Temperature Compensation Based on Hilbert Transform and Instantaneous Phase for Lamb Waves-Based SHM Systems of Aircraft Structures

Abstract

Variation of environmental conditions, e.g. temperature, influence wave propagation and therefore can create serious issues limiting application of guided waves (GW) in SHM systems. Temperature variations cause a change of wave velocity, which can be observed as a signal stretching. Therefore, although no damage occurs, the signal captured during the monitoring process, does not match the baseline signal. This may lead to frequent false alarms if the alarm level is low or to disregarding damage if the alarm level is too high. In this paper a novel method for compensation of the temperature influence on Lamb wave snapshots is presented. The first step of the technique relies on the calculation of signal’s instantaneous phase using Hilbert transform. Next, phase of the resulting complex analytic signal is shifted to match the phase of the baseline signal. Finally, the inverse Hilbert transform is applied to the result, which leads to aligned of the instantaneous phase of both waveforms.