Bolt looseness detection based on ultrasonic wavefield energy analysis using an Nd:YAG pulsed laser scanning system

Summary This paper proposes a bolt looseness detection method based on the ultrasonic wavefield energy in the Lamb wave reflected from a target bolt. The full wavefield inside the area of the specimen containing the target bolt was first visualized. The ultrasonic wave propagation imaging (UWPI) approach was used for visualizing the interaction of the wavefield with the elements on the specimen such as the bolts and the boundaries of the specimen. The interaction between the ultrasonic wave and the airgap between the bolt head and the plate was interpreted based on the microcontact theory. This theory, when applied to the interface of two contacting surfaces such as those joined by a bolt, generally states that by increasing the fastening torque, the true contact area will increase and the wave will propagate across the interface with less energy loss. To apply the theory, the reflected wave from the target bolt was isolated from the incident wave and was further assessed to extract a feature indicating the looseness or tightness. The variation of the energy in the wave reflected from the target bolt with varying magnitudes of torque was determined by performing a continuous wavelet transform‐based energy computation. The reflected wave energies of the different torque values were first compared by considering single time instants starting with the moment the wave was first observed in the UWPI reflecting from the target bolt. Then, energy accumulation was used to obtain the overall energy differences that resulted solely from the torque differences.