External Pressure Buckling Best Practices
Principles for external pressure buckling analysis — eigenvalue screening, imperfection modelling, nonlinear collapse, mesh requirements and margin assessment.
Start with eigenvalue analysis
Run a linear eigenvalue buckling analysis first to identify the critical mode shape and estimate the upper-bound buckling pressure. Use this as a screening tool, not as the design basis.
Always perform nonlinear analysis
Eigenvalue predictions are consistently unconservative for shell buckling. Nonlinear analysis with explicit imperfections is required for a defensible collapse pressure prediction. Include large-deformation effects and nonlinear material behaviour if stresses approach yield.
Model realistic imperfections
Scale the lowest eigenmode to the manufacturing tolerance (typically 0.5-1.5% ovality). Alternatively, use measured imperfection data if available. The imperfection shape and amplitude significantly affect the collapse pressure.
Use arc-length or Riks method
Apply the external pressure incrementally using arc-length or modified Riks method. These methods can trace the load-displacement path through the instability point, capturing the maximum sustainable pressure (collapse pressure).
Perform a sensitivity study
Vary the imperfection amplitude and assess the effect on the collapse pressure. If the collapse pressure is highly sensitive, tighter manufacturing tolerances or more conservative design assumptions may be needed.