5 / 2017-10-13 17:55:15

Bolt Loosening Diagnosis by Measuring Vibration Responses Excited by Laser Induced Plasma

Bolt loosening, Laser excitation, High frequency response, Fault diagnosis

A health monitoring system based on an impulse response excited by laser ablation is proposed to detect a bolted joint loosening. The purpose of this study is to determine an appropriate location of laser irradiation as excitation point and to develop a damage detection method. A high power Nd:YAG pulse laser is used to generate an ideal impulse excitation on a structural surface which offers the potential to measure high frequency vibration responses of the structure. A health monitoring apparatus is set up using this vibration testing system and a damage detecting algorithm. The test piece is irradiated with the pulse laser at two different locations i.e. the bolt and on the flange, and the vibration responses, under the tight and loose bolt conditions, are measured using an accelerometer fixed at a point. In the selection of the laser irradiation location, it can be observed that choosing the excitation point on the bolt surface yields a significant change in vibration properties between tight and loose bolt conditions and provides an appropriate system for bolt loosening detection. It is also observed that the difference in frequency responses in tight and loose bolt conditions is small in the low frequency region and becomes more distinct in the high frequency region. The bolt loosening can be identified by introducing a damage index by statistical evaluation of the frequency response data using the Recognition-Taguchi (RT) method. A comparison can be made between damage index values obtained from the frequency responses with respect to the two points of irradiation further demonstrating the above observation.