Description
As the name suggests, creep-fatigue is the combined action of creep and fatigue failure mechanisms.
Creep is the time-dependant deformation of a material while under an applied constant stress, with that stress below the yield point. In the latter stages of creep, cavities may form at grain boundaries, leading to fracture.
Fatigue is a progressive crack growth mechanism that occurs when a material is subjected to a fluctuating (cyclic) tensile stress. Sources of this cyclic stress may include vibrations in the system, stress cycling, and thermal cycling.
Creep-fatigue may then be considered as the deformation of a material, leading to fracture, under repeated stresses at high temperature. More detailed explanations for both creep and fatigue may be found in other failure mechanisms on the SureScreen Materials website.
Mechanism
During the course of a component’s lifetime, when exposed to high temperature and cyclic loading, there is the risk of crack initiation by creep-fatigue mechanisms.
Creep-fatigue damage depends on a number of factors such as temperature, the rate and range of strain, the time held at both temperature and stress, the ductility, and creep strength of the material. The variation in these parameters will cause changes in the mechanisms of creep-fatigue. Broadly, the mechanisms can then be divided into four main modes;
1. Fatigue dominated. If the time at temperature or strain (the hold time) is short, creep damage mechanisms have little time to develop, and damage is dominated by fatigue.
2. Creep dominated. With prolonged hold times, and at high temperatures, fatigue cracks may still develop, but the creep damage becomes more dominant.
3. Creep-fatigue – when creep is consequential. Fatigue cracks develop and propagate into the material, but under the effects of the presence of the cracks, the stress regime changes in the vicinity of these cracks, leading to associated creep damage.
4. Creep-fatigue – when creep damage is simultaneous with fatigue. Fatigue cracks and creep damage develop independent of each other, with some fatigue cracks following the planes of weakness provided by the creep damage.
Appearance
Creep-fatigue failures may be difficult to diagnose post-failure by examination of the fracture faces. As a crack develops at high temperature, the surface will be exposed to the local environment and given that it will have experienced high temperatures, oxidation of the fracture surfaces will be highly likely. The oxidation will have either wholly or partially destroyed the fracture features. However, in some industrial applications, where the operating conditions are tightly controlled and known, data may be available to date the various stages of the crack growth, based on the thickness and structure of the oxide layers. This data will be derived from laboratory tests to generate oxide growth on the specific materials, and in known conditions. However, this data may not always be available, and dating the stages of crack growth will, at best, be highly subjective.
Understanding of the modes of fracture can be useful in failure analysis and prevention of recurrence, and this may be established by metallographic examination of cross-sections through the material as shown in the following photomicrographs. In this way, the relative balance between creep, fatigue, and creep-fatigue can be better established in addition to examination of the thicknesses of oxide layers, should the oxide data be available for assessment.
The image below shows the early development of voids in the microstructure during Stage 3 creep and there are no signs of fatigue.
This next image shows multiple fatigue cracks propagating from the outer surface into the material, and no creep damage.
The image below shows the latter stages of Stage 3 creep with extension of cracks predominantly along the grain boundaries. In creep-fatigue, fatigue cracking would be observed to extend from the surface, and then to link through the creep voids not dissimilar to this image.
