Prof. Konstantinos V. Spiliopoulos
National Technical University of Athens, Greece
Title: Direct Numerical Assessment of Asymptotic Cyclic States of Inelastic Structures
Abstarct: During their lifetime, civil and mechanical engineering structures are subjected to cyclically repeated mechanical and/or thermal loadings. For cost effective design, their material is pushed to stress beyond its elastic limit, and thus, inelastic time independent (plastic strains), and/or inelastic time dependent (creep strains) develop. The accumulation of these strains over the loading cycles has a serious impact on the safety of the structure, as well as on its lifetime. Depending on the level of loading, plastic straining, for example, may lead the structure to failure, either due to low cycle fatigue or ratcheting, or to safety through elastic shakedown, where the asymptotic strains are fully elastic. The question, whether a cyclic loading may lead the structure to safe or unsafe asymptotic states, may be answered by inelastic step by step analyses, which follow, consecutively, all the loading cycles. Such analyses are very time consuming and often numerically unstable. Alternatively, there are numerical methods, called Direct Methods, that bypass the transient deformation stages, and search the asymptotic states, right from the start of the calculations. This plenary lecture deals with the presentation of such a method, which has many computational advantages against other direct methods, in terms of simplicity and efficiency. It is an iterative procedure and has been called Residual Stress Decomposition method (RSDM). After resolving the total stress into elastic and residual stresses, the method focuses on the physics of the asymptotic state, in which, the sought residual stresses are expected to be cyclic. Thus, these unknown residual stresses are decomposed in Fourier series, whose coefficients are found iteratively. All kinds of behavior, either shakedown, or low cycle fatigue, or ratcheting, may be predicted. The method has also been adapted to estimate the safety factor, when only the variation intervals of the loading are known. The implementation of the approach is realized within the framework of the finite element method. A numerical scheme, which provides the procedure with super-linear convergence, will also be presented.
Bio: Dr. Konstantinos V. Spiliopoulos, is Professor Emeritus of Structural Mechanics in the Institute of Structural Analysis and Antiseismic Research, School of Civil Engineering of the National Technical University of Athens (NTUA), Greece. He is also Adjunct Professor of the Beijing Jiatong University in China. He received his Diploma in Civil Engineering from NTUA and holds a PhD Degree in Structural Mechanics from the Department of Aeronautics of Imperial College of Science & Technology, University of London. He has research publications in many diverse areas of structural mechanics related to numerical algorithms, graph theory, mathematical programming, inelastic analysis of structures, static & dynamic analysis of structures, nonlinear reinforced concrete analysis of structures (NRCA), large displacements, large rotations, large strains, direct methods, limit analysis, shakedown analysis. He has published 35 papers in top rated international journals, 8 chapters in edited refereed books and over 65 papers in international conferences. He has been a reviewer in 33 international journals. Work on NRCA has been adopted by the ADINA software and is also recommended in the 2018 BetonKalender. He has been the main guest editor of the Springer SNAS Journal Topical Collection in Engineering ‘Limit States Analysis and Design of Structures and Materials’, and invited lecturer in 22 International Workshops. He is also co-editor of two edited books.