Perpendicular lines

The term "ratchet marks" is used to describe features that are very useful in identification of fatigue fractures and in locating and counting the number of fatigue origins. These marks are essentially perpendicular to the surface from which fatigue fractures originate. Therefore, in circular, shaft-like parts, the ratchet marks are essentially radial, pointing toward the center; in flat parts, such as leaf springs, they initially are perpendicular to the surface but may curve if the bending is unidirectional.

The ratchet marks are not the origins themselves; each ratchet mark separates two adjacent fatigue fractures. As the cracks become deeper, the cracks from each origin tend to grow together and become essentially one fatigue fracture that has numerous origins. The number of ratchet marks equals or is one less than the number of origins; thus recognition of the number of ratchet marks is important in determining the number of origins.
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