ANALYSIS ON THE MAGNETO-ELASTIC-PLASTIC BUCKLING/SNAPPING OF CANTILEVER RECTANGULAR FERROMAGNETIC PLATES

An analysis of buckling/snapping and bending behaviors of magneto-elastic-plastic interaction and coupling for cantilever rectangular soft ferromagnetic plates is presented.Based on the expression of magnetic force from the variational principle of ferromagnetic plates,the buckling and bending theory of thin plates,the Mises yield criterion and the increment theory for plastic deformation,we establish a numerical code to quantitatively simulate the behaviors of the nonlinearly multi-fields coupling problems by the finite element method.Along with the phenom- ena of buckling/snapping and bending,or the characteristic curve of deflection versus magnitude of applied magnetic fields being numerically displayed,the critical loads of buckling/snapping, and the influences of plastic deformation and the width of plate on these critical loads,the plastic regions expanding with the magnitude of applied magnetic field,as well as the evolvement of deflection configuration of the plate are numerically obtained in a case study.     

Theory of creep deformation with kinematic hardening for materials with different properties in tension and compression

A constitutive model for creep deformation that describes the loading-history-dependent behavior of initially isotropic materials with different properties in tension and compression under stress vector rotations limited by 50–60° is presented within a thermodynamic framework. In the proposed constitutive model a kinematic hardening rule is adopted. This model also introduces an effective equivalent stress in the creep potential that is based on the first and second invariants of the effective stress tensor, and on the joint invariant of the effective stress tensor and eigenvector associated with the maximum principal Cauchy stress. The formulation of the kinematic hardening rule is presented and discussed. All the material parameters in the model have been obtained from a series of proposed basic experiments with constant stresses. These model parameters are then used to predict the creep deformation of the aluminum alloy under multiaxial loading with constant stresses, and under non-proportional uniaxial and non-proportional multiaxial loadings for both isothermal and nonisothermal processes.     

Microscale prediction of deformation in an austenitic stainless steel under uniaxial loading

In this study, the deformation behaviour of polycrystalline austenitic 316H stainless steel under uniaxial loading is investigated by means of in-situ neutron diffraction (ND) measurement and crystal plasticity-based finite element (FE) modelling. Data have been obtained for the macroscopic stress–strain response and the lattice strain evolution in the longitudinal and transverse direction relative to the uniaxial loading axis. Comparison between the model predictions and the ND measurements suggests that in most cases the FE model can predict the lattice strain evolution at the microscale and capture the general trends observed in the experiments. Both ND measurements and FE modelling simulations identify little effect of micromorphology effect on the longitudinal lattice strain evolution, while the transverse lattice strain response appears to be sensitive to the microstructure, in particular the initial crystallographic orientation of the material.     

The excitation of waves by piezoelectric patch actuators arranged symmetrically on both surfaces of an elastic layer

An electromechanical system consisting of an elastic waveguide and flexible symmetrically arranged piezoelectric patch actuators attached to both of its surfaces is considered. In the mathematical model employed, which takes into account both the dynamic contact interaction of the patch with the waveguide and the presence of higher modes of oscillation of the layer, the effect of the geometrical and physical parameters of the system on the amount of energy delivered by the piezoelectric actuators in the substrate and its distribution between the excited Lamb waves is investigated. The analysis is carried out using the solution of a system of integro-differential equations, to which the boundary-value problem considered is reduced. In particular, it is shown that the maximum radiation of energy, transferred by antisymmetric and symmetric normal modes, is reached when the width of the patch is equal to a half-integer number of wavelengths of one of the normal modes of the patch-layer-patch triple-layer structure.     

Complex dynamics in Duffing system with two external forcings

Duffing's equation with two external forcing terms have been discussed. The threshold values of chaotic motion under the periodic and quasi-periodic perturbations are obtained by using second-order averaging method and Melnikov's method. Numerical simulations not only show the consistence with the theoretical analysis but also exhibit the interesting bifurcation diagrams and the more new complex dynamical behaviors, including period-n (n=2,3,6,8) orbits, cascades of period-doubling and reverse period doubling bifurcations, quasi-periodic orbit, period windows, bubble from period-one to period-two, onset of chaos, hopping behavior of chaos, transient chaos, chaotic attractors and strange non-chaotic attractor, crisis which depends on the frequencies, amplitudes and damping. In particular, the second frequency plays a very important role for dynamics of the system, and the system can leave chaotic region to periodic motions by adjusting some parameter which can be considered as an control strategy of chaos. The computation of Lyapunov exponents confirm the dynamical behaviors.     

Vibration serviceability of footbridges under human-induced excitation: a literature review

Increasing strength of new structural materials and longer spans of new footbridges, accompanied with aesthetic requirements for greater slenderness, are resulting in more lively footbridge structures. In the past few years this issue attracted great public attention. The excessive lateral sway motion caused by crowd walking across the infamous Millennium Bridge in London is the prime example of the vibration serviceability problem of footbridges. In principle, consideration of footbridge vibration serviceability requires a characterisation of the vibration source, path and receiver. This paper is the most comprehensive review published to date of about 200 references which deal with these three key issues.The literature survey identified humans as the most important source of vibration for footbridges. However, modelling of the crowd-induced dynamic force is not clearly defined yet, despite some serious attempts to tackle this issue in the last few years.The vibration path is the mass, damping and stiffness of the footbridge. Of these, damping is the most uncertain but extremely important parameter as the resonant behaviour tends to govern vibration serviceability of footbridges.A typical receiver of footbridge vibrations is a pedestrian who is quite often the source of vibrations as well. Many scales for rating the human perception of vibrations have been found in the published literature. However, few are applicable to footbridges because a receiver is not stationary but is actually moving across the vibrating structure.During footbridge vibration, especially under crowd load, it seems that some form of human–structure interaction occurs. The problem of influence of walking people on footbridge vibration properties, such as the natural frequency and damping is not well understood, let alone quantified.Finally, there is not a single national or international design guidance which covers all aspects of the problem comprehensively and some form of their combination with other published information is prudent when designing major footbridge structures. The overdue update of the current codes to reflect the recent research achievements is a great challenge for the next 5–10 years.     

A study of microstructural length scale effects on the behaviour of FCC polycrystals using strain gradient concepts

Grain size is a critically important aspect of polycrystalline materials and experimental observations on Cu and Al polycrystals have shown that a Hall–Petch-type phenomenon does exist at the onset of plastic deformation. In this work, a parametric study is conducted to investigate the effect of microstructural and deformation-related length scales on the behaviour of such FCC polycrystals. It relies on a recently proposed non-local dislocation-mechanics based crystallographic theory to describe the evolution of dislocation mean spacings within each grain, and on finite element techniques to incorporate explicitly grain interaction effects. Polycrystals are modeled as representative volume elements (RVEs) containing up to 64 randomly oriented grains. Predictions obtained from RVEs of Cu polycrystals with different grain sizes are shown to be consistent with experimental data. Furthermore, mesh sensitivity studies revealed that, when there is a predominance of geometrically necessary dislocations relative to statistically stored dislocations, the polycrystal response becomes increasingly mesh sensitive. This was found to occur especially during the early stages of deformation in polycrystals with small grains.     

Adaptive numerical simulation of pulsating planar flames for large Lewis and Zeldovich ranges

We study numerically the behaviour of pulsating planar flames in the thermo-diffusive approximation. The numerical scheme is based on a finite volume discretization with an adaptive multi-resolution technique for automatic grid adaption. This allows an accurate and efficient computation of pulsating flames even for very large activation energies. Depending on the Lewis number and the Zeldovich number, we observe different behaviours, like stable or pulsating flames, the latter being either damped, periodic, or a-periodic. A bifurcation diagram in the Lewis–Zeldovich plane is computed and our results are compared with previous computations [Rogg B. The effect of Lewis number greater than unity on an unsteady propagating flame with one-step chemistry. In: Peters N, Warnatz J, editors, Numerical methods in laminar flame propagation, Notes on numerical fluid mechanics, vol. 6. Vieweg; 1982. p. 38–48.] and theoretical predictions [Joulin G, Clavin P. Linear stability analysis of nonadiabatic flames: diffusional-thermal model. Combust Flame 1979;35:139–53]. For Lewis numbers larger than 6 we find that the stability limit is again increasing towards larger Zeldovich numbers and not monotonically decreasing as predicted by the asymptotic theory. A study of the flame velocities for different Zeldovich numbers shows that the amplitude of the pulsations strongly varies with the Lewis number. A Fourier analysis yields information on their frequency.