Federico Guarracino

On Elastoplastic Buckling Analysis of Cylinders Under Nonproportional Loading by Differential Quadrature Method

The paper investigates the elastoplastic buckling of thin circular shells subjected to nonproportional loading consisting of axial tensile stress and external pressure. The governing equations of buckling for cylindrical shells derived by Flugge serve as the basis of analysis. To capture the elastic/plastic behavior, two plasticity theories are considered; the flow theory and the deformation theory of plasticity. Plastic buckling pressures for cylinders with various combinations of boundary conditions are presented for which no analytical solutions are available. The results obtained from the flow and deformation theories confirm that, under over-constrained kinematic assumptions, the deformation theory tends to provide lower values of buckling pressure and the discrepancies in the results from the two plasticity theories increase with increasing thickness-to-radius ratios, tensile stresses, boundary clamping and E/σy ratios. The plastic buckling results obtained by means of the differential quadrature method are compared with carefully conducted FEA results for both the flow and the deformation theory of plasticity. The reasons underlying the apparent plastic buckling paradox are thus investigated for a vast class of boundary conditions and loads.

Stability Analysis of Circular Beams with Mixed-Mode Imperfections under Uniform Lateral Pressure

The elastic-plastic collapse of circular beams under uniform lateral pressure with an initial imperfection represented by a combination of different modes and amplitudes and with varying material properties is analysed from a computational viewpoint. The work is stimulated by a number of accurate experimental tests recently performed and it is found that both the initial imperfection and the material inhomogeneity along the beam axis can affect the collapse and produce a sensible variation in the carrying capacity of the structure on account of the changes between the underlying buckling modes. This can give reason for some apparently anomalous observed experimental results.

An Analytical Approach to the Analysis of Inhomogeneous Pipes under External Pressure

Pipes for deep-water applications possess a diameter-to-thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that prior to failure the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding. This paper presents an investigation into the mechanics of this specific problem and develops an analytical approach that accounts for the effects of geometrical and material data on the collapse pressure of inhomogeneous rings under external hydrostatic pressure. The analytical expressions have been correlated to numerical and experimental test data, proving their accuracy.

Numerical Evaluation of Plastic Buckling of Short Cylinders Under Combined Loading

The so-called “plastic buckling paradox” originates from the fact that the Prandtl–Reuss’ flow theory of plasticity overestimates the plastic buckling load of plates and shells, whereas Hencky’s deformation theory of plasticity provides results that are more accurate. However, it has been shown that this problem can be overcome by introducing certain initial imperfection in accurate finite element (FE) simulations based on the flow theory of plasticity.1–4 The present study goes deeper into the problem and reveals that in the case of short cylinders under combined loading, which have long been the object of extensive research in the elastic range,5 a different modeling of the material behavior can also trigger a mode jumping from the initial imperfection, which may even reverse the reported predictions by the flow and deformation theories of plasticity. This fact must be taken in maximum consideration when performing nonlinear FE analyses for estimating the plastic buckling of thin and moderately thin short cylindrical shells.