Ratchet Marks
Definition
Radial steps or ridges on a fatigue fracture surface separating adjacent regions that initiated from separate sites. They run from the component surface into the fracture and indicate multi-site initiation or high stress amplitude.
- Location
- Fatigue fracture surface
- Appearance
- Radial steps or ridges
- Orientation
- Run from surface into the fracture
- Indicates
- Multiple crack initiation sites or high stress amplitude
Common questions
Why do ratchet marks matter for determining the cause of a mechanical failure?+
Their presence tells an investigator that fatigue cracking began at more than one point on the component simultaneously, which usually points to a higher applied stress or a stress concentrator such as a notch, weld defect, or corrosion pit rather than a single random initiation.
How are ratchet marks distinguished from beach marks on the same fracture surface?+
Beach marks are concentric curved bands that record the crack front's progressive advance over time, while ratchet marks are radial steps that mark the boundary between fracture surfaces from separate initiation sites converging. The two are usually visible together but describe different features.
Related terms
- Beach Marks
- Macroscopic curved bands on a fatigue fracture surface, visible without magnification, produced by changes in crack growth rate during service (load changes,...
- Fatigue Fracture
- Fracture resulting from repeated cyclic loading, typically at stresses below the static yield strength. The fracture surface has characteristic markings (beach marks,...
- Fatigue Striations
- Microscopic parallel marks on a fatigue fracture surface, formed during individual loading cycles in Stage II crack growth. Visible by SEM at...
- Multi-Site Damage (MSD)
- The simultaneous presence of fatigue cracks at multiple adjacent fastener holes or structural features, characteristic of ageing aircraft fuselage and other riveted...
- Stress Intensity Range (ΔK)
- The difference between the maximum and minimum stress intensity factor in one loading cycle, ΔK = K_max - K_min. The Paris law...