Fracture mechanics and fatigue
The fracture surface of a part that failed by fatigue shows shell-like patterns visible to the naked eye (beach marks) and fine lines visible in an electron microscope (striations). Which statement about them is correct?
- ABoth form one line per load cycle, so counting them gives the number of cycles to failure
- BStriations roughly mark the crack advance in each cycle, while beach marks record history such as shutdowns
- CBoth form in the final fracture zone that broke all at once, gathering at the place farthest from the origin
Show answer and explanation
Answer: B. Striations roughly mark the crack advance in each cycle, while beach marks record history such as shutdowns
Striations are lines left as the crack tip opens and blunts, then closes and sharpens again in each cycle, and their spacing corresponds roughly to the crack growth per cycle, da/dN. The spacing is fine, at the µm level or below, and cannot be seen without an electron microscope. Beach marks are patterns that record positions of the crack front through shutdowns and restarts, changes in load level, differences in how corrosion progresses and so on, and each band contains many cycles. Both appear in the region where the fatigue crack advanced little by little, and the common center of the patterns shows the origin. The final fracture zone is where the remaining cross-section broke all at once, and it shows a ductile or brittle fracture surface.