Assessment of fatigue life of square threads using SN curve analysis

Abstract

When coil tubing is used, thread failure in Bottom Hole Assemblies (BHA) at high depths is a worry due to tough well paths. Square threads made from medium carbon steel (AISI 1045) are investigated to find out if they can enhance the fatigue resistance in extreme drilling environments. The study investigates if square threads, with their flat geometry, may permit steady and balanced loading of BHA parts, in contrast to traditional thread forms. A four-point rotating bending fatigue machine was used to test samples at different stress levels. To determine the performance of square threads, fatigue life and endurance limits were calculated by analyzing the data with the S-N methodology. The minimum load we used is 130N and we get maximum number of cycles i.e. 773250, whereas, maximum load is 450N at which we got 4037 number of cycles. Stress Concentration factor K_t is 1.72. Analysis concluded that, even if their fatigue life is just below that of rounded threads, square threads are still a better alternative than traditional thread designs for BHA components working in tough conditions.