Research on Loosening Diagnosis of Flange Bolts of Wind Turbine Tower Barrel Based on Strain Monitoring Theory

Wind turbines affected by alternating loads may cause their bolts to loosen due to fatigue. As an important part of connecting the wind turbine tower, flange bolt connection directly affects the safety and stability of the overall structure. Therefore, it is necessary to monitor the health status of flange bolts in real time. Based on strain monitoring theory, a strain based method of flange bolt loosening diagnosis was proposed, which can realize real-time monitoring of bolt health status. Firstly, the monitoring principle of flange bolts of tower barrel was proposed by using equivalent static method, and the relations between bolt loosening and dynamic strain near flanges were expounded. In order to reflect the strain changes before and after bolt loosening and eliminate the instantaneous error, the strain data were discretized and the RMS difference ratio RMSCR was proposed. The zone loosening index RDI was proposed to reflect the degree of bolt loosening in the zone. Through numerical simulation, a series of working conditions were designed to carry out flange bolt loosening research, and the feasibility of monitoring theory and the effectiveness of damage indexes were verified. The results showed that when the preload loss was small, the index change was not sensitive, as the preload decreased, the index began to change slowly when it reached 40%, and as the preload decreased, the change amplitude was more obvious. For the reverse balanced flange, the index was still applicable, and the monitoring effect in the tension area was better.