Stages of fatigue failure

The definition above refers to fracture "under repeated or fluctuating stresses having a maximum value less than the tensile strength." (The final fracture may have either brittle or ductile characteristics, depending upon the metal involved and the circumstances of the stress and the environment.).
There are three stages of fatigue fracture: initiation, propagation, and final rupture. Indeed, this is the way that most authors refer to fatigue fracture, for it helps to simplify a subject that can become exceedingly complex.

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Stage 1- Initiation. Initiation is the most complex stage of fatigue fracture and is the stage most rigorously studied by researchers.The most significant factor about the initiation stage of fatigue fracture is that the irreversible changes in the metal are caused by repetitive shear stresses. The accumulation of microchanges over a large number of load applications, called "cumulative damage," has been the subject of study over the years." (Obviously, if this stage can be prevented, there can be no fatigue fracture.)The initiation site of a given fatigue fracture is very small, never extending for more than two to five grains around the origin. At the location of a severe stress concentration, the number depends on the geometry of the part as well as on environmental, stress, metallurgical, and strength conditions, as will become apparent.

Stage 2 - Propagation. The propagation stage of fatigue causes the microcrack to change direction and grow perpendicular to the tensile stress. The second, or propagation, stage of fatigue is usually the most readily identifiable area of a fatigue fracture.

Stage 3 - Final Rupture. As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area of the part or test specimen, it eventually weakens the part so greatly that final, complete fracture can occur with only one more load application. The fracture mode may be either ductile (with a dimpled fracture surface) or brittle (with a cleavage, or perhaps even intergranular, fracture surface) or any combination thereof, depending upon the metal concerned, the stress level, the environment, etc. Stage 3 represents the "last straw" that broke the camel's back, to borrow a metaphor.