Creep-Fatigue Interaction
Creep-Fatigue Interaction: Theoretical Foundations
Creep-Fatigue Interaction
Professor, what happens when creep and fatigue occur simultaneously?
Under repeated loading at high temperatures (above 350°C for steel), creep damage and fatigue damage accumulate simultaneously. Evaluation considering both is necessary.
ASME NH Linear Damage Rule
Sum of fatigue damage $D_f = n/N_f$ (Coffin-Manson) + creep damage $D_c = t/t_r$ (time fraction). ASME NH specifies the allowable damage using a damage envelope (Creep-fatigue interaction diagram).
Summary
Fundamentals of Creep-Fatigue Interaction
In high-temperature fatigue, "pure fatigue" and "creep" progress simultaneously, and simple linear addition significantly overestimates life. The ASME Boiler & Pressure Vessel Code (Section III, Appendix T) introduced the "Creep-Fatigue Interaction Diagram" in 1974. It established the concept of plotting the fatigue usage fraction Df (= Σni/Nf) on the vertical axis and the creep usage fraction Dc (= Σti/tr) on the horizontal axis, and determining failure when their sum exceeds the design allowable curve.
Computational Methods for Creep-Fatigue Interaction
FEM
1. Elastoplastic + creep analysis using Chaboche + Norton (*VISCO)
2. Obtain $\Delta\varepsilon$ from stabilized hysteresis loop → fatigue damage $D_f$
3. Obtain $\dot{\varepsilon}_{cr}$ from high-temperature hold time → creep damage $D_c$
4. Evaluate $D_f + D_c$ using the damage envelope
Summary
Difference Between Time-Fraction Rule and Strain-Fraction Rule
Methods for evaluating creep damage are broadly divided into two. ① Time-Fraction Rule: Quantifies damage by the sum of ratios Σ(ti/tr), where hold time ti is divided by rupture time tr(σ,T). ② Ductility Exhaustion Method: Calculates damage from the ratio of creep strain rate to ductility. A 1998 report by UK AEA Technology showed that for stainless steel 316H, the strain-fraction method gives 30-50% more conservative predictions than the time-fraction method. The time-fraction method is more commonly adopted in nuclear reactor pressure vessel design.
Creep-Fatigue Interaction in Practice
Practical Checklist
Design Example: Gas Turbine First-Stage Rotor Blade
The GE Aviation GE9X (mounted on Boeing 777X) first-stage rotor blade is manufactured from TBC (Thermal Barrier Coating)-coated single-crystal nickel superalloy CMSX-4 and exposed to combustion gas temperatures above 1700°C. Creep-fatigue life analysis of the blade uses a proprietary interaction diagram, evaluating design life by summing takeoff/landing cycles (LCF) and cruise creep. Overhaul intervals are determined in combination with EHM (Engine Health Monitoring) data, typically whichever comes first: about 3000 cycles or 15,000 flight hours.
Creep-Fatigue Interaction: Software & Solver Comparison
Tools
Abaqus Creep-Fatigue Analysis Capabilities
Dassault Systèmes Abaqus analyzes creep deformation by combining the CREEP card with time-dependent plasticity (PLASTIC with TIME DEPENDENT). Creep-fatigue damage evaluation can be performed using the Creep-Fatigue Interaction (CFI) module of fe-safe (a fatigue-specific plugin integrated with Abaqus). Ansys Mechanical 2024 R1 added a Creep Fatigue Analysis (CFA) wizard that automatically plots the ASME Section III code interaction diagram. Since SIMcenter Nastran does not support creep, migration to Abaqus/Ansys is common in practice for high-temperature creep problems.
Advanced Technologies
Advanced
Creep-Fatigue Integration via Continuum Damage Mechanics
Continuum Damage Mechanics (CDM) is a framework developed by Lemaitre & Chaboche (1985, École Normale Supérieure de Cachan, France), incorporating a damage variable ω as an internal variable into the constitutive equations. By describing creep damage and fatigue damage with independent damage variables ωc, ωf and solving their evolution equations simultaneously, the interaction can be expressed naturally. Implementation examples of this model in Abaqus User Subroutine (UMAT or CREEP) have been reported for high-temperature fatigue evaluation of Inconel 617 by MPa Forming (2021, Tohoku University).
Creep-Fatigue Interaction: Common Issues & Debugging
Trouble
Related Topics
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