| 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):
- Stick regime: no slip; no wear
- Mixed stick-slip: partial slip; fretting fatigue cracks; wear debris at edges
- Gross slip: full sliding; higher wear; oxidative wear
- 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).
