Buckling strength sensitivity for thin-walled tubes with complex imperfections

Thin-walled tubes are widely used for civil infrastructure, such as wind turbine support towers. These tubes have inevitable defects arising from manufacturing. Like all shells, these imperfections can trigger a variety of collapse mechanisms and lead to highly variable strength. Classically this is understood as deriving from a complex interplay between the imperfection shape and the many nearly coincident buckling modes that exist in such shells. The conventional numerical approach to assess the influence of these imperfections is to include structural defects using the primary buckling modes (e.g., modes 1-3) in a geometric and material nonlinear shell finite element analysis on the imperfect structure, i.e., a GMNIA solution. Measured imperfections typically do not take the form of the primary buckling modes. Instead, imperfections have more complex shapes that can be understood as combinations of several higher-order modes that potentially induce failures. However, it remains unclear which combinations of higher modes result in significant reductions in buckling strength, so there is little guidance as to which modes should be included in imperfection models. Assessing the sensitivity of buckling strength to imperfections from mode combinations is, therefore, critical to understanding which higher modes should be included in imperfection models. In this study, we impose imperfections as combinations of the first one hundred axial and bending buckling modes, then perform GMNIA collapse simulations under uniform bending with different magnitudes to assess the strength sensitivity. In addition, we also geometrically categorized the mode shapes to assess sensitivity according to a characteristic length-scale (wavelength) of each mode and observe the specific imperfection features that cause large drops in strength. The intent of the work is to build imperfection characterizations that are optimally informed by structural features that drive strength.

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