Thermal fatigue is a degradation process that occurs in materials subjected to cyclic thermal loading, often in environments where components experience rapid or repeated temperature fluctuations. Unlike mechanical fatigue, thermal fatigue arises from the thermal expansion and contraction that induce cyclic strains, leading to crack initiation and propagation over time.
This phenomenon is particularly significant in high-temperature applications such as boilers, aerospace, automotive engines, and heat exchangers, where service conditions involve frequent heating and cooling cycles.
On June 27, 2016, a significant explosion and fire occurred at the Enterprise Products gas processing plant in Pascagoula, Mississippi, attributed to thermal fatigue. The incident was triggered by a major loss of containment in a heat exchanger, leading to the release of methane, ethane, propane, and other hydrocarbons. The released gases ignited, resulting in multiple fires and secondary explosions. The event caused extensive damage and led to a site shutdown lasting nearly six months. Fortunately, no injuries were reported.
Figure 1 – Aftermath of an explosion and fire at the Enterprise Products gas processing plant in Pascagoula, Mississippi
Thermal fatigue is influenced by several key factors, primarily the magnitude of the temperature swing and the frequency of thermal cycling—defined as the number of thermal cycles per second, minute, day, or other time intervals. These cycles, particularly when large in temperature differential or frequent in occurrence, place significant stress on materials.
The time to failure due to thermal fatigue is closely linked to both the magnitude of the cyclic stress and the number of cycles a component undergoes. As either the stress or the cycling frequency increases, the time to failure decreases accordingly. This relationship highlights the cumulative damage that repeated thermal stress can cause over time.
Because numerous variables influence the onset and location of thermal fatigue cracking, there is no universal threshold for allowable temperature swings. For instance, components with rigid attachments may crack under relatively small temperature differentials. However, as a general rule of thumb, thermal fatigue cracking becomes more likely when temperature fluctuations exceed approximately 110 °C to 165 °C.
Thermal fatigue damage is also promoted by structural inflexibility that inhibits accommodation of differential thermal expansion. Additionally, rapid surface temperature changes—such as cold water impinging on a hot metal tube—can create steep thermal gradients, further increasing the risk of cracking through the component’s thickness or along its length.
Stress concentrators like notches, sharp corners, and weld toes are common initiation sites for thermal fatigue cracks. These features amplify local stresses and make materials more vulnerable under thermal cycling conditions. Furthermore, operational patterns such as frequent equipment or unit start-ups and shutdowns can significantly increase the susceptibility to thermal fatigue, depending on how often they occur.
Thermal Fatigue Crack Morphology
Thermal fatigue cracks typically initiate at the surface of a component, occurring on either the internal (ID) or external (OD) surface. These cracks are generally wide and often contain oxides due to the elevated temperatures involved, which facilitate oxidation along the crack faces. Cracks may appear individually or as multiple parallel cracks, depending on the severity and nature of the thermal cycling.
Figure 2 – Thermal fatigue crack example.
Once initiated, thermal fatigue cracks tend to propagate in a direction perpendicular to the principal stress. These cracks are commonly transgranular, dagger-shaped, and oxide-filled. Depending on the stress distribution, cracking may present in axial, circumferential, or even both orientations simultaneously at the same location.
In steam-generating equipment, thermal fatigue cracking is frequently observed along the toe of fillet welds. The abrupt change in section thickness at these locations acts as a stress riser, promoting crack initiation. Cracks often originate at the end of attachment lugs, and when bending moments are introduced due to mechanical constraints, these cracks can grow circumferentially into adjacent tubing.
In systems utilising soot blowers, the presence of water can lead to a distinctive crazing pattern. In such cases, circumferential cracks are typically the dominant feature, while smaller axial cracks form as secondary features, further contributing to component degradation.
Figure 3 – Tube sheet leaks attributable to thermal fatigue
Prevention
Thermal fatigue is best mitigated through careful design and controlled operation aimed at minimising fluctuating thermal stresses and reducing the frequency and severity of thermal cycling. Preventive strategies vary depending on the specific application, but several core principles are widely applicable.
From a design perspective, reducing stress concentrators is essential. This includes the use of smooth geometric transitions, blend grinding of weld profiles, and avoiding sharp corners or abrupt changes in section thickness. Designs should also allow for sufficient flexibility to accommodate differential thermal expansion, especially when dissimilar materials are joined.
Operational controls are equally important. Implementing controlled heating and cooling rates during equipment start-up and shutdown can significantly reduce thermal stresses. For steam-generating equipment, the use of slip spacers that allow movement and the avoidance of rigid attachments can help absorb thermal expansion without introducing excessive stress. Additionally, drain lines should be installed on soot blowers to remove condensate and prevent thermal shock during the initial stages of operation.
In some applications, particularly where cold fluids may come into contact with hot pressure boundary walls, liners or sleeves can be installed as a thermal barrier to reduce direct exposure and minimise localised thermal gradients. These preventive measures, when combined, play a vital role in extending component life and ensuring reliable, fatigue-resistant performance.
For more information or expert advice on thermal fatigue and its prevention, feel free to contact us at info@r-techmaterials.com. Our team of specialists can assist with material integrity challenges and provide support to ensure the safety and longevity of your engineering systems.