14 research outputs found
A harmonic one-dimensional element for non-linear thermo-mechanical analysis of axisymmetric structures under asymmetric loads: The case of hot strip rolling
No full textThis work presents a one-dimensional harmonic finite element for the transient elasto-plastic analysis of axisymmetric structures loaded by non-axisymmetric thermal and mechanical loads. The one-dimensional element exploits a semi-analytical approach, based on Fourier series decomposition of the applied loads. The initial stress method is used for the non-linear solution of elasto-plastic analysis. As a case study, the proposed one-dimensional harmonic element is applied for modelling a two-dimensional circle under thermal and mechanical loadings rotating over its surface, which is used as an approximation of a work roll in hot strip rolling. With the one-dimensional harmonic element, the cyclic thermo-mechanical behaviour of the work roll can be simulated by considering localized plasticity caused by thermo-mechanical loads representative of strip and back-up roll. Compared to two-dimensional models already used in the literature, the one-dimensional element allows a significant reduction in the computational time to be achieved; it follows that the whole transient thermo-mechanical response can be simulated, thus permitting a more complete evaluation of the stress-strain response that is necessary for fatigue life assessment. © IMechE 2016
A semi-analytical finite element approach in machine design of axisymmetric structures
No full textNon
Validation of compact models of microcantilever actuators for RF-MEMS application
No full textElectromechanical behavior of microcantilever specimens for in-plane and out-of-plane bending tests, currently designed by industry for Radio-Frequency application, are here analyzed. Main features of these two layouts are discussed. In particular, a comprehensive experimental validation of 2D and 3D numerical models implemented to predict the coupled electromechanical behavior of these microsystems is performed. Effectiveness of plane models to predict pull-in, in presence of geometric non-linearity, due to large tip displacement and initial curvature of microbeam, is investigated. Three dimensional models are then used to investigate the local effects of the electric field and the limits of the two dimensional approach. In addition, this paper investigates the effectiveness of 2D models to be used as compact numerical tools in substitution of some known Model Order Reduction techniques, which unfortunately are unsuitable to predict simultaneously the effects of both the electromechanical and geometric non-linearitie
Shaft design: A semi-analytical finite element approach
No full textThis work deals with the practical use of semi-analytical finite elements in the machine design. The case of mechanical shafts is considered. The most usual loading condition characterized by the presence of axial, torsional, bending, and shear loads can be modeled by over imposing an axi-symmetric, an axi-antisymmetric and a harmonic load, corresponding to the first three terms of the Fourier series expansion, if semi-analytical plane finite element is used. A practical case is presented and the advantages, with respect to the three-dimensional approach in terms of computational time and accuracy for stress and displacement evaluation, is put in evidence. © 2017 Taylor & Franci
A Special Finite Element for Static and Dynamic Study of Mechanical Systems under Large Motion”, part 2
No full textInternational audienceIn the first part of the paper the theory of the 3D dynamics of mechanical systems composed by elastic beams, structures and mechanisms, was studied. These systems are divided into so-called macro-elements and the movement equations of one macro-element were established. Only the Euler-Rodrigues parameters are used to describe the global motion of the system. In this second part of the paper a special finite element (SFET) having four degrees of freedom per node, the Euler-Rodrigues parameters, is described in details. The stiffness and mass matrices are expressed only in nodal Euler-Rodrigues parameters. The most important aspect of the proposed approach is that the exact equations, written for the deformed configuration, are solved. Therefore an extremely accurate and very fast convergent method results. To validate the SFET finite element finally several 2D and 3D, static and dynamic examples are presented and the accuracy of the results is discussed
Experimental characterization of electrostatically actuated in-plane bending of microcantilevers
No full textExperimental validation of numerical models developed by the authors to predict the static behaviour of microelectrostatic actuators is described. Cantilever microbeams, currently used in connection with RF-MEMS and micro-scale material testing were analysed. A set of microcantilevers, bending in the plane of the wafer, i.e. in the same plane as the profiling system's target, was tested. This differs from the popular case of out-of-plane microbeams, usually studied in the literature. Geometry nonlinearity caused by large deflection of the microbeam was investigated and nonlinear coupled formulation of electromechanical equilibrium was performed. Coupled-field analysis was implemented using the Finite Element Method (FEM), to predict displacements and pull-in voltage measured by Fogale Zoomsurf 3D, subsequently plotting the displacement-versus-voltage curve to complete model validation. FEM nonlinear analysis, based on iterative approach with mesh morphing, and FEM non-incremental approach, including a special element proposed by the authors, are compared to the linear solution and to experimental results. Geometry nonlinearity appears relevant in microbeam modelling and requires a nonlinear solution of the coupled problem. Investigative work, which compared the results of 2D and 3D models to experimental data, revealed that some three dimensional effects are significant in model validation, but the 2D approach may be effective in predicting static behaviour provided that at least a microbeam thickness equivalent is adopte
Validation of compact models of microcantilever actuators for RF-MEMS application
No full textMicrocantilever specimens for in-plane and out-ofplane bending tests are here analyzed. Experimental validation of 2D and 3D numerical models is performed. Main features of in-plane and out-of-plane layouts are then discussed. Effectiveness of plane models to predict pull-in in presence of geometric nonlinearity due to a large tip displacement and initial curvature of microbeam is investigated. The paper is aimed to discuss the capability of 2D models to be used as compact tools to substitute some model order reduction techniques, which appear unsuitable in presence of both electromechanical and geometric nonlinearitie
FEM modelling and experimental characterization of microbeams in presence of residual stress
No full textOut-of-plane bending tests are here used to
experimentally validate some numerical models of microbeams
actuated by the electric field. Out-of-plane bending
microcantilevers and clamped–clamped microbeams often
suffer the presence of residual strain and stress, respectively,
which affect their static and dynamic behaviour and
pull-in voltage. In case of microcantilever an accurate
modelling has to include the effect of an initial curvature
due to microfabrication process, while in double clamped
microbeams constraints may impose a pre-loading caused
by a tensile stress. So-called geometrical nonlinearity
sometimes occurs, when microcantilever exhibits large
displacement, or because of the mechanical coupling
between axial and flexural behaviours in double clamped
microbeams. Modelling this kind of nonlinearity is an
additional goal of this study. Experiments demonstrated a
good agreement with results of FEM approaches proposed.
In the case of microbridges numerical models are used to
identify the residual stress. A reverse analysis is
implemented, the axial pre-stress is calculated by means of
the measured pull-in voltage
RF-MEMS beam components: FEM modeling and experimental identification of pull-in in presence of residual stress
No full textIn this paper an experimental validation of numerical approaches aimed to predict the coupled behaviour of microbeams for out-of-plane bending tests is performed. This work completes a previous investigation concerning in plane microbeams bending. Often out-of-plane microcantilevers and clamped-clamped microbeams suffer the presence of residual strain and stress, which affect the value of pull-in voltage. In case of microcantilever an accurate modelling includes the effect of the initial curvature due to microfabrication. In double clamped microbeams a preloading applied by tensile stress is considered. Geometrical onlinearity caused by mechanical coupling between axial and flexural behaviour is detected and modelled. Experimental results demonstrate a good agreement between FEM approaches proposed and tests. A fairly fast and accurate prediction of pull-in condition is performed, thus numerical models can be used to identify residual stress in microbridges by reverse analysis from the measured value of pull-in voltage
