Fatigue Performance Comparison of S135 Drill Pipe Steel under Uniaxial Loading

The fatigue life of S135 drill pipe steel was tested by tension-compression loading and torsion loading under uniaxial loading during the fatigue test. The quantitative formulas of uniaxial fatigue life were obtained by regression analysis method, and the fatigue fracture mechanism was analyzed. The results show that a linear correlation was found between fatigue life and effective stress in the double logarithmic plots. The ratio of torsion fatigue limit to tension-compression fatigue limit was 0.682. Cracks initiated from the samples surface under tension-compression loading as well as torsion loading. For the tension-compression fatigue and the torsion fatigue samples, the main characteristics for cracks propagation were fatigue striations and sheared ripple marks, respectively.