Thermal fatigue

---

Description

Fatigue occurs when a material is subjected to a fluctuating (cyclic) tensile stress and after a period of time, a small microcrack initiates and then grows progressively through the material until the crack reaches a point where the remaining section of material suddenly fractures. Sources of this cyclic stress may include vibrations in the system, loading, or stress cycling, but with thermal fatigue, the source is from thermal cycling i.e. changes in temperature. Fluctuations in temperature will cause a component or assembly to expand and contract, with a resulting change in the strain, and hence stress. In some metal combinations or assemblies, due to either excessive temperature changes or poor design, the generated stresses may be greater than the yield or tensile strength of the material leading to deformation or fracture.

Mechanism

Fatigue cracking can only occur when stress fluctuates, and this is usually referred to as stress cycles. These stress cycles progressively develop cracks. Thermal fatigue can develop in components that are subjected to changes in temperature. In large components, such as castings, variations in the thickness and general geometry, combined with variations in the location of heat input, generate variations in heating rates, or temperature gradients. As a metal expands due to increase in temperature, it may be partially restrained by the surrounding (colder) material. Strains may increase to a point where plastic yielding occurs. On cooling, the area that had been heated contracts and again, it is restrained by the surrounding material and contraction may result in tensile stresses that are sufficient to generate cracks. As this cyclic thermal input continues, with sufficient strain, the crack can propagate in a staged manner.

Appearance and Examples

Since thermal fatigue cracks are associated with heating, cracks may often be heavily oxidised, with the degree of oxidation dependent on the environment. The example below shows the thermal fatigue cracking of a cast iron engine component.

The extent of the oxidation attack of both the surface of the casting (along the lower edge of the image) and within the crack show that oxidation rates are high due to significant heating. The image also shows a smaller thermal fatigue crack on the right. With this particular component, the outer surfaces exhibited multiple areas of such cracking that were readily observed, as shown below.

It is interesting to note that these cracks were generally located at changes in section in the material, that would be expected to be locations subjected to increased stress due to thermal gradients in the component.

Like high cycle fatigue (discussed in another white paper), cracking is relatively slow and progressive. As such, variations in the appearance of the fracture face may be observed to show this staged progression. In high cycle fatigue, this may manifest as linear marks that show the position of the crack front at different times. These marks, termed ‘beach marks’, can be caused by changes in loading cycle, corrosion, or ingression of contamination. Like high cycle fatigue cracks, at high magnifications, the surfaces of thermal fatigue cracks may also exhibit similar marks showing crack progression, called striations. These marks are caused by each stress cycle and are only observed at the magnifications obtained with scanning electron microscopes.

However, due to the high temperatures experienced with thermal fatigue, striations and beach marks may not always be particularly clear, and in the example above, the degree of oxidation had destroyed all such features. For example, the image below shows the exposed fracture face of the casting referred to above, with brittle characteristics, heavy oxidation, and no evidence of progressive cracking.

Avoidance

A key factor in avoiding thermal fatigue failures is in the design of the component, and this should include a good understanding of the thermal gradients that may occur. Whilst thicker sections will reduce the nominal sectional stress for a given load, thick sections may increase the thermal gradient and increase the potential for thermal fatigue cracking. Thinner sections are more preferable for this very reason and so a balance needs to be made regarding the strength and fatigue resistance of the component, between static or dynamic loading (and hence stress) and thermal fatigue. Since thermal fatigue cracks initiate from a free surface, these will generally occur at the surface of a component, although internal defects may also provide initiation sites. If these surfaces are accessible, they may be readily inspectable using non-destructive testing (NDT) techniques such as dye/liquid penetrant (LP) and magnetic particle inspection (MPI). For areas that are difficult to access, such as the internal surface of a casting, or for volumetric cracks where they have initiated from an internal defect, techniques such as ultrasonic testing (UT) or radiography (e.g. X-ray) may be appropriate.