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Bolted joints are used in many machines/structures and some of them have been loosened during long time use,
and unluckily these bolt loosening may cause a great accident of machines/structures system. These bolted joint,
especially in important places, are main object of maintenance inspection. Maintenance inspection with human-
involvement is desired to be improved owing to time-consuming, labor-intensive and high-cost. By remote and
full automation monitoring of the bolt loosening, constantly monitoring of bolted joint is achieved. In order to
detect loosening of bolted joints without human-involvement, applying a structural health monitoring technique
and smart structures/materials concept is the key objective. In this study, a new method of bolt loosening
detection by adopting a smart washer has been proposed, and the basic detection principle was discussed with
numerical analysis about frequency equation of the system, was confirmed experimentally. The smart washer
used in this study is in cantilever type with piezoelectric material, which adds the washer the self-sensing and
actuation function. The principle used to detect the loosening of the bolts is a method of a bolt loosening
detection noted that the natural frequency of a smart washer system is decreasing by the change of the bolt
tightening axial tension. The feature of this proposed method is achieving to identify the natural frequency
at current condition on demand by adopting the self-sensing and actuation function and system identification
algorithm for varying the natural frequency depending the bolt tightening axial tension.
A novel bolt loosening detection method by adopting adaptive observer is proposed in this paper. The
numerical simulations are performed to verify the possibility of the adaptive observer-based loosening detection.
Improvement of the detection accuracy for a bolt loosening is confirmed by adopting initial parameter and
variable adaptive gain by numerical simulation.
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