Evaluation of the stability and load-carrying capacity of double-curved unstiffened shells under compression through analysis of different initial imperfection sources

• Numerical analysis of doubly curved unstiffened shells under axial compression. • Comparative study of MSI, LBMI, and SPLI imperfection scenarios is presented. • LBMI is the most conservative; MSI and SPLI show correlated, realistic prediction potential. • Considered convex shells are imperfection-sensitive but carry higher loads. • Some concave shells show imperfection insensitivity with higher prebuckling stiffness. The article concerns the use of various sources of initial imperfections to numerically assess the stability and load-carrying capacity of thin-walled segments of isotropic and composite toroidal shells subjected to compression. The considered sources of initial imperfections are: Linear Buckling Mode-shaped Imperfection, Single Perturbation Load Imperfection and actual maps of mid-surface (MSI) and thickness imperfections, originally measured for unstiffened cylindrical and conical shells. The most conservative Knock-Down Factor estimates were determined using Linear Buckling Mode approach, while the results provided by Single Perturbation Load approach were consistent with those obtained using measured MSI but were slightly more conservative. The studies demonstrated that a slight convexity can increase the load-carrying capacity of a composite doubly curved structure compared to the nominal cylinder or cone structure. Some of the analysed concave structures exhibited characteristics of imperfection-insensitive structures, where the load-carrying capacity significantly exceeded the critical buckling load determined from linear stability analysis.