Striations or beachmarks?

  • Both striations and beachmarks identify the position of the tip of the fatigue crack at a given point in time.
  • Both striations and beachmarks expand from the fatigue origin or origins, often in a circular or semicircular fashion.
  • Both striations and beachmarks are relatively parallel ridges which do not cross similar features from another origin.
Some fatigue fracture surfaces have neither striations nor beachmarks.
Artifacts, or false features, can confuse observation of both striations and, beachmarks.

Differences Between Striations and Beachmarks
  • The most obvious difference between striations and beachmarks is size. Striations are extremely small ridges, visible only with an electron microscope. Beachmarks are much larger than striations. If they are present, they are normally visible to the unaided eye.
  • The other difference between striations and beachmarks, as previously mentioned, is the factors that cause them. Striations represent the advance of the crack front by one load application in many ductile metals, whereas beachmarks locate the position of the crack front when repetitive, fluctuating loading was stopped for a period of time.
  • Fatigue Under Compression Forces. A seemingly puzzling type of fatigue crack is one that grows in a part, or a region of a part, that is stressed in compression when the part is under load. It may be recalled that the general principle of mechanically induced residual stresses is that "tensile yielding under an applied load results in compressive residual stresses when the load is released, and vice versa." This type of fatigue fracture can be illustrated with the example of the Belleville spring washer, a hardened spring steel washer that is cone-shaped in order to give high spring rates in limited space.When this type of spring washer is pressed flat, the upper surface and corner of the hole are heavily compressed in a triangular pattern around the hole. The compressive stress may be so high that the metal yields compressively around the hole. That is, the circumferential compressive stress on the upper side of the plate at the hole forces the metal to yield compressively when the spring washer is pressed flat. When the fatigue load is released, the washer springs back to its original position and the upper surface now is stressed in residual tension as a result of compressive yielding when the load was applied. Consequently, the fatigue action causes the necessary tensile stress at this location when the load is released, not when it is applied. With continued operation, radial fatigue cracks will form at the upper corner. The solution for this "compression" type of cracking often lies in the field of design, as well as in heat treatment to increase yield strength.
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