- Was It Pulled Apart, or Bent Out of Shape?
· Cause of Death #1: Clean Break (Insufficient Tensile Strength)
· Crime Scene: Bolt snapped cleanly; shaft fractured at the root. The most dramatic end.
· Design Lesson: You sized it for bulk, but forgot to calculate tensile stress. Strength is the baseline for survival – never let your guard down.
· Cause of Death #2: Bent and Permanently Deformed (Yield Strength Exceeded)
· Crime Scene: A lifting arm, after just one mild overload, took a slight set and could never be straightened again.
· Design Lesson: Stress exceeded the material’s yield strength. The part didn’t break, but it’s scrap. Always apply a safety factor – don’t treat yield as your working limit.
· Cause of Death #3: Shaking Like Parkinson’s – Not Broken, Not Bent, but Unstable (Insufficient Stiffness)
· Crime Scene: A gantry stands fine, but vibrates violently at speed, ruining machining accuracy.
· Design Lesson: A classic stiffness deficit. Often, the design limiter isn’t strength (will it collapse?) but stiffness (will it stay steady?).
- Was It Worn Down, or Shaken to Death?
· Cause of Death #4: Worn Smooth by Time (Insufficient Hardness)
· Crime Scene: Guideways scored; punch tips rounded after only a few cycles.
· Design Lesson: Surface hardness dictates wear life. For moving parts, remember the “tough core, hard shell” rule – a ductile core for impact resistance, a hard surface for wear resistance.
· Cause of Death #5: Cumulative Fatigue (Fatigue Strength Failure)
· Crime Scene: A motor mount cracks quietly from an inconspicuous corner – no major impact, just constant vibration.
· Design Lesson: This is a fatigue strength issue. Static strength may be adequate, but that doesn’t guarantee survival under cyclic stress. For high-cycle components, always consult the S‑N curve.
- Was It Killed by Impact, or by Buckling?
· Cause of Death #6: Shattered on Impact (Poor Toughness / Brittleness)
· Crime Scene: A cylinder striker hit a hard stop and chipped a piece right off.
· Design Lesson: Brittle materials (e.g., certain cast irons, over‑hardened steels) fear shock loads. For areas prone to impact, choose tougher materials that allow some “deformation cushion.”
· Cause of Death #7: Tall and Slender – Buckled Under Compression (Stability Failure)
· Crime Scene: A long leadscrew, under compression, twisted into a spiral; a column suddenly bowed sideways under load.
· Design Lesson: It’s not crushed by insufficient strength – it buckled due to inadequate stability. Slender members must be checked against Euler’s formula – buckling often strikes more suddenly than strength failure.
- Quantitative Details: Deflection, Elastic Modulus, and Plasticity
· Cause of Death #8: Precision Lost to “Nose‑Dive” (Excessive Deflection)
· Crime Scene: A cantilever beam sags by a few tenths of a millimeter under any load, skewing camera images.
· Design Lesson: Deflection is the numerical value of deformation. Don’t just say “it feels sturdy” – calculate exactly how much it bends. For precision‑critical applications, obsess over this number.
· Cause of Death #9: Won’t Spring Back (Elastic Failure)
· Crime Scene: A spring clip, once compressed, never returns; a seal loses its compensation and starts leaking.
· Design Lesson: The elastic range was exceeded, entering the plastic zone. For springs, always ensure working deformation stays within the material’s elastic limit.
· Cause of Death #10: Cracking at the Bend (Poor Plasticity)
· Crime Scene: A sheet‑metal part cracked at the outside radius when bent to 90°.
· Design Lesson: The material lacks sufficient plasticity for such large permanent deformation. For deep‑drawn parts or complex bends, always select materials with high elongation.
· Cause of Death #11: Too Limp (Wrong Elastic Modulus Choice)
· Crime Scene: A nylon rod and a steel rod of the same dimensions – the nylon bows like a bow under load.
· Design Lesson: Elastic modulus is the material’s inherent stiffness – it’s “born” with it. Changing material (e.g., nylon to steel) can increase stiffness; but heat‑treating steel (which raises yield strength) won’t increase stiffness – the elastic modulus of steel is essentially fixed.
Design Review: Think of It as a “Health Checkup” Checklist
If you treat these 12 indicators as a physical exam sheet, then during design review, you should run through each question:
- Will it collapse? – Strength, Yield Strength, Stability
- Will it perform? – Stiffness, Deflection
- How long will it last? – Hardness (wear), Fatigue Strength (cyclic resistance), Toughness (shock/accident resistance)
- Can we make it? – Plasticity (bending), Brittleness (machinability / cracking risk)
Final Word of Truth:
Design isn’t just about mechanics calculations – it’s about failure simulation. Mentally “kill” your part in every possible way, identify its most likely cause of death, then reinforce it against those specific 12 indicators. Do that, and your equipment will be damn hard to break.
(Thanks for reading – feel free to share with that colleague who always mixes up “stiffness” and “strength.”)
