DIRECT EVALUATION OF THE WORST IMPERFECTION SHAPE IN SHELL BUCKLING

For the evaluation of the imperfection-sensitivity of elastic and elas tic-plastic shells a fully nonlinear finite-element-method has been de veloped that directly gives the 'worst' imperfection shape connected t o the ultimate limit load. The approach uses no approximations such as asymptotical theories or computations in the deep postcritical range. The key point of the method is the description of imperfections as no dal degrees of freedom at the element level. These unknown quantities are implemented by isoparametric shape functions in a finite shell ele ment including finite rotations and thickness stretch. The ultimate bu ckling load and the corresponding 'worst' imperfection shape is define d by two different criteria and numerically determined by an extended system of nonlinear equations. In the computation of the structurally stable collapse the deflections, the imperfection shape, the eigenvect or and the lowest possible load-level are obtained. The method is illu strated by several numerical examples.