What are the best practices and tools for monitoring and inspecting creep and fatigue damage in service?

3 How do creep and fatigue differ?

Creep and fatigue differ in several aspects, such as the stress level, the temperature, the time scale, and the deformation mechanism. Creep usually occurs under high stress and high temperature, while fatigue can occur under low or high stress and any temperature. Creep is a time-dependent process that can take years or decades to manifest, while fatigue is a cycle-dependent process that can occur in minutes or hours. Creep involves viscous or plastic deformation of the material, while fatigue involves elastic or reversible deformation of the material.

4 How do creep and fatigue interact?

Creep and fatigue can interact in complex ways, depending on the stress conditions, the temperature, the material properties, and the loading history. Creep and fatigue can enhance or reduce each other's effects, resulting in different types of damage and failure modes. For example, creep can increase the crack growth rate in fatigue, while fatigue can increase the creep rate in creep. Creep and fatigue can also cause combined damage mechanisms, such as creep-fatigue cracking, creep-fatigue ratcheting, or creep-fatigue oxidation.

6 What are the best tools for inspecting creep and fatigue damage?

Inspecting creep and fatigue damage requires specialized tools that can detect and characterize the defects, cracks, and deformations in the material. Scanning electron microscopy (SEM) is an excellent tool for this purpose, as it can reveal the microstructural features, such as grain boundaries, dislocations, precipitates, or phases, and the fracture surfaces, such as dimples, striations, or intergranular facets. X-ray diffraction (XRD) can measure the residual stress, the phase composition, the crystal structure, and the lattice parameter of the material. Digital image correlation (DIC) is another effective tool for inspecting creep and fatigue damage; it can measure the full-field strain distribution and displacement of the material under load using high-resolution cameras and image processing techniques.