Fatigue fracture is a fracture that occurs when a material is subjected to cyclic loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture.
Rotating shafts, connecting rods, aircraft wings and leaf springs are some examples of structural and machine components that are subjected to millions of cycles of alternating stresses during service. Majority of fractures in such components is due to fatigue.
Fatigue fracture occurs by crack propagation. The crack usually initiates at the surface of the specimen and propagates slowly at first into the interiors. At some critical stage, crack propagation becomes rapid culminating in fracture.
The fatigue behavior can be understood from results of fatigue test, which are presented in from of S-N curves. Samples of material are subjected to alternating stresses of different levels. The number of cycles of stress reversals N required to cause fracture is plotted against the applied stress level S. Some materials such as mild steel show a clearly defined fatigue limit. If the applied stress is below the fatigue limit, (aka Endurance Limit) the material will withstand any number of stress reversals. If materials don't show clearly defined limit, the fatigue limit is defined as stress that would cause failure after a specified number of stress reversals. The above info is taken from Material Science and Engineering by Raghavan and the picture shown here is taken from http://www.fea-optimization.com/. Please do refer to them for more info.
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