substrates are widely used in electronic devices. When layers having different thermal or mechanical properties are subjected to processes with temperature variations, warpage and residual stresses may develop incurred by the mismatch of volume change, possibly leading to the failure of the device. In order to analyze the thermo-mechanical behavior of glass including the warpage in multilayered structures with glass substrates, the thermo-mechanical constitutive equations of glass are developed here by coupling the stress relaxation (for mechanical behavior) and structural relaxation models (for thermal behavior). The constitutive equations for structural relaxation are generalized for the case with temperature dependent thermal expansion coefficients, while considering both formulations based on the specific volume and the fictive temperature. Besides, the numerical formulations of the constitutive equations for finite element analysis are derived for three-dimensional and plane stress conditions. The developed constitutive model is validated with experiments for the warpage of silicon-glass multilayered structures undergoing thermal cycles. [ABSTRACT FROM AUTHOR]

Localized necking is often considered as precursor to failure in metal components. In modern technologies, functional components (e.g., in flexible electronic devices) may be affected by this necking phenomenon, and to avoid the occurrence of strain localization, elastomer substrates are bonded to the metal layers. This paper proposes an investigation of the development of localized necking in both freestanding metal layers and elastomer/metal bilayers. Finite strain versions of both rigid–plastic flow theory and deformation theory of plasticity are employed to model the mechanical response of the metal layer. For the elastomer, a neo-Hookean constitutive law is considered. Localized necking is predicted using both bifurcation (whenever possible) and Marciniak–Kuczynski analyses. A variety of numerical results are presented, which pertain to the prediction of localized necking in freestanding metal layers and metal/substrate bilayers. The effects of the constitutive framework and the presence of an elastomer substrate on strain localization predictions have been specifically highlighted. It is demonstrated that the addition of an elastomer layer can retard significantly the occurrence of localized necking. It is also demonstrated that the results of the Marciniak–Kuczynski analysis tend towards the bifurcation predictions in the limit of a vanishing size for the geometric imperfection. [ABSTRACT FROM AUTHOR]

Highlights: [•] A single rolling pass of the accumulated roll bonding process in which a Cu/Nb layered composite is examined. [•] Experimental morphological and crystallographic texture data is presented and used to initialize numerical models. [•] A single crystal model based upon thermally activated dislocation motion is used and the material parameters presented. [•] Forty morphological and crystallographic plane strain compression numerical model realizations of the material. [•] Numerical simulations were compared with the experimental data and the results suggest very good agreement. [Copyright &y& Elsevier]

Highlights: [•] Multiscale computational model for semi-crystalline polymer is proposed. [•] Laminar composite model and homogenization method are used in the model. [•] Material’s parameters used in the model are defined to represent experimental data. [•] Macroscopically uniform and non-uniform deformations of HDPE are investigated. [•] Obtained results are compared to the experimental results. [Copyright &y& Elsevier]

Abstract: When a lattice glide dislocation approaches a bi-metal interface with relatively low shear strength, it causes the interface to shear. Interfacial shearing is accommodated by the nucleation and growth of interfacial dislocations, which have an attractive interaction with the incoming dislocation. Thus a critical length scale exists at which the net force on the incoming lattice glide dislocation can transition from being initially repulsive to attractive. In this paper, we develop dislocation-based interface shear models in order to represent this mechanism of interface/dislocation interaction at the continuum scale. Three versions are devised with different degrees of complexity and hence computational cost: the continuous shear model (CSM), simplified-CSM model (SCSM), and single dislocation shear model (SDSM). We simulate the interaction processes with these three models by means of a Green’s function method for an anisotropic bimaterial. All three models find that the critical length scale at which the dislocation becomes attracted to the interface increases as the interfacial shear resistance decreases. While the most complex model of the three, the CSM, performs the best, the SCSM and SDSM are more advantageous for implementation into higher-length scale dislocation dynamics models. [Copyright &y& Elsevier]

Abstract: The aim of this work is to construct yield surfaces to describe initial yielding and characterize hardening behavior of a highly anisotropic material. A methodology for constructing yield surfaces for isotropic materials using axial–torsion loading is extended to highly anisotropic materials. The technique uses a sensitive definition of yielding based on permanent strain rather than offset strain, and enables multiple yield points and multiple yield surfaces to be conducted on a single specimen. A target value of 20×10−6 is used for Al2O3 fiber reinforced aluminum laminates having a fiber volume fraction of 0.55. Sixteen radial probes are used to define the yield locus in the axial–shear stress plane. Initial yield surfaces for [04], [904], and [0/90]2 fibrous aluminum laminates are well described by ellipses in the axial–shear stress plane having aspect ratios of 10, 2.5, and 3.3, respectively. For reference, the aspect ratio of the Mises ellipse for an isotropic material is 1.73. Initial yield surfaces do not have a tension–compression asymmetry. Four overload profiles (plus, ex, hourglass, and zee) are applied to characterize hardening of a [0/90]2 laminate by constructing 30 subsequent yield surfaces. Parameters to describe the center and axes of an ellipse are regressed to the yield points. The results clearly indicate that kinematic hardening dominates so that material state evolution can be described by tracking the center of the yield locus. For a nonproportional overload of (σ, τ)=(500,70)MPa, the center of the yield locus translated to (σ, τ)=(430,37)MPa and the ellipse major axis was only 110MPa. [ABSTRACT FROM AUTHOR]

Abstract: The strain gradient work hardening is important in micro-indentation of bulk metals and thin metallic films, though the indentation of thin films may display very different behavior from that of bulk metals. We use the conventional theory of mechanism-based strain gradient plasticity (CMSG) to study the indentation of a hard tungsten film on soft aluminum substrate, and find good agreement with experiments. The effect of friction stress (intrinsic lattice resistance), which is important in body-center-cubic tungsten, is accounted for. We also extend CMSG to a finite deformation theory since the indentation depth in experiments can be as large as the film thickness. Contrary to indentation of bulk metals or soft metallic films on hard substrate, the micro-indentation hardness of a hard tungsten film on soft aluminum substrate decreases monotonically with the increasing depth of indentation, and it never approaches a constant (macroscopic hardness). It is also shown that the strain gradient effect in the soft aluminum substrate is insignificant, but that in the hard tungsten thin film is important in shallow indentation. The strain gradient effect in tungsten, however, disappears rapidly as the indentation depth increases because the intrinsic material length in tungsten is rather small. [Copyright &y& Elsevier]

The flow stresses developed during elevated temperature extrusion in laminated metal composites are analyzed. The laminates consist either of ultrahigh carbon steels (UHCSs) or NiAl jacketed in a mild steel can. Flow stresses in the individual components of the laminate were determined by utilizing an isostrain model and by establishing the true coefficient of friction during extrusion. The average coefficient of friction was found equal to about 0.38 for UHCSs and 0.32 for NiAl. The results reveal that plastic flow of the UHCSs is a diffusion-controlled dislocation creep process. An increase in carbon content leads to a decrease in the flow stress because iron atom mobility is increased. An increase in substitutional solid solution alloy additions leads to an increase in the flow stress because of a decrease in the stacking fault energy. The influence of strain rate sensitivity of each component on the extrusion of laminated composites is modeled. It is shown that striking differences in extrusion response are achieved by altering the volume fraction and strain rate sensitivity of the component materials. The analysis of extrusion is extended to interpret Pearson and Smythe''s 1931 data on extrusion of lead, tin and cadmium. These data are also shown to be well characterized by a diffusion-controlled dislocation creep process. Coefficients of friction for extrusion were determined for these three metals. These results, together with those for the UHCSs and NiAl, indicate that the friction coefficient is a function of the flow stress of the metal during extrusion, increasing in value as the flow stress decreases. It is proposed that this trend is principally related to microstructural changes that are unique to the process of extrusion. [Copyright &y& Elsevier]