C. W. Lim

Exact variational nonlocal stress modeling with asymptotic higher-order strain gradients for nanobeams

This article presents a complete and asymptotic representation of the one-dimensional nanobeam model with nonlocal stress via an exact variational principle approach. An asymptotic governing differential equation of infinite-order strain gradient model and the corresponding infinite number of boundary conditions are derived and discussed. For practical applications, it explores and presents a reduced higher-order solution to the asymptotic nonlocal model. It is also identified here and explained at length that most publications on this subject have inaccurately employed an excessively simplified lower-order model which furnishes intriguing solutions under certain loading and boundary conditions where the results become identical to the classical solution, i.e., without the small-scale effect at all. Various nanobeam examples are solved to demonstrate the difference between using the simplified lower-order nonlocal model and the asymptotic higher-order strain gradient nonlocal stress model. An important conclusion is the discovery of significant over- or underestimation of stress levels using the lower-order model, particularly at the vicinity of the clamped end of a cantilevered nanobeam under a tip point load. The consequence is that the design of a nanobeam based on the lower-order strain gradient model could be flawed in predicting the nonlocal stress at the clamped end where it could, depending on the magnitude of the small-scale parameter, significantly over- or underestimate the failure criteria of a nanobeam which are governed by the level of stress.

Dynamics and stability of transverse vibrations of nonlocal nanobeams with a variable axial load

This paper investigates the natural frequency, steady-state resonance and stability for the transverse vibrations of a nanobeam subjected to a variable initial axial force, including axial tension and axial compression, based on nonlocal elasticity theory. It is reported that the nonlocal nanoscale has significant effects on vibration behavior, which results in a new effective nonlocal bending moment different to but dependent on the corresponding nonlocal bending moment. The effects of nonlocal nanoscale and the variation of initial axial force on the natural frequency as well as the instability regions are analyzed by the perturbation method. It concludes that both the nonlocal nanoscale and the initial tension, including static and dynamic tensions, cause an increase in natural frequency, while an initial compression causes the natural frequency to decrease. Instability regions are also greatly influenced by the nonlocal nanoscale and initial tension and they become smaller with stronger nonlocal effects or larger initial tension.

ANALYTICAL SOLUTIONS FOR VIBRATION OF SIMPLY SUPPORTED NONLOCAL NANOBEAMS WITH AN AXIAL FORCE

This paper presents exact, analytical solutions for the transverse vibration of simply supported nanobeams subjected to an initial axial force based on nonlocal elasticity theory. Classical continuum theory is inherently size independent while nonlocal elasticity exhibits size dependence. The latter has significant effects on bending moment, which results in a conceptually different definition of a new effective nonlocal bending moment with respect to the corresponding classical bending moment. A sixth-order partial differential governing equation is subsequently obtained. The effects of nonlocal nanoscale on the vibration frequencies and mode shapes are considered and analytical solutions are solved. Effects of the nonlocal nanoscale and dimensionless axial force including axial tension and axial compression on the first three mode frequencies are presented and discussed. It is found that the nonlocal nanoscale induces higher natural frequencies and stiffness of the nano structures.

Nonlocal thermal-elasticity for nanobeam deformation: Exact solutions with stiffness enhancement effects

The nanomechanical response for a nanobeam under thermal effects is investigated by using the nonlocal elasticity field theory, which was first proposed by Eringen in the early 1970s. The nonlocal constitutive relation is adopted to determine the strain energy density which considers the history of nonlinear straining with respect to an unstrained state. Based on the variational principle and integrating the straining energy density over the entire domain of interest influenced by a temperature field, a new higher-order differential equation and the corresponding higher-order boundary conditions are derived. The thermal-elastic effects of typical nanobeams are presented where new exact analytical solutions with physical boundary conditions are derived. Subsequently, the effects of the nonlocal nanoscale and temperature on the nanobeam transverse deflection are analyzed and discussed. It is observed that these factors have a significant influence on the transverse deflection. In particular, the nanobeam st...

Twisting statics and dynamics for circular elastic nanosolids by nonlocal elasticity theory

The torsional static and dynamic behaviors of circular nanosolids such as nanoshafts, nanorods and nanotubes are established based on a new nonlocal elastic stress field theory. Based on a new expression for strain energy with a nonlocal nanoscale parameter, new higher-order governing equations and the corresponding boundary conditions are first derived here via the variational principle because the classical equilibrium conditions and/or equations of motion cannot be directly applied to nonlocal nanostructures even if the stress and moment quantities are replaced by the corresponding nonlocal quantities. The static twist and torsional vibration of circular, nonlocal nanosolids are solved and discussed in detail. A comparison of the conventional and new nonlocal models is also presented for a fully fixed nanosolid, where a lower-order governing equation and reduced stiffness are found in the conventional model while the new model reports opposite solutions. Analytical solutions and numerical examples based on the new nonlocal stress theory demonstrate that nonlocal stress enhances stiffness of nanosolids, i.e. the angular displacement decreases with the increasing nonlocal nanoscale while the natural frequency increases with the increasing nonlocal nanoscale.

TORSIONAL BUCKLING OF FUNCTIONALLY GRADED CYLINDRICAL SHELLS WITH TEMPERATURE-DEPENDENT PROPERTIES

Based on Hamiltons principle, a new accurate solution methodology is developed to study the torsional bifurcation buckling of functionally graded cylindrical shells in a thermal environment. The effective properties of functionally graded materials (FGMs) are assumed to be functions of the ambient temperature as well as the thickness coordinate of the shell. By applying Donnells shell theory, the lower-order Hamiltonian canonical equations are established, from which the eigenvalues and eigenvectors are solved as the critical loads and buckling modes of the shell of concern, respectively. The effects of various aspects, including the combined in-plane and transverse boundary conditions, dimensionless geometric parameters, FGM parameters and changing thermal surroundings, are discussed in detail. The results reveal that the in-plane axial edge supports do have a certain influence on the buckling loads. On the other hand, the transverse boundary conditions only affect extremely short shells. With increasing thermal loads, the material volume fraction has a different influence on the critical stresses. It is concluded that the optimized FGM mixtures to withstand thermal torsional buckling are Si3N4/SUS304 and Al2O3/SUS304 among the materials studied in this paper.

A SYMPLECTIC HAMILTONIAN APPROACH FOR THERMAL BUCKLING OF CYLINDRICAL SHELLS

The paper deals with the thermal buckling of cylindrical shells in a uniform temperature field based on the Hamiltonian principle in a symplectic space. In the system, the buckling problem is reduced to an eigenvalue problem which corresponds to the critical temperatures and buckling modes. Unlike the classical approach where a predetermined trial shape function satisfying the geometric boundary conditions is required at the outset, the symplectic eigenvalue approach is completely rational where solutions satisfying both geometric and natural boundary conditions are solved with complete reasoning. The results reveal distinct axisymmetric buckling and nonaxisymmetric buckling modes under thermal loads. Besides, the influence for different boundary conditions is discussed.

Vibration and Stability of an Axially Moving Beam on Elastic Foundation

The free vibration of an axially moving elastic beam on simple supports resting on elastic foundation is investigated in this paper. Analytical expressions are derived by considering the ordinary differential equations obtained by the Galerkin truncation method. Critical axially moving velocity is computed for different foundation and supporting conditions. The phenomena of divergence and flutter are found beyond the critical velocity. The stability in the vicinity of critical state is studied and instability regions are discussed accounting for the parameters of axial tension and the foundation stiffness. The complex natural frequencies of the system are obtained by considering the governing partial differential equation without truncation. Some numerical examples are presented to illustrate the contributions of axial speed, tension, and the foundation stiffness.

Nonlinear vibration of a cantilever with a derjaguin-müller-toporov contact end

In this paper, the principal resonance is investigated for a cantilever with a contact end. The cantilever is modeled as an Euler–Bernoulli beam, and the contact is modeled by the Derjaguin–Muller–Toporov theory. The problem is formulated as a linear nonautonomous partial-differential equation with a nonlinear autonomous boundary condition. The method of multiple scales is applied to determine the steady-state response. The equation of response curves is derived from the solvability condition of eliminating secular terms. The stability of steady-state responses is analyzed by using the Lyapunov-linearized stability theory. Numerical examples are presented to highlight the effects of the excitation amplitude, the damping coefficient, and the coefficients related to the contact.

HAMILTONIAN SYSTEM FOR DYNAMIC BUCKLING OF TRANSVERSELY ISOTROPIC CYLINDRICAL SHELLS SUBJECTED TO AN AXIAL IMPACT

This paper investigates the prebuckling dynamics of transversely isotropic thin cylinder shells in the context of propagation and reflection of axial stress waves. By constructing the Hamiltonian system of the governing equation, the symplectic eigenvalues and eigenfunctions are obtained directly and rationally without the need for any trial shape functions, such as the classical semi-inverse method. The critical loads and buckling models are reduced to the problem of eigenvalues and eigensolutions, in which zero-eigenvalue solutions and nonzero-eigenvalue solutions correspond to axisymmetric buckling and nonaxisymmetric buckling, respectively. Numerical results reveal that energy is concentrated at the unconstrained free ends of the shell and the buckling modes have bigger bell-mouthed shapes at these positions.