During the fatigue process, the formation of the main crack expends most of the whole fatigue life, and once the main crack forms, the crack propagates rapidly and fractures immediately.
疲劳过程中主裂纹的形成消耗了大部分的疲劳寿命，一旦主裂纹形成就快速扩展瞬间断裂。
2
Therefore it can be judged that the fractures in the tubing are fatiguefractures and the tubing failure is caused by fatigue fracture.
由此可以断定油管断口属疲劳断口，油管的断裂失效性质是疲劳断裂。
3
Crack spreading area and transient breaking are observed, typical fatigue striations, fatigue steps and secondary cracks are observed on some of the fractures.