Microscopic characteristics of fatigue failure

Striations are the most characteristic microscopic evidence of fatigue fracture, although striations are not always present on fatigue fracture surfaces, as will be seen.
However, each time the crack is opened by a tensile stress of sufficient magnitude, creating a tiny ridge, or striation, on each of the mating fracture surfaces.
If the maximum cyclic load remains constant, the striations near the fatigue origin are extremely small and closely spaced; the crack grows at a slow rate because the part is still quite strong. However, as the crack gradually propagates, the spacing between striations increases and the crack grows at an increasingly rapid rate because the crack greatly weakens the section. Eventually, complete final fracture (stage 3) and separation occur.
Unfortunately, striations are not always visible on fatigue fracture surfaces for a variety of reasons:
  1. On very hard or very soft metals.
  2. Artifacts caused by rubbing or other postfracture damage may produce parallel ridges that resemble striations. Certain lamellar microstructures in metals, resemble fatigue striations. However, careful study in the electron microscope will reveal that the orientation of the platelets varies randomly from one location to another, whereas true striations are generally concentric around the origin.
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