The subject of this paper is nonlinear vibrations of beams, strings (defined as beams with very thin uniform cross sections), plates and membranes (defined as very thin plates) without initial tension. Such problems are of great current interest in minute structures with some dimensions in the range of nanometers (nm) to micrometers (m). A general discussion of these problems is followed by finite element method (FEM) analyses of beams and square plates with different boundary conditions. It is shown that the common practice of neglecting the bending stiffness of strings and membranes, while permissible in the presence of significant initial tension, is not appropriate in the case of nonlinear vibrations of such objects, with no initial tension, and with moderately large amplitude (of the order of the diameter of a string or the thickness of a plate). Approximate, but accurate analytical expressions are presented in this paper for the ratio of the nonlinear to the linear natural fundamental frequency of beams and plates, as functions of the ratio of amplitude to radius of gyration for beams, or the ratio of amplitude to thickness for square plates, for various boundary conditions. These expressions are independent of system parameters-the Young's modulus, density, length, and radius of gyration for beams; the Young's modulus, density, length of side, and thickness for square plates. (The plate formula exhibits explicit dependence on the Poisson's ratio.) It is expected that these results will prove to be useful for the design of macro as well as micro and nano structures.
SUMMARYMicro-electro-mechanical (MEM) and nano-electro-mechanical (NEM) systems sometimes use beamor plate-shaped conductors that can be very thin-with h=L ≈ O(10 −2 -10 −3 ) (in terms of the thickness h and length L of a beam or the side of a square pate). Conventional boundary element method (BEM) analysis of the electric ÿeld in a region exterior to such thin conductors can become di cult to carry out accurately and e ciently-especially since MEMS analysis requires computation of charge densities (and then surface tractions) separately on the top and bottom surfaces of such objects. A new boundary integral equation (BIE) is derived in this work that, when used together with the standard BIE with logarithmically singular kernels, results in a powerful technique for the BEM analysis of such problems with thin beams. This new approach, in fact, works best for very thin beams. This thin beam BEM is derived and discussed in this work.
It has been shown in previous research that, relative to (the usually considered case of) a single impact, multiple impacts (clattering) of rigid casings can greatly enhance the probability of failure of fragile components mounted on or inside them. This paper addresses the important issues of the roles of casing flexibility and contact model in the above situation. A finite element analysis of clattering of a Timoshenko beam is carried out here. Dependence of the maximum change in average velocity due to impact, on the beam stiffness and coefficient of restitution, are studied here.
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