Development of a mechanical strain sensor based on time reversal of ultrasonic guided waves

The development of a strain sensor based on the time reversal focusing technique is presented. The sensor is composed by a strip of aluminum plate with two ultrasonic piezocomposite transducers bonded at the ends of the plate. The time reversal technique acts as a dispersion compensator of the guided waves propagating in the plate, allowing time recompression of the waves. When the plate is subjected to a longitudinal traction, a time reversal focusing is performed between the transducers in order to detect the change in the focus due to strain. The strain can be evaluated by measuring the change of the amplitude and shift in the time of flight, and comparing them with a reference signal obtained at zero strain state. In order to improve the systems sensitivity, 2-2 piezocomposite transducers designed to operate between 0.2 to 3.0 MHz are used. Experiments are conducted by applying strain up to 150 μ-strain. The results show an increase in sensitivity when compared with the results of the conventional mono-element transducer greater than 200%. Results presented here can be used in the project of stress monitoring transducers and structural health systems.