Plastic buckling of circular tubes under combined internal pressure and axial compression

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Paquette, J. (Joshua)

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This thesis deals with the problem of plastic buckling and collapse of moderately thin-walled tubes under combined internal pressure and axial compression. The problem is investigated through a combination of experiments and analysis. Experiments were conducted on stainless steel tubes with D/t ratios of 28.3 and 39.8. The specimens were designed to mimic an infinitely long tube. The tubes were pressurized to pressures ranging from 0-70% of the yield pressure and then compressed under constant pressure. All tubes buckled in the plastic range The first buckling consisted of axisymmetric wrinkling which occurs at increasing stress. Further compression caused the wrinkle amplitude to grow. This reduced the axial rigidity of the tube and eventually caused a limit load instability representing the onset of collapse. The onset of buckling and the onset of collapse were established for each tube D/t ratio as a function of pressure. The bifurcation problem was analyzed using deformation plasticity to find the critical stress and wavelength, and flow theory plasticity to find the critical strain. Both plasticity theories were modified to include plastic anisotropy, which was found to be present in the pipe stock from which the tube specimens were cut. Inclusion of the anisotropic plasticity was essential in making the predictions agree with the measured values. The postbuckling behavior of the tubes was analyzed by considering a shell with a uniform axisymmetric imperfection. Anisotropic flow theory was used in this model. The model reproduces the measured responses up to and including the limit point. For pressurized tubes, predictions of the limit strain were in good agreement with the experiments. For pure compression, the measured and calculated values differed somewhat due to a second nonaxisymmetric bifurcation that was encountered in the experiments but that was not included in the model. The calculated limit stresses were somewhat lower than the measured values due to finite deformation effects not included in the model. Modeling of the anisotropy was essential for the success of this analysis also


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