Mechanical Strain Sensing by Broadband Time Reversal in Plates

In a previous work, the ultrasonic measurement of longitudinal strain in a plate using the time-reversal technique was proved. One drawback of this measurement is the low sensitivity of the signal against changes in strain. This problem can be solved using inverse filter signal processing. This technique increases the sensitivity but also reduces the energy of the signal and, consequently, the signal-to-noise ratio (SNR). Thus, a physical solution is presented to improve the sensitivity of the system. Additionally, the one-bit time reversal is introduced to simplify the hardware used in this technique. The strain sensing system is composed of a pair of piezocomposite transducers bonded to the surface of the tested plate and used to generate and sense the ultrasonic waves guided through the specimen. The use of time reversal provides phase compensation for dispersion and edge reflections in the propagation of the guided waves in the plate, allowing time recompression of the waves. The measurement principle is based on the detection of changes in the amplitude and time-of-flight of the focused signal when the plate is subjected to longitudinal strains. System sensitivity is improved using 2-2 piezocomposite transducers designed to operate between 0.2 and 3.0 MHz. In the signal processing, the one-bit time reversal is compared with the conventional time reversal in 12-bit resolution. A figure of merit is introduced to evaluate the influence of the transfer function on strain sensitivity. This figure of merit relates the energy concentrated at the time-reversal focus with the total energy of the signal. This value represents the ability of the time-reversal process to recompress the signal at the focus. The experiments were conducted by applying strains up to 150 μm/m. The results show a linear response in the change of the focus amplitude. The sensitivity depends on the transducers and it can be related to the proposed figure of merit. The focus quality is kept when one-bit time reversal is used, showing to be also feasible for the measuring technique. All the results agreed with the numerical time-reversal implementation.