Strain sensitivity enhancement of ultrasonic waves in plates using phase filter

Ultrasonic guided waves can be used to measure mechanical strain in plates due to the stress-induced velocity change. Strain monitoring is usually performed by measuring the time-of-flight shift. However, interpretation of results can be difficult due to wave mixing, mainly when several dispersive modes propagate, or in the presence of reflections. In these cases, the time-reversal focusing technique can be used to monitor the strain level, by observing the peak of the focused time-reversal signal, which changes proportionally to the strain level. However, not all components of the spectrum contribute with the same sensitivity to strain changes. In this paper, we developed a signal filtering procedure based on the phase of the Fourier spectrum that increases the time-reversal strain sensitivity. The time-reversal process is modified by using a new signal as reference which is synthesized relying on prior knowledge of the impulse response at some non-null strain level. The technique was evaluated with different pairs of transducers in an aluminium plate, effectively producing more strain-sensitive signals. However, high strain-sensitive signal presents poor energy concentration which, in turn, can be difficult to detect. The technique can be adapted to provide strain-robust signals.