Beach mark
Macroscopic lines on a fatigue fracture surface caused by operational pauses or load variations. They indicate the progressive progression of crack growth over time.
Beach marks on a fracture surface are the typical sign of a fatigue fracture. The crack doesn't grow all at once, but in steps with each load cycle. If the part is at rest in between, the fresh fracture area slightly oxidizes or gets polished by friction, and with the next load cycle a visible line is created. Several such interruptions result in the typical stripe pattern. In welded structures, these cracks almost always start at notches – at the weld transition, at the root, or at defects like pores. If you see such a picture, you know: The part has been repeatedly loaded, the fracture didn't happen suddenly. Look closely at the weld to see if there was a sharp notch, and whether the weld was post-treated. By grinding the transitions and removing spatter, you can prevent a lot, because every sharp edge is a potential starting point for such a crack.