Reliability Prediction Method of Fatigue Life for Rod String

Based on the wave equation of longitudinal vibration, the simulation model of cyclic stress properties for sucker rod is proposed. The regression model for stress concentration factor of rods with defects is established by the finite element method. According to the influence of transition section on fatigue limit and service life on the degree of damage, combining the randomness of material mechanical properties, defect size and cyclic stress, the reliability prediction model for fatigue life is put forward. The corresponding reliability simulation software is developed. The software not only analyses the effect of actual stress using factor (SUF) and service life on rod string´s fatigue life and reliability, but also analyses the weak link of rod string´s reliability. Meanwhile, several simulation results are shown as follows: Transition section is the weak link of the whole rod string; Service life and the actual stress level have significant influence on fatigue life and reliability; For multistage rod string designed according to equal-strength theory, different stage rod string has different reliability, and the reliability of the rod string´s bottom-part is lower than its top-part.