Strain Sensitivity Enhancement of Broadband Ultrasonic Signals in Plates Using Spectral Phase Filtering

Abstract

The focused signal obtained by the time-reversal or the cross-correlation techniques of ultrasonic guided waves in plates changes when the medium is subject to strain, which can be used to monitor the medium strain level. In this paper, the sensitivity to strain of cross-correlated signals is enhanced by a post-processing filtering procedure aiming to preserve only strain-sensitive spectrum components. Two different strategies were adopted, based on the phase of either the Fourier transform or the short-time Fourier transform. Both use prior knowledge of the system impulse response at some strain level. The technique was evaluated in an aluminum plate, effectively providing up to twice higher sensitivity to strain. The sensitivity increase depends on a phase threshold parameter used in the filtering process. Its performance was assessed based on the sensitivity gain, the loss of energy concentration capability, and the value of the foreknown strain. Signals synthesized with the time–frequency representation, through the short-time Fourier transform, provided a better tradeoff between sensitivity gain and loss of energy concentration.

Improved Stress Estimation with Machine Learning and Ultrasonic Guided Waves

“…Usually, one works at low-frequency, before the cut-off frequency of higher-order modes, so there are fewer propagating modes [8]. By contrast, when several guided wave modes propagate simultaneously, which happens due to a broadband excitation, for instance, the received signal is composed by the superposition of the modes' signal which, depending on several factors, such as, the receiver position and excitation signals, can mix in the received signal [32,33]. A broadband signal generates modes that present different stress-induced time shifts, including modes that may be even virtually non-sensitive to stress, which does not contribute to stress monitoring.…”

Special Issue on Ultrasonic Transducers and Related Apparatus and Applications

“…This can be used to monitor the strain level of the structure. Martinho et al [5] proposed the sensitivity to strain enhancement by a post-processing procedure which preserves only the strain-sensitive spectral components. The proposed technique was demonstrated on the aluminum plate, obtaining up to twice higher sensitivity to strain.…”