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FATIGUE FAILURE 

 

Fatigue failure is the phenomenon leading to fracture under repeated or fluctuating stresses that are less than the tensile strength of the material.  Fatigue fractures are progressive, beginning as minute cracks that grow under the action of fluctuating stress.  There are three stages of fatigue failure: initiation, propagation, and final fracture. 

The initiation site is minute, never extending for more than two to five grains around the origin.  The location of the initiation is at a stress concentration and may be extremely small and difficult to distinguish from the succeeding stage of propagation, or crack growth.  The crack initiation site is always parallel to the shear stress direction. 

As repetitive loading continues, the direction of the crack changes perpendicular to the tensile stress direction.  [It should be noted that ductile fracture is caused by shear stress components, whereas brittle fracture is caused by tensile stress components.]  After the original crack is formed, it becomes an extremely sharp stress concentration that tends to drive the crack ever deeper into the metal with each repeating of the stress.  The local stress at the tip of the crack is extremely high because of the sharp “notch,” and with each crack opening, the depth of the crack advances by one “striation” under many (but not all) circumstances.  Striations are very tiny, closely spaced ridges that identify the tip of the crack at some point in time (Figure 1).  Although striations are the most characteristic microscopic evidence of fatigue fracture, they are not always present on fatigue fracture surfaces.   

Whenever there is an interruption in the propagation of a fatigue fracture a unique feature of macroscopically visible marks or ridges may be found.  These marks are described as “beachmarks” or “growth rings.”   Figure 2 is an example of beachmarks in a fatigue failure.    Beachmarks must not be confused with striations, although they frequently are present on the same fracture surface; there may be many thousands of microscopic striations between each pair of macroscopic beachmarks.  Beachmarks will not be present if the part was operated continuously, or with only brief interruptions in service. 

As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area, it eventually weakens the material so greatly that final, complete fracture occurs.   The final fracture may be either ductile (with a dimpled surface) or brittle (with a cleavage surface), or a combination of the two.

 

Figure 1. Fatigue striations observed with a scanning electron microscope (1000X Original Magnification) Figure 2.  Examples of beachmarks caused by fatigue, see arrow (7X Original Magnification)

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