Component:
Problem:
- Extensive ligament cracking after 4 years service (~400 starts). This lay predominantly in the circumferential direction in the bottom pair of each set of four ligaments. Crack depths were up to 80% of wall.
Approach:
- Fracture mechanics assessment
Results:
- A leak-before break analysis showed that, under the pressure stress alone, a through-wall crack spanning the bottom pair of ligaments would be stable - with a reserve factor greater than 2. Even a crack spanning all four ligaments would be stable with a 10% reserve.
- The strong directionality of the cracking implied the presence of significant additional system loading.
- A simple fatigue crack growth calculation indicated that a stress range of around 150MPa was required to explain the observed crack history.
- Under loads compatible with the crack development, a crack spanning the bottom pair of ligaments would be unstable, failing by collapse.
Conclusions:
- The headers could not continue in service
- Replacement should include a measure of redesign to reduce the system loads