Multiaxial Fatigue Life Research Of Buttress Casing Thread Connection

Casing drilling technique which has been dramatically developing is a revolution in petroleum industry and has aroused great concern at home and abroad. But it presents a new demand for the casing connections' property. The fatigue failure of casing thread connections is a critical issue and hasn't be solved. It is therefore of great engineering and scientific value to systematically study the fatigue failure of commonly used casing connection. In this paper, the notched specimens are designed, according to the character of the stress and strain distribution of the connections. The stress-strain state in the critical position for notched specimens was calculated and analyzed by elastic-plastic finite element method, and then the obtained results were used to predict the multi-axial fatigue life of notched specimens.First, through the tensile testing of J55 casing material, the foundamental mechanics parameters is tested, such as field stress, tensile intension etc.Second, the stress and strain for the connections subjected to proportional loading were analyzed by elastic-plastic finite element method. Point out the critical position of thread connection. The stress-strain state for notched specimens subjected to constant amplitude proportional multiaxial loadings was also calculated and analyzed by the finite element model and the theoretical stress concentration factor of the notched specimens had obtained.Third, a series of multiaxial fatigue tests were carried out on MTS858 material test system for notched specimens. The fatigue fracture surfaces of notched specimens were observed and analyzed. According to the observed results of the fatigue fracture surfaces, the mechanisms of fatigue crack initiation and propagation were studied.Last, on the basis of the results of finite element analysis, some multiaxial fatigue models were used to multiaxial fatigue life. The predicted lives were compared with the experimental data. The results of fatigue life prediction showed that a good agreement was demonstrated with experimental data.