Cracks found in machine and structural components raise two questions: Will the crack cause an immediate failure, and if not, how long until the crack grows large enough for failure?
To address the first question, two methods are used to evaluate cracked components:
- Ductile tearing analysis
- Failure Assessment Diagram (FAD)
Computing critical flaw sizes helps to ensure damage-tolerant components and set appropriate inspection schedules when combined with fatigue analysis. The historic Shippingport reactor vessel (Fig. 1) provides an example to compare the two analysis methods.
1. Shippingport reactor pressure vessel.
A crack in a pressure vessel is analyzed with quarter symmetry to simplify the model. The top head and flange are omitted, since the circumferential flange is much stiffer than the region around the nozzle. A stress concentration by the base of the nozzle is a good location to examine a postulated crack.