A New Identification Method for Bolt Looseness in Wind Turbine Towers

Table 5

Damping ratio of the first-order natural frequency of each wind turbine tower.

Tower ID	Test results before tightening (Hz)				Test results after tightening (Hz)
	Working condition 1	Working condition 2	Working condition 3	Working condition 4	Working condition 5	Working condition 6
1	0.0549	0.0552	0.538	0.0534	0.0542	0.0539
2	0.0572	0.0579	0.0566	0.0560	0.0563	0.0567
3	0.0485	0.0483	0.480	0.0481	0.0487	0.0490
4	0.0427	0.0421	0.0428	0.0430	0.0433	0.0430
5	0.0437	0.0429	0.0428	0.0423	0.0478	0.0473
6	0.0494	0.0483	0.0492	0.0498	0.0496	0.0492