What Does Beach Marks Mean?
Beach marks are macroscopic progression marks on a fatigue fracture or stress-corrosion cracking (SCC) surface that indicate successive positions of the advancing crack front. They take the form of crescent-shaped macroscopic marks on fatigue fractures representing positions of the crack propagation, radiating outward from one or more origins.
Beach marks are also known as clamshell marks, arrest marks or growth rings.
Corrosionpedia Explains Beach Marks
Beach marks are concentric rings in a fatigue region which resemble tide marks on a beach. Crack advancements are caused by changes in environment and load.
Whenever there is an interruption in the propagation of a fatigue fracture, a unique feature of macroscopically visible marks or ridges may be found. Beach marks must not be confused with striations, although they frequently are present on the same fracture surface; there may be many thousands of microscopic striations between each pair of macroscopic beach marks. Beach marks do not occur if the part has been operated continuously, or with only brief interruptions in service.
Since they indicate various arrest points during periods of crack advance and are always oriented perpendicular to the direction of the stress, failure analysis can determine how the failed part was loaded and where the crack started. This information is very useful in failure analysis of material. It may be seen in fatigue failures of materials that are used for a period of time, allowed to rest for an equivalent time period and then loaded again as in factory usage.
Beach marks arise from two unrelated sources:
- Selective oxidation at the tip of the crack when the component is in an unloaded state
- Rapid changes in applied-stress intensity
Beach marks from oxidation are created only when the loading is intermittent. There is no opportunity for beach mark formation at a constant load in a dry environment.
Beach marks are not as well defined or may be absent in cast products, and may be obliterated in situations where the crack closes on itself during the loading cycle. Additionally, beach marks are not unique to fatigue cracking. Beach marks can also occur from changes in crack advance rates during SCC.