Strain sensitivity model for guided waves in plates using the time-reversal technique

The use of the time-reversal technique to detect variations in the external applied traction in a strip of aluminum plate is presented. Experiments have been performed in transmission-reception mode, using two fixed ultrasonic transducers near the ends of the plate. Two different parameters of the time-reversed signal are used for monitoring the strain state, the peak amplitude and the focusing time. The inverse filter technique was used in the transmitted signal to equalize the amplitude spectrum, enhancing the sensitivity in the focus amplitude by 5 times. The external traction sensitivity was experimentally verified by using an aluminum plate (800 × 100 × 3 mm) and two 5-MHz ultrasonic transducers spaced 700 mm apart. At the maximum strain state (180 μstrain), the peak value was reduced by about 10% in the conventional process and by 50% using the inverse filter. To evaluate the effects of the strain in the time-reversal signal, a theoretical model was constructed. This model is successful in predicting the changes in the group delay and, consequently, the focusing time using a linear equation. This relationship can also be used to determine the strain level quantitatively. The experimental results show that the time-reversal signal technique can be used in practical monitoring of changes at the strain level in mechanical structures.