Thermal Fatigue

Thermal fatigue is a damage mechanism that occurs in materials subjected to repeated cycles of heating and cooling. In this case, the material undergoes cyclic stresses solely due to expansion and contraction induced by temperature changes, without the need for an external mechanical load. In other words, unlike mechanical fatigue, thermal fatigue generates stresses through thermal expansion, which over time lead to crack initiation and propagation in the material. This phenomenon is particularly significant in high-temperature systems that frequently heat and cool, such as boilers, gas turbines, aircraft engines, automotive engines, and heat exchangers. Indeed, in components exposed to such cyclic thermal loads, thermal fatigue can accumulate over time and result in critical failures.

Mechanism of Thermal Fatigue

During thermal cycles, expansion and contraction within the material generate thermal stresses if they are constrained. For example, when the surface of a component rapidly heats and expands while the interior remains cooler, the difference in expansion between the surface layer and the core restricts surface movement. As a result, compressive stresses develop on the surface during heating, and tensile stresses arise during cooling. This periodic stress repetition accumulates cumulative damage, similar to mechanical fatigue. When the magnitude of thermal stresses exceeds the material’s yield strength at the relevant temperature, microcracks begin to form on the surface. Observations in metal injection molds have shown that the difference in expansion between the hot surface layer and the cooler interior generates tensile and compressive stresses that produce fine, mesh-like cracks (thermal fatigue cracks). Thermal fatigue differs from thermal shock: while thermal shock can cause immediate fracture due to a single abrupt temperature change, thermal fatigue typically involves the gradual accumulation of cracks over many repeated, lower-rate temperature fluctuations.

In repeated thermal cycles, crack initiation generally occurs at the surface because surface regions experience greater heat exchange with the environment and undergo the largest temperature variations. Each heating-cooling cycle may leave behind some plastic or elastic deformation in the material. Especially under large temperature ranges, the material experiences strains near its yield limit during each cycle, resulting in low-cycle fatigue-like damage accumulation. Through the successive action of tensile and compressive stresses, microscopic cracks or localized weaknesses in the material structure initially form.

Material Behavior and Damage Evolution

The behavior of materials under thermal fatigue is closely related to their internal structure and mechanical properties. Thermal fatigue cracks typically initiate at the material surface and, when observed, appear as wide, oxide-filled fissures. The interiors of surface cracks exposed to high temperatures may become coated with oxides, indicating that oxidation has occurred at the crack surfaces and that the environment has reacted with the material. Cracks may appear as isolated features or, depending on the intensity and frequency of thermal cycles, develop as multiple parallel cracks on the surface. From a microstructural perspective, thermal fatigue cracks generally propagate in a transgranular manner, traversing grain interiors and extending primarily perpendicular to the dominant applied stress. These cracks may leave behind sharp, spear-like propagation traces described as “dagger-shaped” and can grow in longitudinal or transverse directions (axial or circumferential), independent of any external load orientation.

Heterogeneities in the material’s internal structure can affect resistance to thermal fatigue. For instance, if different phases or particles within an alloy have mismatched coefficients of thermal expansion, localized stresses can develop around these regions during thermal cycles. These localized stresses can generate zones of plastic deformation and initiate microcracks. However, a specific distribution of second-phase particles can also hinder crack propagation; literature indicates that a uniform dispersion of hard second-phase particles in alloys makes thermal fatigue crack growth more difficult. During thermal fatigue, the hardness and strength of materials typically decrease with increasing cycle number, as repeated heating and cooling induce softening and cumulative damage in the microstructure.

The high-temperature behavior of materials—such as ductility, yield strength, and creep tendency—is among the critical factors determining the rate of thermal fatigue crack formation. ^【1】 For example, a highly ductile material can absorb a portion of thermal stresses through plastic deformation, thereby delaying crack initiation. Similarly, alloys that maintain high-temperature strength—by preserving their hardness and yield strength even at elevated temperatures—accumulate less damage during thermal cycling. Materials with high thermal conductivity reduce temperature gradients by minimizing the temperature difference between the surface and interior, thereby lowering thermal stresses and slowing fatigue damage. Indeed, a study on a hot-work tool steel demonstrated that high-temperature yield strength, high ductility, and high thermal conductivity collectively delay the initiation and propagation of surface cracks caused by thermal fatigue.

Applications and Prevention Strategies

Thermal fatigue is encountered in numerous engineering applications, particularly in equipment that undergoes periodic heating and cooling. Regions in steam boilers and piping systems where hot steam contacts cold water, as well as hot-cold fluid transition surfaces in heat exchangers, are classic examples of thermal fatigue. Start-stop cycles of gas turbines and aircraft engines induce thermal stresses in turbine blades and combustion chamber components, reducing their service life. In automotive engines, components such as cylinders are highly susceptible to crack formation due to continuous thermal cycling. Literature reports that thermal fatigue is one of the life-limiting damage mechanisms in critical aluminum alloy components used in engines.

One of the most prominent areas where thermal fatigue is observed is in metal injection and hot forging dies. During each casting cycle, the die surface experiences an abrupt thermal shock upon contact with molten metal, followed by cooling. This continuous cycling initially generates very fine capillary cracks (heat checking) on the surface; as usage progresses, these cracks grow and compromise the die’s structural integrity, significantly reducing its service life. Field studies have reported that approximately 58% of damage in such dies is attributable to thermal fatigue.

Three primary prevention strategies are emphasized: (i) design optimization, (ii) controlled operating conditions, and (iii) appropriate material selection and surface engineering. In design, avoiding sharp corners and abrupt section changes reduces stress concentrations. In operation, gradual heating and cooling procedures and preheating techniques reduce the temperature difference between surface and core, lowering the risk of crack initiation. At the material level, properties such as good thermal conductivity, high toughness, and oxidation resistance are preferred; surface treatments such as nitriding or thermal barrier coatings significantly extend thermal fatigue life.

Thermal fatigue is a multifactorial damage mechanism directly influenced by design, material selection, and operational parameters. ^【2】 Without appropriate engineering measures, it significantly shortens the service life of critical components and imposes substantial maintenance and cost burdens across industry.