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Return to
Failure Analysis Case Histories
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FATIGUE FAILURE |
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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.
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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.
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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.
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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.
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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. |
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| 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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