Engineering Design and Analysis
This domain tests the transition from a requirement to a verified design: identifying constraints and criteria, selecting a concept, sizing components, checking failure modes, and documenting traceability. It rewards explicit assumptions and penalizes calculations that ignore tolerances, stress concentrations, or the difference between verification and validation.
Core rules and concepts
- Separate requirements (what the system must do) from constraints (limits such as budget, code, or interface) and criteria (measures used to compare options).
- Use a factor of safety deliberately: state whether it applies to yield, ultimate, fatigue, or buckling, and avoid double-counting it with separate load factors.
- Check all credible failure modes: yielding, fracture, fatigue, wear, buckling, deflection, thermal expansion, and corrosion.
- Perform a tolerance stack before declaring an assembly fits; worst-case and statistical stacks answer different questions.
- Verification confirms the design meets stated requirements; validation confirms the requirements reflect the user need.
Worked example
A tie rod carries 50 kN and uses a material with yield strength 250 MPa. A factor of safety of 2.5 on yielding is required. Required area A = FS × F / S_y = 2.5 × 50,000 N / 250 N/mm² = 500 mm². For a solid circular rod, d = √(4A/π) = √(4 × 500/π) = 25.2 mm. Select the next standard size above 25.2 mm and verify the actual stress and deflection.
Traps
Using ultimate strength when the requirement specifies yield; applying both a load factor and a factor of safety without justification; ignoring stress concentration at a shoulder; and treating a nominal dimension as the worst-case dimension.