Fracture analysis on ball head hanging ring of cross arm insulator for 220kV transmission tower

The fractured ball head hanging ring used for cross arm insulator of 220kV transmission tower was investigated by means of macro-morphology inspection, chemical composition analysis, microstructure analysis, SEM morphology analysis and finite element analysis. The result showed that the pre tightening force of the connecting bolt between the U-shaped hanging plate and the ball head hanging ring was too high and the ball head hanging ring had been stuck, so that it could not swing freely around the bolt hole at the lower end of the U-shaped hanging plate. In addition, due to improper hot working process, a large number of network ferrite had been formed in the structure, resulting in a significant decline in its fatigue resistance and toughness. In this way, in case of wind deflection swing of the conductor, the rod of ball head hanging ring would bear large alternating bending stress and tensile stress, and finally fatigue fracture would occur at the joint between the ball head and the rod with obvious stress concentration.