Which type of defect can appear as fine cracks that lead to surface degradation on turbine components?

The type of defect that can manifest as fine cracks leading to surface degradation on turbine components is best characterized as fatigue. Fatigue occurs due to the cyclic loading and unloading of materials, which eventually leads to the initiation and propagation of small cracks. Over time, these cracks can grow and lead to significant surface degradation and ultimately component failure.

In gas turbine engines, components are subjected to varying loads over numerous cycles during operation, including thermal cycling and mechanical stresses. This environment makes fatigue a particularly relevant concern. The fine cracks generated through fatigue are often difficult to detect in the early stages, making it an insidious problem in turbine maintenance and operation.

Corrosion, stress rupture, and oxidation can also create issues in turbine components, but they do not primarily present as the fine cracks associated with fatigue. Corrosion typically leads to surface material loss and pitting rather than fine cracking. Stress rupture relates more to the failure of materials under constant load over time, often influenced by temperature, but is not characterized by the fine cracking typical of fatigue. Oxidation primarily causes degradation through chemical reactions with oxygen, which can affect the surface but does not specifically result in the fine crack propagation associated with fatigue.