Mechanical Engineering Design and Analysis
Core analysis toolkit
You must move fluently between stress, deflection, fatigue and failure-mode reasoning. Know the difference between a design factor (a policy choice covering unknowns and consequence of failure) and the calculated factor of safety on a specific limit state. Pressure containment follows the thin-wall membrane equations; vessel and piping rules (ASME VIII, ASME B31 series) add corrosion allowance, mill tolerance and joint efficiency E. Shafts are sized on combined torque and bending, with keys, shoulders and grooves treated as stress raisers. Bolted joints are preloaded to a target fraction of proof load, not to the service load. Bearings are rated on basic rating life L10; fillet welds on throat thickness (0.707 x leg for a 45 degree fillet).
Traps
- Hoop stress is twice the longitudinal stress in a closed cylinder; picking the wrong one halves the answer.
- Using yield strength where the code specifies ultimate multiplied by a factor, or forgetting allowable stress falls with temperature.
- Using nominal thickness instead of corroded thickness - always ask what it is at end of life.
- Treating a fatigue problem as static: 30% of yield still fails after millions of cycles at a sharp notch.
- Unit traps: MPa = N/mm2; 1 kgf about 9.81 N; mixing mm and m inside one formula.
Worked example
Cylindrical shell: p = 1.5 MPa, D = 1200 mm, t = 10 mm, E = 0.85. Hoop: pD/(2tE) = (1.5 x 1200)/(2 x 10 x 0.85) = about 106 MPa. Longitudinal: pD/(4tE) = about 53 MPa. If the allowable at design temperature is 138 MPa, the design passes with margin about 1.3. State the limit state and code clause - markers award the basis, not just the number.