fretting-wear

name	fretting-wear
description	Fretting wear and fretting fatigue — stick-slip micromotion, fretting wear maps, Archard wear law, contact mechanics at fretting interface, dovetail joints, blade-disk interfaces, fretting fatigue life reduction, prevention strategies.
metadata	{"priority":7,"promptSignals":{"phrases":["fretting wear","fretting fatigue","fretting damage","micromotion contact","dovetail fretting","blade disk fretting"],"minScore":3}}

Fretting Wear and Fretting Fatigue — Complete Skill

Fretting Fundamentals

Fretting: small-amplitude oscillatory relative motion (1–100 μm) between contacting surfaces; occurs under cyclic loading Fretting damage types:

• Fretting wear: material removal; dimensional loss; debris generation
• Fretting fatigue: cracking from fretting-induced stress concentration; more dangerous than fretting wear alone
• Fretting corrosion: oxidation of wear debris (especially iron → Fe₂O₃ red debris); common in steel contacts

Fretting conditions: Partial slip: contact center sticks; edge zones slip (annular slip zone); most common for fretting fatigue Gross slip: whole contact slides; more wear but less fatigue cracking Stick: no relative motion; no fretting (fully stuck or sticking throughout)

Fretting amplitude classification: < 5 μm: stick regime; minimal damage 5–50 μm: partial slip; fretting fatigue critical

50 μm: gross sliding; wear dominant; fretting fatigue less severe

Contact Mechanics at Fretting Interface

Hertz Contact Stress

Cylinder-on-flat (line contact): Half-contact width: b = √(4PR/(πLE*)) [P = load per unit length; R = cylinder radius; L = contact length] Peak contact pressure: p₀ = 2P/(πbL) Maximum tangential traction at edge (partial slip): q_edge = μ × p(x) at slip edge

Stick-slip boundary (partial slip under tangential load Q): Stick region half-width: c = b × √(1 - Q/(μP)) Slip zone: [c, b]; stick zone: [-c, c]

Stress intensity at slip zone tip (potential crack initiation site): K_fretting ≈ Δq × √(π × a_micro) [Δq = tangential traction range; a_micro = crack length]

Fretting Wear Map (Vingsbo-Söderberg)

Four regimes (fretting displacement amplitude vs. contact load):

1. Stick regime: no slip; no wear
2. Mixed stick-slip: partial slip; fretting fatigue cracks; wear debris at edges
3. Gross slip: full sliding; higher wear; oxidative wear
4. Reciprocating sliding: cyclic gross sliding

Most damaging regime for fatigue: mixed stick-slip (partial slip) Transition from partial to gross slip: δ_transition ≈ b × μ × (2-ν)/(G/p₀) × p₀ [approximate]

Fretting Wear Quantification

Archard Wear Law

Volume wear rate: V_wear = k_fretting × P × s / H [mm³; k_fretting = wear coefficient [mm²/N]; P = normal load [N]; s = sliding distance [mm]; H = hardness [MPa]]

Fretting wear coefficient k_fretting: Steel-on-steel: k = 10⁻⁴–10⁻² mm²/N (highly variable; depends on regime and surface condition) Ti-on-Ti (common in aerospace): k ≈ 10⁻³ mm²/N (poor fretting resistance → major concern in dovetail joints) With DLC coating: k ≈ 10⁻⁵ mm²/N (90% reduction)

Wear depth per cycle: h_cycle = k_fretting × p₀ × 2δ / H [mm/cycle; δ = displacement amplitude; 2δ = total sliding distance per cycle]

Total wear depth after N cycles: h_total = N × h_cycle [mm; track depth for maintenance prediction]

Fretting Fatigue Life Prediction

Stress-Based Approach

Fretting fatigue life reduction: Fretting reduces fatigue limit of steel by 50–70%; titanium by 30–60% Fretting fatigue limit: σ_ff = σ_e × (1 - η_fretting) [η_fretting = fretting reduction factor ≈ 0.3–0.7]

Critical plane approach for fretting fatigue: Smith-Watson-Topper (SWT) parameter at fretting contact edge: SWT = σ_max × Δε/2 [maximize over all planes at contact edge; compare to uniaxial SWT-N data]

Stress intensity at crack tip from fretting: K_max_fretting = σ_bulk × √(πa) + K_fretting(Q/P, contact geometry) Fretting adds stress concentration at contact edge → effective K > bulk-only K

Nucleation Life

Fretting crack nucleates at contact edge within partial slip zone: Nucleation controlled by tangential traction range Δq = 2μ × p₀ (at edge) Crack angle: cracks nucleate at ~45° to contact surface then turn perpendicular to max principal stress Nucleation life (Ruiz criterion): f_Ruiz = Δτ × δ [fretting damage; nucleation when f_Ruiz exceeds threshold]

Short Crack Propagation

Non-propagating cracks (trapped at contact): Short cracks arrested by residual stress or diminishing K_I with depth Crack depth for arrest: a_arrest ≈ b (order of contact width) For deep enough cracks: Paris law propagation takes over

Paris law integration with fretting K-range: da/dN = C × (ΔK_total)^m [ΔK_total = ΔK_bulk + ΔK_fretting] Life = ∫ da / (C × ΔK_total^m) from a_nucleation to a_critical

Dovetail Joint and Blade-Disk Fretting

Aerospace turbine blade attachment — major fretting fatigue application: Contact: blade root (titanium) against disk dovetail (titanium or nickel); centrifugal load P_centrifugal + vibration Q

Key parameters: Contact half-width b ≈ 0.5–2 mm (blade geometry dependent) Centrifugal load P/L = 50–200 MPa (normal contact stress) Vibration tangential load: Q/P ≈ 0.1–0.4 (fretting fatigue range)

Critical location: contact edge at blade root → highest tangential traction → crack initiation site

Design improvements:

• Reduce friction coefficient: Ti-N coating, DLC, MoS₂, shot peen + burnish
• Increase contact area: widen dovetail flank (reduces p₀)
• Introduce compressive residual stress: shot peening + low plasticity burnishing (LPB)
• Reduce slip amplitude: stiffen disk, change blade natural frequency

Prevention and Mitigation

Surface treatments: Shot peening: induces −400 to −800 MPa compressive residual stress; Almen intensity A8–A16; fretting life increase 2–5× Low Plasticity Burnishing (LPB): deeper residual stress than shot peen (−500 to −1000 MPa to 1 mm depth) Nitriding: increased surface hardness → reduced wear; may reduce fretting fatigue if RS > surface damage DLC (diamond-like carbon): k reduced by 1–2 orders of magnitude; excellent for Ti-Ti contact

Coatings for fretting: Nickel-phosphorus electroless: good for steel; moderate fretting improvement Chrome plating: hard; good wear; concerns over Cr(VI) (RoHS/REACH restricted) Thermal spray (WC-Co, Cr₂C₃-NiCr): high hardness; good fretting resistance

Design modifications: Reduce contact stress: increase contact area; add intermediate compliance layer (soft insert) Eliminate slip: tight fit (zero clearance); adhesive bonding; welding Add damping: reduce vibration amplitude → reduce fretting slip amplitude Isolating layer: PTFE, polyimide shim → low-friction separation; eliminates metal-to-metal contact

Standards

Standard	Scope
ASTM STP 1425	Fretting fatigue — advances in basic understanding and applications
ASME BPVC Section III	Nuclear component fretting allowances
SAE ARP5765	Aircraft engine blade retention — fretting considerations
ISO 15243	Bearing failure analysis including fretting
AS 3978	Aircraft engine titanium component fretting assessment

Output

Provide: contact geometry (cylinder-on-flat/flat-on-flat/conforming), material pair, normal load P [N] and contact pressure p₀ [MPa], slip amplitude δ [μm] and slip regime (stick/partial/gross), stick-slip boundary position (c/b ratio), fretting wear coefficient k_fretting, wear rate [mm³/cycle] and depth after N cycles [μm], fretting fatigue life reduction factor (%), estimated fatigue life N_f with fretting vs. unfretting, critical location (contact edge coordinates), mitigation measure (shot peen/DLC/redesign), fretting reduction achieved after mitigation, and applicable standard (ASTM STP 1425, SAE ARP5765).