Active sensing and characterization of nonlinear damages by modulated phases and envelopes of scattered waves

Abstract

In this paper, a sensing methodology is developed and experimentally investigated to detect and characterize damages, which are essentially accompanied by changes in the micro/macroscopic condition of surface contact. The proposed technique is developed mainly for early detection of loose bolted joints, but also may be applicable to kissing bonds in adhesive joints and breathing cracks under the operational condition. The presented sensing system consists of PZT patches attached on the structural surface, one of which acts as a transmitter of high frequency harmonic wave. The incident harmonic wave is scattered by the contact surfaces which potentially involve damages, and received by the other patches. When the structure is subjected to the operational or ambient load at low frequencies, it vibrates, and the inherent damages may introduce a nonlinear effect to the vibro-acoustic dynamics that induces an interaction between the low frequency structural vibration and the high frequency transmitted wave. This nonlinearity is observed as the amplitude and phase modulation of the received wave due to the changes in the scattering characteristics synchronous with the structural vibration. By investigating the relationship between the modulations and the structural vibration, the nonlinear characteristics of the damages can be specified. Experiments using a beam with a bolted joint are conducted for illustrative purpose.