Abstract:Six cracks occurred in the oil inlet pipe of a main pump near the same location. The fracture properties of the parts were determined by macroscopic examination, fracture observation, composition analysis, hardness examination and microstructure examination.The test results show that all the cracks are fatigue cracks. The results show that the vibration stress at the crack location is larger than the design value；The larger vibration stress is the main reason for the initiation of fatigue crack;At the same time, welding residual stress and stress concentration also promote the initiation of fatigue crack.When the vibration stress value of the external field catheter exceeds 30 Mpa, the crack can be avoided by adjusting or increasing the clamp.