International Journal of Engineering Science最新文献

Graphene-metal nanolayered composites (GMNCs) are a new generation of nano-structural composites characterized by a very high density of graphene reinforced interfaces (GRI) between metal nanolayers. Compared to traditional graphene flake reinforced composites, GMNCs have much higher strength, toughness and ductility due to the excellent ability of GRI on constraining dislocation motion and crack propagation. Despite numerous experimental and numerical studies on the mechanical behavior of GMNCs, the underlying strengthening mechanism is still not fully understood due to the absence of appropriate theoretical model. This paper proposes a continuum mechanics based theoretical model to explain the strengthening mechanism in GMNCs. In this model, the metal matrix and the GRI are simulated as homogenous elastic medium of infinite extend and inextensible thin membrane of zero thickness, respectively. Using the theoretical model, two boundary value problems namely (i) A circular prismatic dislocation loop approaching to the GRI and (ii) A mixed mode I/II penny-shaped crack near the GRI are formulated to reveal the two key strengthening mechanism: dislocation blocking and crack shielding, respectively. The two problems are solved analytically by the Generalized Kelvin's Solution (GKS) based method for 3D elasticity and Fredholm integral integration technique. Exact closed form solution for the Peach-Koehler (P-K) force on the dislocation loop is obtained. An efficient numerical scheme is developed to solve the Fredholm integral integration for the crack problem with very high accuracy. It is shown that our theoretical model can well capture and explain the strengthening mechanism observed in experiments. Moreover, the dominant role of Poisson's ratio on the strengthening efficiency is also revealed by our model. This finding implies the exciting possibility that the strength of GMNCs can be tailored by controlling the Poisson's ratio of the metal matrix. The present theoretical modeling can provide valuable insights into the mechanics-based design of GMNCs.

This paper presents an algorithm based on asymptotic methods for computing the effective ultimate and high cyclic fatigue strength of heterogeneous periodic plates, shells, and textiles. The rigorous analysis and convergence proof of this asymptotic method builds upon a series of our previous papers. The method allows to decompose the local stresses as products of periodic stress-concentrations, given as functions of unit cells or graphs/lattices in them, and the macroscopic strain components.

In addition, this paper establishes bounds for the applicability of the method and presents several examples to demonstrate the qualitative advantages of this approach, e.g. for the standard shear and compression tests for plates. The main objective of this paper is to substantially reduce the problem dimension and complexity, thereby enabling more efficient computations.

Soft dielectric elastomers respond to electric stimuli by undergoing large deformations and changes in their material properties. The actuation with deformable electrodes attached to the material originates Coulomb and dipole forces that convert the electric field into a mechanical response. Applications at large deformations can entail crack onset and propagation. Within this context, the response of a soft polymer to an applied electric field may serve to influence the fracture behavior of such materials, potentially enhancing it. Here we explore the fracture performance of an ultra-soft dielectric elastomer. To do so, we conduct tensile tests while applying electrical actuation on samples with pre-cuts. Additionally, we examine the elastomer filled with piezoelectric BaTiO3 particles to ameliorate the fracture performance beyond the limits observed in the unfilled material. In conjunction with the experiments, we employ a bespoke fracture phase-field model to analyze the stress triaxiality near the crack tip. The results indicate that the electric actuation induces beneficial crack tip blunting and stress de-concentration, enhancing the fracture toughness up to a 125 % and delaying crack propagation. Our work provides a route for applications of soft dielectric elastomers that require improved fracture properties or, more broadly, the modulation of fracture behavior.

The classical continuum mechanics faces difficulties in solving problems involving highly inhomogeneous deformations. The proposed theory investigates the impact of higher-order microscopic deformation on modeling of material behaviors and provides a refined interpretation of strain gradients through the homogenized strain energy density. Only one scale parameter, i.e., the size of the Representative Volume Element (RVE), is required by the proposed theory. By employing the variational approach and the Augmented Lagrangian Method (ALM), the governing equations for deformation as well as the numerical solution procedure are derived. It is demonstrated that the homogenized energy theory offers plausible explanations and reasonable predictions for the problems yet unsolved by the classical theory such as the size effect of deformation. The concept of homogenized strain energy proves to be more suitable for describing the intricate mechanical behavior of materials. And higher order partial differential equations can be effectively solved by the ALM by introducing supplementary variables to lower the highest order of the equations.

Recently, Arumugam et al. (2023) developed a constitutive relation for the response of isotropic inhomogeneous compressible elastic solids in order to describe the response of the trabecular bone. Since porous solids such as bones, cement concrete, rocks, metallic alloys, etc., are anisotropic, in this short note we develop a constitutive relation for such bodies that exhibit transverse isotropy and also having two preferred directions of symmetry. Another characteristic of bones is that they exhibit different response characteristics in tension and compression, and hence any constitutive relation that is developed has to be capable of describing this. Also, the material moduli depend on both the density and the mean value of the stress (mechanical pressure), as is to be expected in a porous solid. In the constitutive relation that is developed in this paper, though the stress and the linearized strain appear linearly in the constitutive relation, the relationship is nonlinear. We also derive the response of such solids when undergoing uniaxial extension and compression, simple shear and torsion.

Fluid-conveying nanotubes have become important components of nanoelectromechanical systems (NEMS) working in fluid environments, exciting extensive research on the dynamics of flow-conveying nanotubes. This paper systematically reviews the research progress of mechanics of fluid-conveying nanotubes from several aspects, including tube displacement field, non-classical continuum theory models, modeling, governing equations, boundary condition treatments, and dynamic behaviors. First, a refined displacement field for the tube structure considering curvature nonlinearity is presented. Based on the generalized continuum theory, a size-dependent constitutive model of nanotubes is established that fully considers surface effects, non-local stress and strain gradient effects, as well as the slip flow model for modeling the size-dependency of nanofluid is derived. Subsequently, three types of planar nonlinear vibration problems related to boundary conditions of flow-conveying nanotubes are reviewed. Based on the different nonlinear characteristics caused by different boundary conditions, including curvature nonlinearity, inertia nonlinearity, boundary tension hardening nonlinearity, etc., corresponding assumptions are made and size-dependent longitudinal internal force-displacement relationship is established. The dynamic governing equations and classical and non-classical boundary conditions of flow-conveying nanotubes are derived based on the Hamiltonian variational principle. The current main treatment methods for non-classical boundary conditions are illustrated. Finally, the research status of mechanical behaviors of fluid-conveying nanotubes is reviewed and future research prospects are summarized. This article provides theoretical guidance for linear/nonlinear design of NEMS of next-generation working in fluid environments.

We investigate the nonlinear vibrations of a functionally graded dielectric elastomer plate subjected to electromechanical loads. We focus on local and global dynamics in the system. We employ the Gent strain energy function to model the dielectric elastomer. The functionally graded parameters are the shear modulus, mass density, and permittivity of the elastomer, which are formulated by a common through-thickness power-law scheme. We derive the equation of motion using the Euler-Lagrange equations and solve it numerically with the Runge-Kutta method and a continuation-based method. We investigate the influence of the functionally graded parameters on equilibrium points, natural frequencies, and static/dynamic instability. We also establish a Hamiltonian energy method to detect safe regions of operating gradient parameters. Furthermore, we explore the effect of the functionally graded parameters on chaos and resonance by plotting several numerical diagrams, including time histories, phase portraits, Poincaré maps, largest Lyapunov exponent criteria, bifurcation diagram of Poincaré maps, and frequency-stretch curves. The results provide a benchmark for developing functionally graded soft smart materials.

Collagen fibers, a primary structural protein in the extracellular matrix, provides essential scaffolding for tissues. Functionally, these fibers are essential for providing mechanical support, ensuring tissues like tendons effectively transfer force from muscles to bones. Moreover, collagen is a dynamic component that plays a crucial role in mediating cell signaling, influencing various cellular behaviors and functions.

The intricate network of collagen fibers in tissues forms a highly interconnected system, highlighting the tissue's structural resilience. This complexity, especially when considering interactions between collagen fibers or with cells, presents challenges for detailed analyses.

Our study introduces a homogenization framework for 3D collagen networks with diverse number of connectivity (C ∼ 7 and 4), bridging micro-to-macro scale behaviors. We employed a numerical strategy to homogenize the RVE, incorporating boundary periodicity and uniaxial loading to determine elastic properties. Systematic evaluations yielded a stress-stretch curve, reflecting micro-scale material behavior. This behavior aligned with hyperelastic models for both highly and moderately connected collagen networks, mirroring experimental findings. Collectively, these insights enhance our understanding of collagen mechanics, setting the stage for more nuanced analyses, particularly in cellular interactions within collagen matrices.

Surface effects usually remarkably affect the mechanical response of ultra-thin micro-/nano-structures. However, the mechanisms of surface effects on the fracture characteristics of ultra-thin films are still not fully understood. To this end, this paper develops a modeling framework to investigate the fracture of ultra-thin films at microscales or below. Such a framework couples the Gurtin–Murdoch theory with a phase-field fracture model, in which the former is adopted to introduce the surface effects, i.e., the surface residual stress and surface elasticity of a thin film, and the latter is able to model crack evolution without requiring predefined crack paths or any criteria. Furthermore, a novel crack driving force is introduced, which encompasses the tensile components of both bulk elastic energy and surface elastic energy. Several numerical examples including the biaxial tension test as well as the single-edge notched tension/shear test are performed. The simulation results indicate that the surface strain energy plays a major role in the total elastic strain energy of an ultra-thin film when its thickness is at a micro level, thus demonstrating the significance of surface effects. Moreover, the mode-I fracture test shows that the surface elasticity and surface residual stress have a remarkable influence on the displacement at failure, while for the mode-II fracture test, the surface residual stress significantly influences the fracture characteristics such as the crack path and failure displacement. The developed model paves the way for revealing the fracture mechanisms of ultra-thin micro-/nano-films and conducting their safety assessment.

Residual stress widely exists in soft materials. Besides growth, inhomogeneous thermal expansion is also a primary cause of residual stress. However, establishing a proper hyperelastic constitutive relation is a great challenge since the existing theories cannot capture the change of underlying mechanical responses triggered by temperature variations. In this paper, a general hyperelastic constitutive relation for soft elastomers with thermally-induced residual stress is developed. We first reveal the initial temperature dependence of conventional thermoelastic models. This property attributes the alteration of the underlying thermoelastic response to free thermal expansions. Then, a compatibility-broken curvature compensation (CBCC) framework is established based on finite thermoelasticity. It generates a free thermal expansion to eliminate the Riemannian curvatures of the virtual stress-free configuration derived from the isothermal stress release. Such a mechanism indicates the non-local effect of the residual stress, which fundamentally modifies the traditional view that invariant formulations cover all the possible functional dependence of residual stress. Also, the obtained governing equations are similar to Einstein field equations of the general theory of relativity. This similarity may deeply imply a standard mechanism concerning the curvature compensation leading to residual stress genesis. We finally conduct comparative analyses of the spherically symmetric and axisymmetric problems between the current constitutive relation and the existing models. The influences of adopting distinct residual stresses, the performance of the non-local effect, and the availability of the new constitutive relation are investigated in detail. This framework can shed some light on the constitutive modeling of soft materials.