## 1 Introduction  
Mechanical strain measurement and monitoring are vital in several areas of engineering [1, 2, 3]. Ultrasonic waves can be used for monitoring strain by measuring the time-shift of received signals [4] since the propagation speed of an ultrasonic wave is proportional to the stress level of the medium, according to the acoustoelastic theory [5, 6]. Ultrasonic guided waves are generally dispersive and multimodal but are widely used in the non-destructive evaluation and structural health monitoring since they can propagate long distances and inspect a large area [7]. The influence of strain on ultrasonic guided waves is, however, more complex than on bulk waves: each guided wave mode presents different sensitivity to strain, which also depends on the frequency [8, 9, 10, 11, 12, 13]. That is, at each frequency, the time-shift that each mode may experience is different. Experimental strain measurement with ultrasonic guided waves is usually performed by observing the time-shift of a single guided wave mode [8, 14, 15, 16]. This, however, requires very accurate time-shift measuring and single-mode excitation, which may not be straightforwardly achievable in different types of structure. A wideband excitation in a plate can, in principle, generate several guided wave modes and thus identification of individual time-shifts can be complicated. If the medium presents features that allow scattering [17] and mode conversion [18, 19] the received signals can be even more complex. The time-reversal technique can be used to compensate this complex behavior creating a focused signal. If the received signal, which may be composed of several modes and scattered waves, is reversed in time and retransmitted,then propagation related phenomena, such as scattering and dispersion, are compensated producing a sharp peak in the next received signal [20]. Several researchers used the time-reversal technique on guided waves in order to identify the presence of defects in plates [21; 22; 23; 24; 25; 26]. In previous work, the time-reversal signal processing, either physically or numerically implemented via cross-correlation, was applied to evaluate the variation of the longitudinal tensile stress in plates [27]. The main principle of the technique lies in exploiting the complex propagating behavior, in which several wave modes distributed over a wide frequency range are excited. These several components potentially present different sensitivity to strain, therefore mismatching the system transfer function due to the presence of stress, which reduces the focusing capability of the process and, in turn, decreases the peak of the final signal, being therefore easily observable. A similar procedure was applied to concrete pillars [28] showing that different time-intervals of the received signal present different sensitivity to strain. [PERSON] et al. [29] excited several guided wave modes in a rod and pipe subjected to axial and bending stress, respectively. It has been observed that, due to the different speed variation for the several modes presented in the received signal, stress evaluation through speed measurements was indeed a complex task. They adopted a principal component analysis in order to monitor stress. When several wideband modes propagate, the received signal may be quite complex to analyze since the individual signals from each mode may be distributed over time and with different frequency content. Thus, identification of individual wave packets in either time or frequency domains is usually not possible. However, they can be better distinguished through time-frequency representations [30; 31; 32; 33] such as the short-time Fourier transform (_STFT_). Time-frequency analysis was applied to ultrasonic guided waves before, to increase the detectability of defects in plates [34; 35] and pipes [36; 37]. In this paper, we modify the original cross-correlation strain monitoring procedure [27] aiming to investigate whether the sensitivity to strain could be increased.