Proof-of-concept study of high-order sideband for bolt loosening detection using vibroacoustic modulation method

The significance of continuous loosening detection for bolted joints in engineering structures cannot be overemphasized. Vibroacoustic modulation (VAM) is a popular loosening detection method exploiting nonlinear features. Existing left and right sidebands can only reflect partial contact nonlinearity caused by loose bolted joint. In this paper, we proposed the concept of high-order sideband and established the theoretical model of high-order sideband for the first time. Multi-order sidebands were regarded to contain more information of contact nonlinearity and a novel loosening index (HSLI) integrating the first four high-order sidebands was developed. The VAM experiment was conducted to clearly display multi-order sidebands in the response spectrum for the concept validation and the good universality of high-order sideband was also proved by changing the material and roughness of clamped plates. Moreover, the experiment also found that the HSLI decreased almost linearly with the increase of tightening torques, demonstrating the excellent loosening detection performance compared with the existing nonlinear and linear loosening indexes. Taking a step further, the effects of input voltages of high-frequency and high-frequency excitations on high-order sidebands were discussed in the experiment. The proposed high-order sideband and nonlinear loosening index provide a reliable method of continuous evaluation of bolt loosening, as well as other contact-type damage, such as crack, debonding and bond-slip.