Design and Analysis
Design intent and verification
Design and Analysis tests whether you can turn requirements into a safe, buildable solution and prove it. The exam favors questions on design criteria, failure modes, factors of safety, tolerances, and the validation of calculations or models.
Core rules
- Define the design basis first: loads, environmental conditions, service life, interfaces, and acceptance criteria.
- Check every credible failure mode: yielding, fracture, fatigue, buckling, creep, corrosion, and excessive deflection.
- Factor of safety FS = failure strength/allowable stress, or ultimate load/working load. Do not mix ultimate and yield strengths in one comparison.
- In limit-state design, compare factored resistance with factored load effects; in allowable-stress design, compare service stress with allowable stress.
- Validate FEA with hand checks, equilibrium, mesh convergence, and sensitivity to boundary conditions; a colorful contour plot is not verification.
Worked example
A 20 mm diameter tie rod of yield strength 250 MPa is designed with FS = 2.0 against yielding. Area A = πd²/4 = π(0.020)²/4 = 3.142 × 10⁻⁴ m². Allowable stress = 250/2 = 125 MPa. Allowable load P = σA = (125 × 10⁶)(3.142 × 10⁻⁴) = 39.3 kN. If the calculated service load is 45 kN, the design fails and the rod diameter or material must change; citing FS = 2.0 without checking the actual load is not enough.
Traps
Applying the safety factor to load instead of stress in a nonlinear problem; using nominal dimensions when corrosion or wear allowance is required; ignoring stress concentrations at threads; and accepting an FEA result without checking reaction forces.