failure-mode-analysis

name	failure-mode-analysis
description	Systematic failure mode analysis — given geometry and loading, identify ALL possible failure modes ranked by criticality. Covers static, fatigue, buckling, wear, corrosion, creep, fracture. Like FMEA but for a single component.
metadata	{"priority":9,"promptSignals":{"phrases":["failure mode","how will it fail","failure analysis","FMEA","what could go wrong","failure check","failure risk"],"minScore":4}}

Failure Mode Analysis — Complete Skill

Process: Systematic Failure Mode Identification

For every component analysis, work through ALL categories below. Never skip a category without explicitly ruling it out with reasoning.

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Category 1: Static Failure (Single Load Application)

Ductile Materials (Sy known)

• Tensile yielding: σ_max > Sy → permanent deformation
• Compressive yielding: σ_c > Sy (for ductile, same as tensile)
• Shear yielding: τ > Ssy (Ssy = 0.577Sy von Mises, or 0.5Sy Tresca)
• Bearing failure: σ_b = F/(projected area) > Sy
• Combined stress: σ_von Mises > Sy (check with Mohr's circle)

Brittle Materials (Sut, Suc known — cast iron, ceramics, glass)

• Tensile fracture: σ_1 > Sut (principal stress)
• Compressive fracture: σ_3 < -Suc (usually Suc >> Sut for brittle)
• Modified Mohr (brittle): if σ_1 ≥ 0 and σ_2 ≥ 0: σ_1 ≤ Sut if σ_1 ≥ 0 ≥ σ_2 and |σ_2/σ_1| ≤ 1: σ_1 ≤ Sut if σ_1 ≥ 0 ≥ σ_2 and |σ_2/σ_1| > 1: σ_1/Sut - σ_2/Suc ≤ 1

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Category 2: Fatigue (Cyclic Loading)

• High-cycle fatigue: σ_a/Se + σ_m/Sut > 1 (Goodman)
• Low-cycle fatigue: εp_a > ε_f'(2N_f)^c (Coffin-Manson, N < 10³)
• Fatigue at stress concentration: Kf applied to alternating stress?
• Surface fatigue (pitting): contact stress > S_c (gears, bearings, cams)
• Fretting fatigue: micro-slip at press fits, keyways under vibration
• Variable amplitude: Miner's rule Σ(n_i/N_i) < 1?
• Thermal fatigue: cyclic thermal stresses → check ΔT × α × E

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Category 3: Buckling / Instability

• Global column buckling: P > P_cr = π²EI/(KL)²
• Local buckling: thin plates/flanges — b/t exceeds code limit
• Lateral-torsional buckling: unbraced beams under bending
• Shell buckling: thin cylinders under compression or external pressure
• Snap-through buckling: shallow arches, dished heads under pressure
• Shear buckling: thin webs at high shear

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Category 4: Fracture Mechanics (Cracks / Defects)

• Linear elastic fracture: K_I = σ√(πa)·F(a/W) > K_IC? K_IC [MPa√m]: 4340 steel=50, 2024-T3 Al=24, Ti-6Al-4V=55, ceramics=3-6
• Elastic-plastic: K_IC assumption valid only if a, W >> (K_IC/σ_y)²/2π
• Fatigue crack growth: da/dN = C(ΔK)^m (Paris law) ΔK = Δσ√(πa)·F, failure when a reaches a_c = (K_IC/σ_max·√π·F)²/π
• Stress corrosion cracking (SCC): K_I > K_ISCC in corrosive environment?
• Hydrogen embrittlement: susceptible materials in H₂ environment?

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Category 5: Wear

• Adhesive wear (scuffing/galling): V = k·F·L/H (Archard law) k = wear coefficient (10⁻² for like metals unlubricated, 10⁻⁵ lubricated) Prevent: dissimilar materials, hardness differential, lubrication
• Abrasive wear: hard particles or surface asperities cutting softer surface Increase hardness, use harder coatings
• Surface fatigue (rolling contact): Hertz contact stress > S_c → pitting/spalling
• Fretting wear: micro-slip amplitude < 100μm at press fits, bolted joints
• Erosive wear: high-velocity particles impacting surface (elbows, nozzles)
• Cavitation erosion: bubble collapse near surface (pump impellers, propellers)

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Category 6: Corrosion

• Uniform corrosion: material slowly dissolves in electrolyte
• Galvanic corrosion: dissimilar metals in electrolyte → EMF potential Galvanic series: Mg(-1.6V) → Al(-0.75V) → steel(-0.44V) → Cu(+0.34V) → Au(+1.5V) Rule: >0.25V difference in contact → risk
• Pitting corrosion: localized, self-accelerating (304SS in chloride environments)
• Crevice corrosion: oxygen depletion in tight crevices
• Intergranular corrosion: grain boundary attack (sensitized stainless steel)
• Erosion-corrosion: combined attack (piping elbows)
• Dealloying: selective removal of one component (dezincification of brass)
• SCC: combined tensile stress + corrosive environment (dangerous — sudden)

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Category 7: Creep / High-Temperature Failure

Significant above: T > 0.4·T_melt (Kelvin) Steel: > ~400°C, Aluminum: > ~150°C, Polymers: > 60-100°C

• Steady-state creep: ε̇ = A·σⁿ·exp(-Q/RT) (Norton's law) n ≈ 5-8 (metals), Q = activation energy for creep
• Creep rupture: fracture after extended time at elevated T Larson-Miller parameter: C = T·(log(t_r) + C_LM)
• Stress relaxation: stress decreases at constant strain (bolted joints at temperature)
• Thermal ratcheting: progressive deformation under cyclic thermal loading

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Category 8: Environmental / Special Failure Modes

• Impact/shock: strain rate sensitivity, ductile-brittle transition DBTT for steel: -20°C to +20°C depending on grade Use Charpy impact test; ASTM A36 has poor toughness below -10°C
• Vibration/resonance: operating frequency near natural frequency
• Thermal shock: rapid temperature change → thermal stress ΔT_max = σ_f(1-ν)/(E·α) [maximum allowable ΔT]
• Radiation damage: neutron bombardment → embrittlement (nuclear)
• Biological/microbial corrosion (MIC): sulfate-reducing bacteria in pipelines

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Output Format

Rank all identified failure modes by criticality:

Rank	Failure Mode	Governing Equation	Safety Factor	Action
1	Most critical	Formula	n = X.X	Fix/check
2	Second	Formula	n = X.X	Monitor
...

Flag anything with n < 1.5 as requiring immediate design attention. Always state clearly: "No other failure modes apply because..." for any category ruled out.