A Study of the Notch Fatigue Behavior of S135 Drill Pipe Joint

Drill pipe joint fatigue is the main form of drill pipe failure. In order to improve the safety fatigue life of drill pipe, the notch fatigue behavior of S135 drill pipe joint has been studied. The fatigue specimens were designed according to the force characteristics of drill pipe joint, and the influence of stress concentration on the fatigue performance of S135 drill pipe was studied by PQ-6 rotary bending fatigue tester and scanning electron microscope. The results showed that the fatigue of S135 drill pipe exhibited certain sensitivity to the V-notch. The smaller the notch root radius, the larger the stress concentration factor, and the more serious impact on the fatigue performance. The stress concentration factor and the fatigue strength are in accordance with the power function relationship. The process is as follows: the fatigue crack starts from the root of notch, and many sources of cracks existed (in annular arrangement). When the stress is the same, the tearing phenomenon at the root of notch weakens with the increase of stress concentration factor, the fracture becomes smooth and flat, and the fatigue fringes become fine and dense. The research results showed that the optimal design of the drill pipe thread joint could reduce or alleviate the stress concentration of the root, reduce the fatigue sensitivity of S135 drill pipe to the notch, and improve the safety by better mapping the fatigue life of S135 drill pipe.