# Critical Crack Size

## Definition - What does Critical Crack Size mean?

Critical crack size is the length at which a crack becomes unstable at certain applied stress. It is very helpful in determining material safety. In an unstable crack, crack propagation, once started, continues spontaneously without an increase in magnitude of the applied stress. Fatigue failure occurs very rapidly once the advancing crack has reached a critical size.

Fracture mechanics allow analysis of critical crack size for unstable crack growth leading to fracture. Structures must be inspected periodically to ensure that actual crack size is smaller than critical crack size, otherwise the structure is sure to fail.

## Corrosionpedia explains Critical Crack Size

Critical crack size is the size of a flaw (crack) in a structure that causes failure at a particular stress level. Thus, the critical crack size can vary significantly with the type of material.

In an ideally elastic (brittle) material, crack propagation occurs catastrophically if the crack size is above the critical size. At critical crack size, a subcritical crack growth may occur due to environmental effects, such as stress corrosion, resulting in crack growth and failure. Fracture mechanics is concerned with the formation and growth of cracks, which cause such fractures in brittle materials.

Critical crack size is a function of applied stress. The rate of increase in crack size is small at small crack sizes and increases with crack size until the crack becomes unstable at a critical crack size, which depends on the failure criterion.

In stress corrosion cracking, it is not uncommon for growing cracks to be considered acceptable so long as they are substantially less than a critical crack size and can be repaired at the next system shutdown.