Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load

• Published in: Engineering failure analysis 2016-08, Vol.66, p.141-153
• Main Authors: Xie, Zhanshan, Zheng, Yuan, Wang, Anni, Chen, Yuan, Tian, Qian, Kan, Kan
• Format: Article
• Language: English
• Subjects: Amplitudes; Asymmetric load; Boundary element method; Crack propagation; Failure analysis; Fatigue (materials); Fatigue failure; Fatigue test; Fracture mechanics; Mathematical analysis; Microstructure; Stresses; The ball eye
• ISSN: 1350-6307; 1873-1961
• DOI: 10.1016/j.engfailanal.2016.04.021

The ball eye (BE) is a key connecting component between the insulator and transmission tower, whose fatigue characteristics concern the safety of transmission lines. To understand the fatigue mechanism and characteristics of it, the fatigue test was conducted based on the following data: r=0.25, S=500MPa,then plotting of SN and Δεaxis−N, to analyze the fatigue failure of the test specimen from the macro and micro point of views. The research results show that: the life of BE significantly reduces with the increase of the stress amplitude, but the relative reduction in life is not the same; softening and strain amplitude of the specimen change differently before and after the stress amplitude of 300MPa; when S≤300MPa, the fracture is more smooth, the fatigue crack propagation is slow; when S>300MPa, the rate of fatigue crack growth is faster, and the fatigue crack growth zones are not obvious. The cracks are easily detectable appear at the joint of the BE and insulator cap, and the cracks along the fracture cross section are constantly expanding, showing multiple fatigue sources and fatigue steps. The number of fatigue steps increases as the magnitude of the tensile stress increases. When S=500MPa, the yield strength decreases during the lifetime, the decrease rate of the tensile strength and microstructure strength in each stage are different. Axial lengthening and section shrinkage ratio decrease with the development of fatigue, fatigue evolution process is accompanied by phenomenon of crystalline slip, deformation, dislocation, at the same time, dissipation and decomposition of pearlite occur, and carbide precipitates from the matrix, growing and moving to the grain boundaries, the specific phenomenon of grain growth appears. •For the working conditions of the ball eye on the overhead line, it is the first time to carry out the fatigue test which bases on the following data:r=0.25, S=500MPa, then curving the plotting of S-N, analyzing the fatigue mechanism and characteristics of the ball eye.•From the macro point of view, it is the first time to analyze the mechanical properties and fatigue fracture characteristics of the ball eye in the process of fatigue evolution.•From the microscopic point of view, it is the first time to analyze movement law and evolution characteristics of grain in the process of fatigue failure.