What is the main cause of external pressure failure?

• A. Collapse
• B. Rupture
• C. Fatigue
• D. Buckling

Answer: (D)

External pressure failure is driven by instability causing buckling. When a shell is pressed from the outside, especially if it is thin-walled or slender, it can’t support the compressive load through ordinary bending or membrane action alone. As the external pressure increases toward a critical value, any small imperfection grows into a sudden lateral deflection, and the structure loses stiffness. This buckling makes the vessel unstable and can lead to rapid collapse under loads that the material itself would otherwise tolerate. Rupture is a tearing failure from internal tension, fatigue comes from cyclic loading, and collapse can be the end result of buckling but is not the initiating mechanism itself.