Fatigue life cycle estimation and reliability analysis of hydraulic plumbing of a launch vehicle control actuator

One of the single point failure modes identified for the electro hydraulic control actuators of launch vehicles, is the fatigue failure of the hydraulic plumbing and its joints due to severe vibration environments. This paper presents a case study of the fatigue analysis on the hydraulic plumbing with flared coupling joint, subjected to high vibration levels. An evaluation of the vibration response of the system is made through analysis and experimentation. A finite element model of the plumbing is made to capture the dynamic behaviour of the system. The alternating stresses are assessed by dynamic stress analysis using the validated model. Fatigue life cycle estimation and reliability of the joint are determined pragmatically through stress life methods, based on the local peak stress, the corresponding local strength, and their statistical distribution. Based on the study, a simple design solution is suggested to improve the robustness of the critical control actuator of the launch vehicle.