*4.3. The Chirp-Through-Transmission Ultrasound Technique*

As already described, the ultrasound wave transmission-reception techniques with transducers not directly coupled to the material are pulse-echo, pitch-catch and through-transmission [41]. In the pulse-echo technique, a single transducer receives and transmits the ultrasound waves. In the pitch-catch and through-transmission techniques, the transmitter and receiver are different transducers [62]. The method proposed in the present study is an alternative to the through-transmission technique, as it also uses two piezoelectric transducers at the ends of the material under study. However, a chirp signal is injected into the emitter transducer, which results in an acoustic wave that propagates through the material until it is collected by the receiver transducer. The signal is collected by a data acquisition system and processed using digital signal processing techniques in order to diagnose the condition of the structure.

The proposed method is shown in Figure 2, which shows two piezoelectric diaphragms attached at the ends of the material under study, where the right piezoelectric diaphragm was configured as emitter and the left diaphragm as ultrasonic wave receiver. The emitter is excited by a chirp signal of certain amplitude and frequency. Due to the piezoelectric effect of this transducer when excited with the chirp signal, ultrasonic bulk waves are generated and transmitted through the material under study, as illustrated in Figure 2a. The waves that propagate through the material suffer various effects, such as frequency variations, dispersion, reflection, refraction, compression, rarefaction and energy loss. These effects modify the emitted signal, creating a signature of the material under study. The frequency of the waves changes longitudinally (continuous line) and transverse (dashed line) as they propagate through the structure. Figure 2a shows the application of the technique in a healthy structure (without damage), together with the received signal and its respective frequency spectrum.

In the case of a damaged structure, as shown in Figure 2b, where the points and triangles represent, for example, cracks and material removal, the changes in the waves are different from those observed in the healthy structure and therefore its condition can be diagnosed.

Thus, the signal collected from the damaged material will differ from the signal collected from the healthy material, since changes in structure affect the propagation of ultrasound waves. After the signal acquisition, it is possible to identify the structural change of the material by comparing the signals of the healthy and damaged material. However, this identification involves digital signal processing techniques, such as spectral analysis, frequency band selection, RMS and Counts statistics, among others. Figure 2b shows the application of the technique in a damaged structure, together with the received signal and its respective frequency spectrum, both with different characteristics with respect to the healthy structure.

Thus, the proposed method is based on the changes that ultrasound waves undergo when propagating through the material under study, with the difference that this method does not use the traditional parameters commonly used in ultrasound techniques to diagnose the condition of the structure. Instead, the proposed method employs the processing of the receiver transducer signals resulting from the ultrasound waves generated by the emitting transducer. It is worth mentioning that the prevalent waves in this method are bulk waves, which are expected to allow the monitoring of defects such as microstructure alteration or material removal.

**Figure 2.** Chirp-Through-Transmission Technique: (**a**) health structure; (**b**) damaged structure.

The main features of the proposed method are: (1) The use of a chirp signal at the emitter transducer input instead of pulses; (2) Do not depend on the reflection of the emitted wave; (3) Not limited to specific wavelengths to identify damage; (4) Does not require the use of traditional parameters of ultrasound techniques for damage detection, such as wavelength, propagation speed and time of flight; (5) It is possible to identify the frequency bands in the received wave that best characterize the structural damage; (6) Damage detection can be performed using traditional statistics such as RMS and Counts, enabling the study of many other statistics, indexes and parameters used in process monitoring and SHM; (7) Employs low-cost piezoelectric transducers; (8) It consists of a simple experimental setup for both transducer fastening and data acquisition and signal processing system.

It is worth mentioning that in the proposed method, the changes that the ultrasound waves suffer when propagating through the structure are analyzed through digital signal processing, being necessary the comparison of two signatures, the first of the healthy structure, called baseline and the second one of the altered or damaged structure. Thus, the present method can also be classified as a structural health monitoring method.

According to Giurgiutiu et al. [63,64], the electromechanical impedance (EMI) technique considers the direct and inverse electromechanical properties of piezoelectric materials. In addition, according to Baptista et al. [65], the basic concept of this technique is to monitor structural integrity by exciting a piezoelectric transducer attached to the structure at an appropriate frequency range. Due to the piezoelectric effect, there is a relationship between the mechanical impedance of the structure (related to its integrity) and the electrical impedance of the piezoelectric transducer, obtained by the relationship between the excitation and detection signals. The proposed method, when compared to the EMI technique, does not require the measurement of the structure impedance to detect damage but the variations of the received ultrasonic waves. In addition, common digital signal processing statistics are used to detect differences between the baseline and damage signatures, the technique is not limited to the damage indices commonly used in the EMI technique [66]. Thus, the proposed technique can be expanded to the application of new indices. Finally, traditional EMI techniques use a single sensor to excite and detect the impedance variations of the structure, while in the presented method two transducers are used, analyzing only the received signal.
