Thermal Fatigue
Thermal Fatigue: Theoretical Foundations
What is Thermal Fatigue?
Professor, what is thermal fatigue?
Fatigue due to repeated temperature changes. Temperature change → thermal stress → repetition → fatigue failure. A problem in engine cylinder heads, exhaust manifolds, turbine blades, and nuclear piping.
Characteristics of Thermal Fatigue
Summary
Jet Engine Blade Cooling Hole Cracks
Thermal fatigue is fatigue caused by repeated thermal strain due to temperature changes. In the turbine blades of RR's Trent engine, temperatures fluctuate from 900°C during operation to room temperature when stopped, with thermal strain range around cooling holes reaching 0.5%. According to the Coffin-Manson law, this strain range predicts a material life of 5000 to 10000 cycles, forming the basis for major overhauls (C-check).
Computational Methods for Thermal Fatigue
FEM for Thermal Fatigue
1. Thermal Analysis — Calculate time history of temperature distribution
2. Thermal-Structural Coupling — Temperature distribution → thermal stress → Elastoplastic analysis (Chaboche model recommended)
3. Obtain Stabilized Hysteresis Loop — Stable cycle of stress-strain
4. Fatigue Evaluation — Coffin-Manson + creep damage (summed using Miner's rule)
Summary
Choosing Between Isothermal vs. Non-Isothermal Fatigue Curves
Using isothermal fatigue data as-is for thermal fatigue design carries risk. IN718 nickel superalloy shows 40% shorter life in TMF tests at 400-800°C compared to isothermal tests at 600°C. In analysis, one must decide whether to correct isothermal SN/EN curves by multiplying by a "TMF factor" of 0.5-0.7 or to obtain dedicated TMF fatigue curves. The correction factor method is often used for early-stage development due to cost-effectiveness.
Thermal Fatigue in Practice
Thermal Fatigue in Practice
Engine components (cylinder heads, exhaust systems), turbines, nuclear piping.
Practical Checklist
Thermal Fatigue Design of Exhaust Manifolds
Gasoline engine exhaust manifolds are a typical thermal fatigue environment, cycling from 200°C to 900°C from startup to shutdown. For FEM thermal fatigue analysis of SiMo ductile cast iron manifolds, the flow of fluctuating temperature field → thermal strain → elastoplastic stress → strain-life evaluation is essential. Toyota standardized this flow as a design tool from the late 1990s.
Thermal Fatigue: Software & Solver Comparison
Tools
Practical Flow for Abaqus Thermal Fatigue Coupled Analysis
Abaqus has an established three-step flow: heat transfer analysis (Step 1) → thermal stress analysis (Step 2) → fatigue evaluation (integrated with fe-safe). Through collaboration between DASSAULT and HBM, it's possible to launch fe-safe directly from Abaqus CAE and perform TMF fatigue evaluation including temperature history. Renault improved thermal fatigue life prediction accuracy for turbocharger housings to within ±20% using this flow.
Advanced Technologies
Advanced Topics in Thermal Fatigue
Related Topics
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