External pressure design aims to prevent buckling collapse.

• A. Fatigue failure
• B. Yielding
• C. Thermal cracking
• D. Buckling collapse

Answer: (D)

External pressure design focuses on stability of a shell when the outside pressure compresses it. Under external pressure, slender shells can lose stability and buckle, leading to collapse even before the material yields. The aim is to ensure the buckling (collapse) pressure is higher than the service external pressure, so the structure remains stable. This is achieved by selecting appropriate thickness, geometry, end conditions, and stiffeners to delay or prevent buckling. Fatigue failure involves repeated loading and crack growth, which is a different concern from the static stability addressed here. Yielding is plastic deformation due to stress but is not the primary failure mode for the external-pressure instability being designed against. Thermal cracking results from temperature gradients and is not the central issue in exterior-pressure buckling design.