Low Cycle Fatigue Failure Analysis of High Strength Steel CT-110 under Cyclic Loading: Numerical and Experimental Research

The diameter increase and wall thickness thinning of coiled tubing (CT) are easily caused by plastic deformation, which seriously affects the service fatigue life of CT. Currently, the basic data of the CT fatigue life model are obtained from field operation and fatigue testing machines. Among them, the test procedure for the CT fatigue testing machine is complicated and wastes sample CT. As the research object CT-110, this work established a theoretical model of CT low-cycle fatigue life based on shaping strain. A numerical calculation model of the coupling relationship between CT fatigue life and cycle count is established based on the finite element method (FEM). Finally, experimental verification is carried out. The results show that the material on the compression and tension sides of the CT flows in the direction of the neutral axial, which leads to a significant thinning of the wall thickness of the CT under tension and compression. However, the material at the neutral axis is not subjected to plastic strain. Hence, the maximum plastic strain and fatigue-sensitive area of CT appear on the axial tensile plane and compression plane, which is consistent with the field failure of CT. The elastoplastic finite element (FE) results showed good agreement with experimental results. The numerical results make up for the inaccuracy of theoretical and experimental calculations in predicting the fatigue life of CT. The analysis results provide theoretical guidance for the prediction of CT fatigue life in actual operation, and it can provide a reference for field operators to effectively avoid economic losses caused by misoperation.