Loosening Detection of a Bolted Joint Based on Monitoring Dynamic Characteristics in the Ultrasonic Frequency Region

In typical structures and machines, bolted joints are used for easy assembly, disassembly, and maintenance. However, axial force control is critical because a decrease in the axial force (viz., the fastening force) of bolted joints results in fatigue fractures due to the applied vibrations on the bolted joints and chronic degradation. This work assesses the feasibility of detecting weakly torqued bolted joints by studying the correlation between the variation in axial force of the bolt and the shift in the natural frequency of the protruding threaded part of the bolt. It has been shown that the rigidity of the jointed area at the base of the protruding thread part decreases in accordance with the decrease in the axial force of the bolt. This is attributed to a decrease in the natural frequency of the protruding thread part. Under these assumptions, the axial force on bolted joints is assessed. The accuracy with which axial force can be estimated depends on the extent of the shift in the natural frequency of the protruding thread part. To investigate the practical usability of our method on strength classification of bolts, we assessed the performance of loosening detection in three strength classifications (4.8, 8.8, and 10.9).