Thermal Fatigue in Metals and Components

Thermal fatigue produces cracking from cyclic stresses generated purely by repeated temperature change, without any directly applied mechanical load, making it a distinct but closely related problem to conventional mechanical fatigue. This guide covers the thermal stress mechanism, why thermal fatigue behaves as low-cycle fatigue, the Coffin-Manson life prediction approach, creep-fatigue interaction at high temperature, material selection, testing methods, and design strategies to reduce risk.

What Is Thermal Fatigue?

Thermal fatigue is the accumulation of fatigue damage, ultimately leading to crack initiation and growth, from repeated cycles of thermally induced stress rather than from a directly applied cyclic mechanical load. The stress itself is entirely a byproduct of the material’s tendency to expand when heated and contract when cooled, combined with some form of restraint preventing that dimensional change from occurring freely.

Two Origins of Thermal Stress

Thermal stress arises through two related but distinct mechanisms. External constraint occurs when a component is rigidly attached to, or forms part of, a larger structure that prevents free thermal expansion or contraction — a pipe fixed at both ends, a component clamped in an assembly, or a coating bonded to a substrate with a different coefficient of thermal expansion. Internal constraint, or self-constraint, occurs even in an entirely unrestrained, free-standing component whenever a temperature gradient exists within it: a hot surface attempting to expand is restrained by a cooler interior that has not yet expanded to the same degree, and vice versa on cooling, generating internal stress purely from the mismatch in local thermal strain across the part.

Thermal Stress: The E-α-ΔT Relationship

For a fully constrained uniaxial case — a bar or plate whose ends are rigidly prevented from moving as temperature changes — the induced elastic thermal stress follows directly from the material’s stiffness, expansion coefficient, and the temperature change.

Uniaxial constraint:   σ = E α ΔT

Biaxial/equal constraint
(e.g. thin disc, plate):  σ = E α ΔT / (1 - ν)

where:
  E    = elastic modulus (temperature-dependent)
  α    = coefficient of thermal expansion (temperature-dependent)
  ΔT   = temperature change
  ν    = Poisson's ratio

Because real materials have finite yield strength, this elastic estimate is frequently exceeded in practice: once the calculated elastic thermal stress surpasses the material’s yield strength at the relevant temperature, the material yields locally, and the actual response becomes strain-controlled rather than stress-controlled — the induced thermal strain is largely fixed by the geometry and ΔT, but the resulting stress is capped near the material’s flow stress, with the excess accommodated by plastic deformation. This shift from stress control to strain control is precisely why thermal fatigue is normally analysed using low-cycle, strain-based fatigue methods rather than the stress-based (S-N) approach used for high-cycle mechanical fatigue.

Thermal Fatigue as Low-Cycle Fatigue

Because thermally induced strain scales directly with αΔT rather than with an externally controlled applied load, even a modest temperature swing on a material with a typical metallic coefficient of thermal expansion can readily exceed the elastic strain range corresponding to yield, placing thermal fatigue squarely in the low-cycle, plastic-strain-dominated fatigue regime rather than the elastic, high-cycle regime addressed by conventional S-N curve analysis.

The Coffin-Manson Relation

Δεp / 2  =  εf′ (2Nf)c

where:
  Δεp = plastic strain range per cycle
  Nf   = cycles to failure
  εf′  = fatigue ductility coefficient
        (approximated by true fracture ductility, εf)
  c    = fatigue ductility exponent, typically -0.5 to -0.7
        for most metals (steeper/more negative under
        creep or environmental interaction)

Total Strain-Life: Combining Elastic and Plastic Contributions

At the strain ranges typical of thermal cycling, both elastic (Basquin) and plastic (Coffin-Manson) strain components contribute meaningfully to total life, and the two are commonly combined into a single total strain-life relationship, the basis of most modern low-cycle and thermo-mechanical fatigue life prediction methods.

Δε / 2  =  (σf′ / E)(2Nf)b  +  εf′(2Nf)c

where:
  σf′ = fatigue strength coefficient
  b    = fatigue strength exponent, typically -0.05 to -0.12

At short life (high strain range), the plastic (Coffin-Manson)
term dominates; at long life (low strain range), the elastic
(Basquin) term dominates.

Thermal Shock vs Thermal Fatigue vs Thermo-Mechanical Fatigue

Creep-Fatigue Interaction

Components thermally cycled at high homologous temperature, particularly those with sustained hold periods at peak temperature (start-stop power plant operation is the classic example), accumulate creep damage during the hold in addition to conventional fatigue damage from the cycling itself. Because these two damage mechanisms interact rather than simply adding independently, components subject to creep-fatigue typically show shorter life than either pure fatigue cycling or pure sustained creep loading would predict in isolation.

Σ (ni / Nfi)  +  Σ (tj / trj)   ≤  D

where:
  ni/Nfi  = fatigue damage fraction for each cycle type
  tj/trj  = creep damage fraction for each hold period
        (trj = creep rupture time at hold stress/temperature)
  D    = allowable combined damage fraction (often < 1,
        with margin, per governing design code)

This time-fraction approach underlies creep-fatigue design rules in codes such as ASME Boiler and Pressure Vessel Code Section III, Subsection NH, used for high-temperature nuclear and power plant components, and is a central design consideration for creep-resistant Cr-Mo and 9-12%Cr steels such as P91/P92, where the fine-grained HAZ is particularly susceptible to combined creep-fatigue damage during repeated plant startup and shutdown cycling.

Material Factors Governing Thermal Fatigue Resistance

Common Applications and Failure Examples

• Gas turbine blades and discs: severe TMF from startup/shutdown cycling combined with steady-state centrifugal and gas-bending loads.
• Exhaust manifolds and turbocharger housings: repeated heating/cooling with engine cycling, often complicated by dissimilar-material joints and constrained mounting.
• Brake rotors and discs: rapid, localized surface heating during braking events followed by cooling, producing surface-initiated thermal fatigue cracking (heat checking).
• Die-casting and forging dies: repeated contact with molten or hot metal followed by cooling/ejection, a classic thermal fatigue (heat checking) application.
• Boiler tubes and heat exchangers: startup/shutdown cycling and load-following operation in power generation, often coupled with creep-fatigue interaction.
• Weld repairs and dissimilar-metal joints: mismatched coefficients of thermal expansion across a joint amplify thermal stress under cyclic service temperature.

Testing Methods

Design Mitigation Strategies

• Minimize sharp geometric discontinuities (fillets, notches, section changes) that locally concentrate thermally induced strain.
• Provide flexibility, expansion joints, or sliding supports to relieve external mechanical constraint on thermally cycled components where the design allows.
• Reduce internal thermal gradients through more uniform heating/cooling, improved cooling passage design, or reduced heating/cooling rates where process requirements permit.
• Select materials with lower thermal expansion, higher thermal conductivity, and adequate high-temperature ductility for the specific service cycle.
• For creep-fatigue-prone applications, minimize unnecessary hold time at peak temperature and consider combined damage assessment per the governing design code rather than fatigue analysis alone.

Frequently Asked Questions

Recommended Reference Books

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