Passive detection of bolt joint looseness using flow-induced ambient noise

Bolt joints are commonly used in aviation structures. Bolt looseness may pose serious safety risks and its online monitoring is of great importance to structure and air safety. Active guided wave detection methods can accurately identify the tightness status of bolts. However, the excitation of active guided waves requires big and heavy equipment, such as waveform generators and power amplifiers, which are often not allowed due to the lightweight design of aircraft. It is found that random ultrasonic guided waves can be passively excited by the coupling between airflow and airframe, which carries rich structural health information and has great potential for passive online detection of aircraft structure. In this paper, the cross-correlation function between random guided waves measured by two passive receivers is computed to identify the wave propagation paths in a bolt joint structure and the wave energy along each path passing through a bolt, by which the bolt tightness is assessed. Laboratory and wind tunnel experiments show that a broadband random ultrasonic guided wave can be excited due to the interaction of airflow with an airframe structure, from which the proposed method can efficiently identify the looseness of multiple bolts.