Worst Multiple Perturbation Load Approach of stiffened shells with and without cutouts for improved knockdown factors

An optimization framework of determining the worst realistic imperfection was proposed by the present authors to study the reduction of the load-carrying capacity of unstiffened cylindrical shells. However, with regard to stiffened shells, especially when cutouts are included, the dimple combination pattern should be judged in a more rational manner. In this study, node coordinates are utilized to describe the position of each dimple-shape imperfection for Worst Multiple Perturbation Load Approach (WMPLA), which is an improvement of the MPLA using an optimization algorithm to find the application positions that will reduce the buckling load. Further, a novel method to determine the density of possible positions of dimple-shape imperfections is proposed based on eigenmode shape for stiffened shells without cutout. In addition, the effects of cutouts on the proposed method are investigated in detail. The effectiveness of the proposed method is demonstrated by comparison of several conventional methods to obtain improved knockdown factors (KDFs).