| name | creep-testing |
| description | Creep testing — creep curves, Norton power law, Larson-Miller parameter, stress rupture, ASTM E139, turbine blade life, creep-fatigue interaction, ASME Section III NH, strain accumulation. |
| metadata | {"priority":7,"promptSignals":{"phrases":["creep testing","creep rupture","Larson-Miller","creep life","Norton creep law","creep fatigue"],"minScore":3}} |
Creep Testing — Complete Skill
Creep Fundamentals
Creep: time-dependent plastic strain under constant stress at elevated temperature (T > 0.4T_melt)
Creep strain rate: dε/dt [s⁻¹]; minimum (secondary) rate is most important for design
Three Stages of Creep
- Primary creep: decelerating strain rate; strain hardening > thermal recovery
ε_primary = A × t^m [m ≈ 1/3 for metals; A = material constant]
- Secondary (steady-state): minimum strain rate; equilibrium between hardening and recovery
dε/dt = ε̇_min = const
- Tertiary creep: accelerating rate; void coalescence → necking → fracture
Total creep strain:
ε_total = ε_elastic + ε_primary + ε_secondary + ε_tertiary
ε_rupture = A' × ε̇_min^β [Monkman-Grant relation; β ≈ 1.0 for many metals]
Creep Laws
Norton Power Law (Steady-State)
ε̇ = A × σ^n × exp(-Q_c / (RT))
ε̇ = minimum creep rate [s⁻¹]
σ = applied stress [MPa]
n = stress exponent (3–5 for power-law creep; n=1 diffusion creep; n > 7 dislocation climb)
Q_c = activation energy [kJ/mol] (≈ diffusion activation energy; 250–300 kJ/mol for Ni alloys)
R = 8.314 J/(mol·K); T = temperature [K]
A = material constant
n values by mechanism:
n = 1: Nabarro-Herring or Coble diffusion creep (low stress, high T)
n = 3: dislocation glide (viscous glide)
n = 4–5: climb-controlled dislocation creep (most metals above 0.5T_m)
n > 7: power-law breakdown (high stress)
Double Power Law (Wide Range)
ε̇ = A₁ × σ^n₁ + A₂ × σ^n₂ × exp(-Q/RT) [captures low-stress + high-stress regimes]
Omega Method (API 579/ASME NH)
ε̇ = ε̇₀ × exp(Ω × ε) [Ω = material damage parameter]
Ω describes acceleration of rate with accumulated damage
Larson-Miller Parameter (LMP)
Most widely used for design life estimation:
LMP = T × (log t_r + C) × 10⁻³ [T in K or °R; t_r = rupture time in hours; C = material constant]
C values:
C ≈ 20 for most steels; C ≈ 15 for nickel alloys; C ≈ 30 for titanium alloys
Design use:
- From test data at different T and σ: plot LMP vs. log σ → master curve
- At operating T and σ: read LMP → calculate t_r
- Check t_r ≥ design life × safety factor
Alternative parameters:
Manson-Haferd: (log t_r - log t_a) / (T - T_a) = P(σ) [ta, Ta = material constants]
Orr-Sherby-Dorn: log t_r - Q/(2.303RT) = f(σ)
Stress Rupture
Monkman-Grant:
ε̇_min × t_r = M [M ≈ 0.1–0.3 strain; ε̇_min in %/hr; t_r in hours]
Allows life estimation from minimum creep rate (short test)
Rupture criteria for design:
- Limiting creep strain: ε ≤ ε_allow at end of design life
- Rupture life: t_r ≥ t_design × SF (SF = 1.5–3.0 depending on code)
- Both criteria must be satisfied
Stress rupture values (typical, 100-hr strength at T):
| Material | 760°C (1400°F) σ_100h | 870°C (1600°F) σ_100h |
|---|
| IN718 | 690 MPa | 310 MPa |
| IN738LC | 550 MPa | 240 MPa |
| CMSX-4 (SX) | 950 MPa | 550 MPa |
| 316 SS | 80 MPa | 30 MPa |
| 304 SS | 70 MPa | 25 MPa |
ASTM E139 — Creep Testing Procedure
Specimen: dog-bone tensile; L₀ = 50 mm gauge length; verify alignment < 1% bending
Load application: dead weight lever system (constant load); step loading not permitted
Temperature control: ±1°C uniformity over gauge length; TC calibrated per NIST
Strain measurement: extensometer; resolution ≤ 0.001% strain (0.5 μm for 50 mm gauge)
Constant load vs. constant stress:
ASTM E139: constant load (load control) → stress increases as specimen necks (typical)
True creep: constant stress (requires servo control) — for fundamental studies
Data required per test point:
Time, strain at uniform intervals; minimum creep rate ε̇_min; time and strain at rupture
Minimum test duration:
For design data: tests at multiple T; longest test ≥ 1000 hr for extrapolation to 10⁵ hr
Creep-Fatigue Interaction
Applicable when: cycling + hold times at elevated temperature (turbine startup/shutdown, power cycling)
Linear damage rule (ASME Section III NH):
D = Σ (n/N) + Σ (Δt/t_r) ≤ D_allow
n/N = fatigue damage fraction; Δt/t_r = creep damage fraction
D_allow from material-specific bilinear interaction diagram (creep-fatigue diagram)
Creep-fatigue envelope (e.g., 316 SS at 600°C):
Σn/N + Σt/t_r ≤ 1 (conservative); actual limit from code table less than 1 (e.g., 0.3 each axis)
Robinson's rule (linear interaction):
D_total = D_creep + D_fatigue = Σ(Δt_i/t_r,i) + Σ(n_j/N_j) ≤ 1.0
Cycle counting for creep-fatigue:
Rainflow counting for fatigue; sum of hold-time fractions for creep
Temperature correction: separate bins by temperature range
Turbine Blade Creep Life
Critical location: pressure side at mid-airfoil (highest temperature + stress)
Centrifugal stress: σ_c = ρ × ω² × A_tip × (r_tip² - r_root²) / (2A_root) [approximate]
Or: σ_c = ρ × ω² × ∫r dA / A [exact from annular stress]
Blade temperature correction:
T_metal = T_gas - η_cool × (T_gas - T_coolant) [η_cool = cooling effectiveness, 0.5–0.7]
Life fraction used per cycle:
Φ = Δt / t_r(σ, T) [from LMP master curve]
Cumulative: Σ Φ ≤ 0.8 (80% life fraction criterion for inspection interval)
Standards
| Standard | Scope |
|---|
| ASTM E139 | Conducting creep, creep-rupture, and stress-rupture tests |
| ASME Section III NH | Nuclear components at elevated temperature |
| API 579-1 | Fitness for service — creep damage |
| ASTM E2760 | Creep-fatigue crack growth testing |
| ISO 204 | Uniaxial creep testing in tension |
Output
Provide: material (σ_u [MPa], T_melt [K]), operating stress [MPa] and temperature [°C], creep regime (primary/secondary/tertiary), minimum creep rate ε̇_min [%/hr] from Norton law, Norton exponent n, Larson-Miller parameter LMP, predicted rupture life t_r [hr], creep strain at end of design life [%] vs. allowable, design life [hr] with safety factor, Monkman-Grant check, creep-fatigue damage fractions (Σn/N, Σt/t_r) vs. code limit, turbine blade T_metal [°C] and centrifugal stress [MPa], and applicable standard (ASTM E139, API 579, ASME NH).
