Failure, Fatigue & the Structural Design Review

Machines rarely fail as a uniform coupon under one perfect pull. They loosen, wear, buckle, crack at a shoulder, slip at a joint, overload during a jam, corrode, overheat, or drift out of alignment. MIT’s materials curriculum distinguishes elastic response, plastic flow, creep, fracture, and fatigue because each has different evidence and countermeasures ¹.

Screen the limit states

• Yield: permanent deformation under peak combined stress.
• Buckling: instability of a slender compression member, often before material yield.
• Fatigue: crack initiation and growth under fluctuating stress.
• Fracture: unstable crack growth when toughness is insufficient for stress and flaw size.
• Creep: time-dependent deformation at relevant temperature and stress.
• Wear and fretting: surface loss or damage from contact and small oscillatory motion.
• Joint failure: slip, separation, embedment, bolt fatigue, weld cracking, or thread stripping.
• Functional failure: excessive deflection, vibration, backlash, or alignment loss.

Fatigue as an alternating-load problem

For a stress cycle with maximum \(\sigma_{max}\) and minimum \(\sigma_{min}\), define

Both alternating and mean stress matter. Surface finish, size, environment, notches, residual stress, and reliability change fatigue performance. A motor mount seeing 20 to 80 MPa has \(\sigma_a=30\) MPa and \(\sigma_m=50\) MPa. Calling it “an 80 MPa load” loses the cycle information.

Fault loads deserve their own cases

Normal operation may not govern. Include hard stop, e-stop deceleration, dropped payload, jammed conveyor, seized bearing, pressure spike, off-center part, maintenance leverage, and transport. Define whether each is expected repeatedly, rare but survivable, or allowed to cause controlled sacrificial damage.

A compact structural review package

1. Annotated assembly and load path.
2. Load-case table with sources and combinations.
3. FBDs and reaction calculations.
4. Internal-force, stress, deflection, and stability checks.
5. Connection and fatigue screens.
6. Assumptions and uncertainty register.
7. Verification plan: inspection, proof load, strain measurement, deflection test, or teardown.

Safe verification

Use barriers, remote loading, controlled energy, and competent supervision for structural tests. Do not place a person on or near an unproven load-bearing mechanism. Increase load in bounded steps, inspect between steps, define stop criteria in advance, and treat unexpected noise, slip, or permanent set as data—not a dare to continue.

Course project

Complete the review package for a small machine frame or robot pedestal. Your conclusion must name the governing limit state, margin, and verification evidence still missing. Engineering confidence is calibrated: “calculation suggests” and “test demonstrates” are different claims.