Bolted joint torque monitoring using ultrasonic Lamb wave mixing

Various engineering structures have relied in bolted joint connections due to its easy design and execution. Given the extensive use of bolted joints connections, the necessity of appropriate monitoring becomes important if not essential in many cases, where the smallest defect or under-tightened bolts can compromise the overall performance and safety of the structure. Individual load cells have proven efficient but at relatively high inspection and operational costs. Various techniques have been investigated in the literature. Different to traditional methods, Lamb-wave based methods have the ability to propagate over long distance, and over different types of structural elements. They also posses the ability to inspect areas that could be inaccessible and laborious to inspect. Nonlinear Lamb waves techniques showed potential to outperform linear waves techniques and detect damages at earlier stage. This paper investigates the use of nonlinear Lamb wave mixing, which consists of two Lamb waves at different central frequencies and demonstrate its capability in generating a third wave as a result of interaction with bolted joint in plates. This study focuses on using mixed-frequency response for assessing the condition of a bolted joint.