The Study Of Dynamic Critical Stress Determination Method In Aerocraft Hydraulic Pipe System

The hydraulic pipe system in aircraft structure is a typical system, it has high strength and high ratio of stress/weight. The good operation state of hydraulic pipe system is vital to safety flight of aircraft. But the accidents of hydraulic pipe system have been heard sometimes, so more detail research about strength of hydraulic pipe structure, especially vibration strength, is needed.In hydraulic pipe system design of some model aircraft, the dynamic critical stress is 40MPa, but it is not clear how the value is determined. In this thesis, the dynamic critical stress determination method in hydraulic pipe system design has been studied, the S-N curve of lCrl8Ni9Ti used in hydraulic pipe system has been given at cantilevered restraint, the first natural frequency and sinusoidal vibration loading. The fatigue limit of lCrl8Ni9Ti is 218MPa. Considering the effects of stress concentration coefficient, size coefficient and surface coefficient, the fatigue limit of hydraulic pipe is 47.39MPa. The stress and statistic analysis of hydraulic pipe system at some engine operating states indicate that the stress is less than the fatigue limit. So it can be concluded that the dynamic critical stress in hydraulic pipe system design is fatigue limit. Scanning electron microscopy was used to analyze microcosmic fracture surfaces. Fatigue cracks initiate at specimen surface, and grow to center from both surfaces, create a distinct center line. Beach marks can be found on the fracture surfaces that show the fracture is a typical fatigue failure.