Abstract
Taking into consideration of terms and conditions which could actually be assumed in utilization of LNG piping made of Invar alloy, assessment of integrity for fracture of weld joints was examined synthetically. Consequently, the following conclusions were obtained.
As a cause of crack initiation that could be assumed during the time of construction and operation, a weld crack, a fatigue crack and a stress corrosion cracking were mentioned. As for the weld crack and the stress corrosion cracking, the prevention method on construction was already established. Moreover, under the design stress to be assumed, it became clear that the fatigue crack did not result in initiating with sufficient degree of safety. Therefore, it was thought that the possibility of the crack initiation was very small.
Supposing that the crack arose in a certain reason despite fully preventing the crack initiation, the possibility of resulting in structural collapse by both brittle and ductile fracture was examined. Since percent ductile fracture on Charpy impact test pieces was 100%, it could be judged that there was no possibility of brittle fracture. Furthermore, since there was sufficient degree of safety about elastic-plastic fracture toughness, JIC, as compared with J which was determined from the design conditions of LNG piping, it could be said that the possibility of ductile fracture was very small.
Comparison with the maximum allowable crack size and the detectability of the defects in radiographic testing was performed. The former was much larger compared with the latter, and the detrimental defects in actual construction could be discovered certainly.
