## 1 Introduction  
Detecting damage in composite materials is becoming more and more important with the increase in applications for composites. Up to date, there are many nondestructive testing (NDT) methods for evaluating the structural integrity of a target structure [1; 2; 3; 4; 5; 6; 7; 8; 9]. For composites, since bonding of two composite parts with adhesives is the preferred choice over the conventional mechanical fastening approach, detecting debonding is a crucial factor when maintaining a composite structure. Some of the recent studies in this area include effects of sensor debonding failure on mathematical representation of smart composite laminate [10], piezoelectric wafer guided wave-based debonding of carbon fiber reinforced polymer (CFRP) overlay in CFRP-reinforced concrete structures [11], and laser ultrasonic guided waves for detecting debonding of multilayered bonded composites [12]. Although various NDT methods are available for detecting damage, most of the methods require well-trained experts or heavy equipment to perform the procedure. For this reason, a low-cost, portable NDT system that is easy to use can be an important item in the field of NDT. Moreover, with advance in technology, efforts have been made by combining NDT with IoT (Internet of Things) technology, and a well-summarized work in this area can be found in [13]. The electromechanical impedance (EMI) technique is one of the NDT techniques that use a single piezoelectric (PZT) transducer to act as both actuator and sensor. The technique involves measuring the impedance below 500 kHz and monitoring the changes in the signature for damage identification. The one-dimensional equation first introduced by [PERSON] et al. [14] shows that the electrical impedance of the PZT transducer is directly related to the mechanical impedance of the host structure as shown below. Here, the electrical admittance, \(Y(\omega)\), is a combined function of the mechanical impedance of the host structure, \(Z_{s}(\omega)\), and the PZT transducer, \(Z_{a}(\omega)\), respectively. This proves that any change in the host structure can be monitored by measuring \(Y(\omega)\). Other variables in the equation, \(I\), \(V\), \(\omega\), \(a\), \(\varepsilon_{33}^{T}\), \(\delta\), \(d_{3x}\), \(\overline{Y}_{xx}^{E}\), are the PZT output current, PZT input voltage, input frequency, geometric constant, dielectric constant, loss tangent, piezoelectric constant, and Young's modulus, respectively. \[Y(\omega)=i\omega\omega\left(\varepsilon_{33}^{T}(1-i\delta)-\frac{Z_{s}( \omega)}{Z_{s}(\omega)+Z_{a}(\omega)}d_{3x}^{2}\overline{Y}_{xx}^{E}\right) \tag{1}\]  
Currently, there are three ways of conducting the EMI technique. The most-used one is by using an impedance analyzer (e.g., Agilent 4924a), which can measure impedance over 1 MHz depending on specification. However, such equipment can be very costly and weigh over 10 kg making it impractical for field use. Another way of conducting the EMI technique is by combining a function generator with an oscilloscope and using a simple circuit proposed by [PERSON] et al. [15]. Using this approach, one can conduct the EMI technique for a cost less than 25% compared to the conventional approach, and various authors have tested the reliability of this low-cost version. The cheapest way of conducting the EMI technique is by using the AD5933 evaluation board commercialized by Analog Devices Co. The small size of the device gives the possibility of creating a portable system for conducting the EMI technique. The cost of the AD5933 evaluation board can be as low as under 2% of the cost of the conventional impedance analyzer, which makes the EMI technique a very cheap technique to perform. However, one of the downsides of the device is that it can only measure impedance up to 100 kHz.