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• A method for calculating the fatigue life of threaded joint with multi-axis fatigue. • A method for optimizing threaded joint. • Simulated the fatigue life analysis of a threaded joint subjected to 140MPa ultra-high pressure. • Vibration fatigue is the key factor affecting fatigue fracture of the union joint thread. Ultra-high pressure fracturing technology has become more and more important in China's deep Shale Gas extraction. In recent years, fracturing manifold failure accidents have happened frequently and service life has shortened drastically. There are more than 94% of failure accidents that are caused by vibration fatigue of the union joint. In this paper, it was established as a 3d finite element model of the union joint based on finite element theory and fatigue theory. The author proposed a safety evaluation method based on fatigue life, and compared it with that based on strength. The results show that vibration is the key factor affecting fatigue life, and the fatigue life-based safety evaluation method is more suitable to describe the union joint safety. Therefore, set fatigue life as optimization goal and the key parameters of the union joint thread are optimized by orthogonal optimization method. The optimal results are obtained from the study range, namely that the thread turns 8, the pitch is 10.47 mm, the bearing surface angle is 24.5°, and the guide surface angle is 24.5°. In comparison to the initial union joint, the fatigue life of the optimized union joint is increased by two orders of magnitude. These studies improve the fatigue performance of the union joint and have important guiding significance for improving the safety evaluation level of the fracturing manifold system in this paper. [ABSTRACT FROM AUTHOR]

Small-diameter thread is widely used in the connection of aeroengine, nuclear equipment, and petrochemical equipment parts due to its advantages of high connection strength, simple structure, and convenient loading and unloading. Manual tapping is its main production method. But, the stability of product quality is poor; the qualified rate of one-time machining is less than 20%, resulting in a large number of repeated machining and scrap. The existing thread milling method is difficult to adapt to the small-diameter internal thread processing of Waspaloy high-temperature alloy. The deformation of the tool during the processing and the difficulty in heat dissipation from the unreasonable processing technology are the main reasons for the processing accuracy not meeting the standard. In this paper, a 3-D numerical simulation model of small-diameter internal thread milling is established based on the flow stress control theory. The influence of structure parameters and milling machining parameters on milling force and milling temperature is studied by orthogonal experiment, and the range analysis is carried out. The mathematical model of milling force and temperature is calculated by nonlinear regression analysis. Based on Pareto genetic algorithm, multiple objective optimization method, the cutter structure, and machining parameters are optimized. The better machining parameters and cutting cutter parameters are obtained; the deformation of thread milling cutter in X, Y, and Z direction is obtained, and a new structure of thread milling cutter is designed; the feasibility of the new type of thread milling cutter and the machining scheme is verified by the trial cutting experiment of the thread milling cutter. [ABSTRACT FROM AUTHOR]