## 1 Introduction  
With the continuous development of aviation, aerospace, energy, automobile, electronic products and biomedical engineering, the key components of high-end equipment have the characteristics of light weight, high performance and great efficiency. As an important key component, hollow shaft parts are widely used in automobiles, high-speed rail and aerospace equipment [1; 2; 3; 4; 5; 6]. Tubing separation is the first step of tubing parts processing, which directly affects the quality and efficiency of subsequent processing. The original blanks, such as hollow pins, outer rings in bearings, bushings for chains and retaining rings, are obtained through tubing blanking [7; 8; 9].  
There is a direct link between the quality of tubing separation and the quality and efficiency of subsequent processing. The existing tubing separation methods are mainly divided into the cutting method and shearing method, most of which have problems, such as low processing efficiency, poor section accuracy and serious waste of raw materials. The cutting method mainly includes sawing, turning and laser cutting. The shearing method mainly includes the shearing method without mandrel, the shearing method with mandrel and the circumferential shearing method with mandrel [10; 11]. The tubing sections obtained by sawing and shearing separation generally have shortcomings, such as low separation efficiency, large saw loss, noise and long burrs [12; 13]. Due to the hollow structure, metal tubing is prone to deformation when subjected to radial impact or pressure, which seriously affects the blanking quality. This is also considered a major difficulty in tubing separation. New processes and blanking methods are necessary to obtain acceptable section quality. Therefore, tubing precision separation technology with high quality, high efficiency and low energy consumption has become one of the key research directions in the field of machinery manufacturing [14; 15; 16].  
As for the problems of poor section precision and serious waste of raw materials in tubing blanking, this paper adopts the tubing fracture separation method of eccentric wheel rotational fatigue loading, which is a crack-controlled propagation precision separation technology. By prefabricating annular notch on specific tubing sections and applying cyclic loading, the tubing will generate fatigue cracks at the notch and expand along the specific section until the purpose of tubing separation is achieved. Compared with the traditional tubing separation method, this method has the advantages of simple structure, small loading stress, good separation efficiency, high section quality, as well as saving raw materials. In order to study the tubing fatigue fracture under eccentric wheel loading, the extended finite element method is used to simulate and analyze the tubing fatigue crack propagation. In addition, the key issues and fracture mechanism involved in the tubing separation process under eccentric wheel rotational fatigue loading were studied deeply, so as to better realize the controlled precision separation of tubing. Today's development concepts of green manufacturing and efficient and precise forming urgently require new efficient and precise separation methods for tubes to provide technical support. Exploring new methods of separating metal tubes with high efficiency, precision and material saving has important engineering application value. In studying the performance of tubes under complex fatigue, the theory of crack propagation under load has important scientific significance.