A nonlinear ultrasonic wave mixing method for looseness detection of bolted joints

Bolted joints are widely used in various machines and engineering structures. The tightening state of bolts directly affects the reliability and safety of bolted joints. Therefore, the bolt looseness detection is very important to ensure the reliability of bolted joints. In this paper, a novel nonlinear ultrasonic wave mixing method is proposed for looseness detection of bolted joints. The basic theory of the nonlinear ultrasonic wave mixing method for detecting the bolt looseness is established. Based on the detection theory, a nonlinear modulation index is defined to quantify the level of bolt looseness. An experimental setup is built to implement the nonlinear ultrasonic wave mixing detection method on bolted joints. Experiments are conducted on two bolted joints with different materials and sizes. The influences of excitation frequencies and voltages of two ultrasonic signals on the detection performance are investigated experimentally. A method is proposed for the selection of appropriate excitation frequencies to improve the detection performance. The experimental results demonstrate that the proposed method can effectively detect bolt looseness and it has good detection sensitivity to the early bolt loosening.