Pressure Vessels, Welds, Fatigue, GD&T, and Design Safety
Key Takeaways
- Pressure-vessel questions usually require recognizing thin-wall assumptions, hoop stress, longitudinal stress, and code-aware safety context.
- Weld design starts with throat area, load direction, weld group geometry, and allowable shear or combined stress.
- Fatigue questions require alternating stress, mean stress, stress concentration, surface condition, size effect, and safety factor thinking.
- GD&T questions test how form, orientation, location, and datum references control manufacturing variation.
- Design safety is a system decision: material, geometry, load uncertainty, inspection, standards, and consequence of failure all matter.
- FE problems may ask for the governing failure mode rather than a long calculation.
Pressure vessels: identify the wall model
Pressure-vessel questions begin by deciding whether a thin-wall approximation is appropriate. A common FE cue is that wall thickness is small relative to radius. For a thin cylindrical vessel, hoop stress is larger than longitudinal stress, so hoop stress often governs shell sizing. For spherical vessels, membrane stress is lower than the cylindrical hoop stress for the same pressure, radius, and thickness.
| Vessel cue | Likely check |
|---|---|
| Thin cylindrical shell | Hoop stress and longitudinal stress |
| Spherical shell | Membrane stress |
| Thick wall | Lamé-type stress distribution if provided |
| Nozzle or opening | Local reinforcement and stress concentration concept |
| Code compliance | Materials, allowable stress, inspection, testing, and documentation |
Do not treat pressure-vessel design as only a formula lookup. The formula estimates stress. Real safety also depends on material specification, corrosion allowance, joint efficiency, fabrication quality, relief devices, nondestructive examination, and hydrostatic or pneumatic testing requirements when applicable.
Welds: throat and load path
Fillet weld strength is usually based on effective throat area, not the visible leg length alone. If the problem gives throat thickness, use it directly. If it gives leg size for an equal-leg fillet weld, the effective throat is about 0.707 times the leg size. Multiply effective throat by weld length to get area for a simple direct-shear check.
Weld groups can also see moment, torsion, or eccentric loading. In those cases, the most highly loaded point may not be found by simple total load divided by total weld length. FE questions often stay simpler, but the load path still matters: direct shear, tension, bending, eccentric bracket, or torsion.
Fatigue: repeated loading changes the rule
Static safety factors compare stress to yield or ultimate strength. Fatigue safety checks repeated cycles, often using alternating stress and mean stress. Stress concentration, notch sensitivity, surface finish, size, reliability, and loading type can all reduce fatigue strength. A polished laboratory specimen has a different endurance behavior than a keyed shaft with a shoulder fillet.
| Fatigue cue | Meaning |
|---|---|
| Fully reversed | Mean stress near zero, alternating stress dominates |
| Pulsating tension | Positive mean stress increases damage risk |
| Notch, keyway, thread root | Stress concentration matters |
| Infinite-life language | Compare to endurance-style limit if applicable |
| Variable amplitude | Damage accumulation concept may apply if given |
GD&T and design safety
Geometric dimensioning and tolerancing controls how manufactured geometry may vary. Datums establish reference features. Flatness, straightness, circularity, and cylindricity control form. Perpendicularity, parallelism, and angularity control orientation. Position controls feature location relative to datums. Runout controls rotating feature variation.
For FE purposes, focus on the engineering reason. A bearing seat may need circularity and runout control. A bolt pattern may use position tolerance relative to datums. A sealing surface may need flatness. GD&T is not decoration on a drawing; it is a design safety tool because it controls assembly, load sharing, vibration, leakage, and inspectability.
Safety-factor judgment
A design safety factor should reflect load uncertainty, material variability, inspection quality, consequence of failure, environment, fatigue, and code requirements. A low-consequence bracket and a pressure vessel near personnel do not deserve the same risk posture. On the exam, when options include ignoring a standard, hiding an overstress, or reducing inspection to meet schedule, reject them. Engineering design protects users through calculation, standards, verification, and honest reporting.
For a thin-walled cylindrical pressure vessel under internal pressure, which membrane stress is typically larger?
An equal-leg fillet weld is specified by leg size, but the allowable shear check uses effective throat. What approximate factor converts leg size to throat?
Which drawing-control need is most directly addressed by a position tolerance relative to datums?