Critical crack depth
Crack dimension at which unstable crack propagation is predicted to occur based on fracture mechanics calculations. NOTE 1 Crack depth for a given load at which the stress intensity factor equals the plane strain fracture toughness (KIC). NOTE 2 Crack depth is that depth at which the combination of load ratio and toughness ratio are at the limit on the failure assessment diagram.
Source: API TR 1PER15K-1, Protocol for Verification and Validation of High-pressure High-temperature Equipment, First Edition, March 2013. Global Standards