Fatigue refers to the stress caused by cyclic loading of components.  The effect of Fatigue is particularly pronounced in components which are stiff and thick. 

One of the chief causes of fatigue is the frequent  starting and stopping of the boiler.  When the boiler is started and stopped, the temperature gradient causes the expansion and contraction of the components. Fatigue can also result in cracks in welds.   Some of the areas particularly vulnerable to fatigue are the pipe to drum connection, fitting to tube, the welding in the support attachments, etc.