The present paper deals with instabilities of long homogeneous and isotropic thin shells, characterized by geometrical nonlinearities and imperfections.It is therefore of interest for the designer to know how these structural elements behave underdifferent loading conditions (lateral and/or hydrostatic pressure) especially with reference to abent helicoidally geometrical shape of particular interest for the helicoidally Steam Generatorstube bundle.Apparently no data exist in the literature to describe the non-linear buckling behaviour of curvedthin shells under external pressure, thus the theoretical analyses, based on the classical linearelastic theory, and might be inadequate to evaluate the collapse load especially if the curvature israther large.To the purpose of determining the buckling pressure load the effects of a pre-existing level ofgeometrical and technological imperfection, unavoidably caused by various manufacturingprocesses were also considered.A numerical analysis with available computing resources (FEM code) has allowed to take intoaccount the nonlinear geometrical and material properties and to set up appropriate models todescribe the buckling phenomenon.Moreover at Pisa University a rather intense experimental research activity is being carried outon the buckling of thin walled tube specimens in the dimensional range suitable for the abovementioned applications.A validation of numerical evaluations by comparison with test results is performed and the localcharacter of buckling in the circumferential direction is also demonstrated as well as therelationship of the buckling stress on the severity of the initial defects (mainly diameter tothickness or tube/bending diameters ratios) on the collapse loads. A detailed knowledge of thisdependency would lead to a better prediction of the buckling stress of the considered thin shell.
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