SHIP STRUCTURE COLLAPSE ANALYSES DUE TO NONLINEAR EFFECTS, WITH A FOCUS ON CYCLIC LOADS

The thesis investigates the nonlinear effects on ship stiffened panels caused by various applied loading conditions, specifically monotonic loading and cyclic loading. The study also considers their influence on the ultimate capacity. The research investigates the impact of geometric imperfections, including both initial imperfections from production processes and those arising from the structure's exposure to repetitive loading conditions, referred to as residual plastic deformations. In relation to the application of cyclic load conditions, a numerical investigation is conducted to study the accumulation of residual plastic deformation in stiffened panels subjected to compression and tension-compression cyclic loading conditions. The proposed numerical models allow to understand how cyclic loads lead to the accumulation of residual plastic deformations and how these could affect the ultimate load strength value, compared to monotonic loading scenarios. The comparison between results obtained from the only compression cyclic loading case and the alternate traction-compression loading is presented, emphasizing the impact of load sign alteration.. Furthermore, the investigation explores various material hardening models—namely isotropic, kinematic, and combined—and their influence on the results. The numerical models are built with different strategies and they are cyclically subjected to uniaxial loading under different time histories: in particular, the maximum compressive load is applied as a fraction of the ultimate load of the intact structure, which respects the design limits of the panel itself. In addition, the effect of the accumulated plastic deformations on the ultimate strength of the structure is analysed, in order to evaluate the influence of the cyclic load history in terms of geometric changes and consequent residual stresses field. On the other hand, a monotonic load experimental experience has been followed in the laboratory of the University of Genova. The experiment refers to a project by Fincantieri, which deals with buckling check methodologies for HSLA steel marine structures. The results obtained from this experiment are useful to better understand the instability phenomena of the stiffened panels and the effects of some important parameters on the ultimate strength, i.e. geometric imperfections and residual stresses, which are calculated. Finally the experiment is reproduced in a numerical way, trying to elaborate a suitable model and a consistent procedure. As a result, the thesis delivers significant insights into nonlinear phenomena such as plasticization and instability, as well as geometric imperfections. It also draws comparisons across various numerical modelling approaches, loading scenarios, and material hardening laws.