Optimization of fatigue life distribution model and establishment of probabilistic S-N curves for a 165Â ksi grade super high strength drill pipe steel

Fatigue performance of a 165 ksi Grade Super High Strength Drill Pipe Steel was studied by rotary bending fatigue tests and fatigue reliability design method. Three common fatigue life distribution models were analyzed in this paper and the most suitable model was selected by comprehensive coefficient method to describe the distribution of test data. Based on the life distribution model and Basquin's equation, probabilistic S-N curves in middle-long life regime were obtained. The probabilistic S-N curves were also extended to super-long life regime. Test results show that the most match model was the three-parameter Weibull distribution; two-parameter Weibull distribution ranked second; the last was logarithmic normal distribution. Fatigue limits under reliability 50%, 95% and 99% were 648.91 MPa, 632.28 MPa and 618.64 MPa respectively with 99% confidence level. Extended probabilistic S-N curves were lower than fatigue limits in super-long life regime.