Chapter 15 - Combined Internal Pressure and Axial Compression

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This chapter addresses the problem of plastic buckling of long tubes under combined internal pressure and axial compression. Stainless-steel tubes with D/t ranging from about 28 to 40 are compressed to collapse at different levels of internal pressure up to values of 75% of the yield pressure. The behavior is similar to that of pure axial compression: the structure deforms uniformly into the plastic regime and develops axisymmetric wrinkling at an increasing load. With further compression the wrinkle amplitude grows, the axial rigidity is reduced, eventually leading to a limit load instability, which represents the onset of collapse. Internal pressure lowers the yield load and the overall axial response. It also increases the wrinkle wavelength, makes them more stable causing the switch to non-axisymmetric modes less preferred, and increases the axial strain at collapse. In the experiments reported, all tubes remained axisymmetric until the end of the tests past the limit load. The critical stress and wavelength are established using classical plastic bifurcation theory based on the deformation theory of plasticity. An axisymmetric shell analysis is first shown to reproduce the experimental results, and subsequently used to study the effect of internal pressure, the D/t, geometric imperfections, and material parameters on the strain at the onset of instability and at collapse.