Ratchet Marks

Ratchet marks usually occur on a surface where high stress concentration is present, i.e., thread roots fillet corners, etc.  They indicate that cracks have initiated on multiple planes.  Small tear steps or shear walls occur near the origin of a fatigue crack. A small fatigue crack origin (or initiation point) will be located approximately midway between two ratchet marks. An example of ratchet marks is shown in Photograph A below.

As the individual fatigue cracks, which are initiated at different levels, grow a “shear wall” is created joining the separate failure planes.  As the small individual fatigue cracks grow larger, the individual fatigue cracks merge forming a common (and larger) fatigue crack on a common plane and the ratchet marks disappear.