Strain monitoring in metallic plates using the time reversal focusing technique

This paper presents the use of the time reversal technique to detect variations on the external applied traction in a strip of aluminum plate. Experiments have been performed in transmission-reception mode, using two fixed ultrasonic transducers near the ends of the strip of aluminum plate. Two different parameters of the time reversed signal are used for monitoring the strain state, the peak amplitude and the focusing time. An inverse filter technique was used in the transmitted signal in order to equalize the amplitude spectrum, enhancing the sensitivity in the focus amplitude by five times. The external traction sensitivity was verified using an aluminum plate (800mm×100mm×3mm) and two 5 MHz ultrasonic transducers spaced by 700mm. At the maximum strain state (180µstrain), the peak value was reduced about 10 % in the conventional process and 50 % using the inverse filter. The changes in the group delay and consequently the focusing time was well predicted by a linear equation and it can be also used to monitor the strain level. The experimental results show that the time reversal signal technique can be used in practical monitoring of changes in the strain level in strips of plates.