Correlations between Fatigue Strength and Inclusion Size of S135 Drill Pipe Steel

Rotary bending fatigue tests were conducted on S135 drill pipe steel, and then the fatigue performance was studied and the effect of inclusion on the fatigue property was analyzed. The diameters of maximum inclusions in different field-of-views were measured by optical microscope. The fatigue strength of samples was predicted by Murakami model and compared with the tested result. The results show that fatigue cracks originated at the complex aluminum oxide inclusions in the surface or subsurface of samples. The fatigue strength measured by the staircase method was 541.87 MPa. The maximum relative error between the fatigue strength calculated by the Murakami model and the tested value was 5.62%, indicating a relatively high accuracy of the calculation by the Murakami model.