INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING Int. J. Numer. Meth. Engng 2001; 52:1465–1485 (DOI: 10.1002/nme.266) Singular perturbations for sensitivity analysis in symmetric bifurcation buckling Luis A. Godoy 1; *; † and Enrique G. Banchio 2 1 Department of Structures; FCEFyN; National University of C ordoba; P.O. Box 916; C ordoba; Argentina 2 National University of C ordoba; C ordoba; Argentina SUMMARY A direct procedure for the evaluation of imperfection-sensitivity in bifurcation problems is presented. The problems arise in the context of the general theory of elastic stability (Koiter’s theory) for discrete structural systems, in which the total potential energy is employed together with a stability criterion based on energy derivatives. The imperfection sensitivity of critical states, such as bifurcations and tri- furcations, is usually represented as a plot of the critical load versus the amplitude  of the imperfection considered. However, such plots have a singularity at the point with  =0, so that a regular perturbation expansion of the solution is not possible. In this work, we describe a direct procedure to obtain the sensitivity of the critical load (eigenvalue of the bifurcation problem) and the sensitivity of the critical direction (eigenvector of the bifurcation problem) using singular perturbation analysis. The perturbation expansions are constructed as a power series in terms of the imperfection amplitude, in which the expo- nents and the coecients are the unknowns of the problem. The solution of the exponents is obtained by means of trial and error using a least degenerate criterion, or by geometrical considerations. To com- pute the coecients a detailed formulation is presented, which employs the conditions of equilibrium and stability at the critical state and their contracted forms. The formulation is applied to symmetric bifurcations, and the coecients are solved up to third-order terms in the expansion. The algorithm is illustrated by means of a simple example (a beam on an elastic foundation under axial load) for which the coecients are computed and the imperfection-sensitivity is plotted. Copyright ? 2001 John Wiley & Sons, Ltd. KEY WORDS: bifurcation; buckling; imperfection-sensitivity; instability; perturbation techniques; sensitivity analysis 1. INTRODUCTION This paper reports on a new methodology to compute the imperfection sensitivity response of a structure in bifurcation buckling problems. Imperfection-sensitivity is a central topic in * Correspondence to: Luis A. Godoy, Department of Structures, FCEFyN, National University of C ordoba, P.O. Box 916, C ordoba, Argentina † E-mail: lgodoy@com.uncor.edu Contract=grant sponsor: CONICET, Argentina Contract=grant sponsor: Science Research Agency of C ordoba Contract=grant sponsor: National University of C ordoba Copyright ? 2001 John Wiley & Sons, Ltd.