抄録
Break out had happened during field hydraulic test of unused line pipe. The observation of failure pipe indicated presence of the pre-damage at the broken portion. We supposed that the damage would have occurred during manufacturing or shipping. From analysis offracture surface and failure section, follow ing facts were revealed
(1) The cracks propagate in wall thickness direction independently of the metal-flow of hot rolling direction.
(2) The crack initiation sites are the external and the internal surfaces of the pipe and the cracks propagate stepwise with smooth fracture surface.
(3) In the fracture surface, many corrosion products are observed and clear striations are locally recognized.
It was concluded that the failure was caused by fatigue crack during shipment. This mechanism was confirmed by the pulsating bending fatigue test of pipes.
