Fatigue Failure:

The main of engineering failures are caused through fatigue. It is described as the tendency of a material to fracture through means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably under the normal strength. While the fracture is of brittle categories, it might take a few times to propagate, depending on both the intensity and frequency of the stress cycles. However, there is extremely little, if any, warning before failure if the crack is not remembered. The number of cycles needed to cause fatigue failure at a particular peak stress is commonly quite huge, but it decreases as the stress is rised. For a few mild steels, cyclical stresses could be continued indefinitely given the peak stress (sometimes called fatigue strength) is below the endurance limit value.

A good instance of fatigue failure is breaking a thin steel rod or wire along with your hands after bending it back and forth various times in the similar place. Another instance is an unbalanced pump impeller resulting in vibrations which could cause fatigue failure.

The category of fatigue of most concern in nuclear power plants is thermal fatigue. It could arise from thermal stresses generated through cyclic changes in temperature. Huge elements like the reactor vessel, pressurizer, and reactor system piping are subject to cyclic stresses caused through temperature difference during reactor startup, modification in power level, and shutdown.