1,721,051 research outputs found
Coupled instability in thin-walled structures by a FEM implementation of Koiter's approach
No full textUn approccio perturbativo misto per valutare l'ordine di meccanismi infinitesimi in strutture reticolari spaziali
No full textExtrapolation locking and its sanitization in Koiter's asymptotic analysis
No full textThis paper shows that the FEM implementation of Koiter's asymptotic method [W.T. Koiter, On the stability of elastic equilibrium, 1970, Ph.D. Thesis, Delft, 1945. English transl. NASA TT-F10, 883, 1967, AFFDL-TR70-25] outlined by Casciaro et al. [Finite element asymptotic analysis of slender elastic structures: a simple approach, Int. J. Num. Meth. Eng. 35 (1992) 1397-1426] provides accurate and reliable results in the critical and post-critical analysis of non-linear elastic structures. Care, however, does have to be taken in implementing (apparently) minor details to avoid locking effects which adversely affect accuracy and which can destroy the method reliability. As the effects related to the finite element interpolation have been discussed before this paper focuses on the nonlinear locking due to the use, implicit in the method, of finite distance extrapolations. Within this scope, it is shown that perturbation algorithms based on compatible formulations can imply a strong critical and post-critical locking when analysing structures characterized by high stiffness ratios in the presence of moderate pre-critical rotations. On the contrary, perturbation algorithms based on independent extrapolations of displacements and stresses furnish reliable results in excellent agreement with those provided by step-by-step analysis, at a small fraction of its computational cost. (C) 1999 Elsevier Science S.A. All rights reserved
Path-following analysis of thin-walled structures and comparison with asymptotic post-critical solutions
No full textA numerical analysis of infinitesimal mechanisms
No full textThe paper presents a numerical algorithm, based on Koiter's theory of the elastic stability, for detecting the order of infinitesimal mechanisms, i.e. kinematically indeterminate systems of pin-jointed bars. In cases of one degree of indeterminacy the algorithm improves, in terms of computational simplicity and efficiency, an analogous algorithm proposed by Salerno in 1992. This is shown to be due to the vanishing of the terms higher than the third-order of the asymptotic expressions of the energy, owing to the use of the Green strain measure and a mixed (displacement and stress) formulation of the problem. Moreover, the proposed algorithm is able to provide a correct definition of mechanism in cases of several degrees of indeterminacy, mainly for structures like those first studied by Connelly and Servatius in 1994, which the paper will treat in depth
A constrained solid-shell model for the geometric nonlinear finite-element analysis of laminates with alternating stiff/soft layers. Applications to laminated glass
Full text linkSolid-shell models are developed for the geometrically nonlinear analysis of multi-layered composite structures made of alternating layers with large difference in material properties. Exemplificative applications are presented for
aminated glass, in which a number of stiff plies of glass are permanently shear-coupled by soft
interlayers. The sectional warping due to significant transverse shear strains in the soft layers makes theories of laminated plates based on the plane-section hypothesis unreliable. The proposed approach is based on a
geometrically exact solid-shell finite element model with one element per layer in the thickness direction, as
alternative to solid discretization. The element approximation is based on the displacement nodal values at
the top and bottom surfaces of the layers, with a natural C0 continuity. An alternative solid-shell model with
fewer parameters is derived imposing the equal finite rotation of the stiff layers at each surface point by a local rotation-free re-parametrization of the nodal displacements and enforcing the plane stress condition. The approach permits an easy coupling with a fully solid discretization, e.g. to model connections, and is based on a simple strain measure quadratic in the displacement unknowns and suitable for finite strains. Extensive numerical examples for laminated glass plates and curved shells susceptible to large deflections and buckling are provided, comparing the results with those from a fully solid approach
Prediction of limit-cycles oscillations in digitally controlled DC-DC converters using statistical approach
No full textDigitally controlled DC-DC converters are affected by quantization effects on A/D converters and digital pulse-width modulators (DPWMs) which may result in undesirable limit-cycle oscillations. Existing static and dynamic models predict the existence of only a small part of limit cycle oscillations, so that extensive time-domain simulations are usually needed in order to verify the presence of limit-cycle oscillations under different load and input voltage conditions. This paper proposes an alternative approach based on statistical models. Modelling the quantization error as a white noise, including the quantization effects on the controller and converter state variables, and evaluating the correlation between state variables, a statistical prediction of limit-cycle oscillations is obtained. By means of the proposed method, design criteria for the regulator parameters, in terms of achievable bandwidth, location of PID zeros and desired phase margin, can be derived. Simulation and experimental results confirm the validity of the proposed method
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