PSI - Issue 64

Antonella D’Alessandro et al. / Procedia Structural Integrity 64 (2024) 1160–1167 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Setups and Methods 4.1 Electrical tests on mortar samples

The smart mortars are subjected to electrical tests to evaluate the electrical resistance and capacitance through an ad hoc developed electronic device called “ smart material electrometer ” (SME) designed and fabricated by the authors. The device can generate a biphasic square wave signal of ± 10 V and detect the electrical current response of smart mortars doped with carbon fibers at different concentrations. In this regard, Figure 2(a) depicts the 1 Hz voltage signal that powers the specimen, whose electrical current response is analyzed in two ways. Besides, a Python script searches the steady state electrical current , i.e., the electrical current which is reached at 80% of the current transient, as observed in Figure 2(b). On the other hand, Figure 2(c) shows the area under curves to determine the total electrical charge ( ) involved in each current transient, regardless of whether it is negative or positive. Then, the voltage is divided by the steady-state current to determine the electrical resistance, as shown in Figure 2(d). When the cycle is negative, as well as are both negative, making positive the electrical resistance. Besides, the capacitance is also retrieved by dividing ( ) by , as depicted in Figure 2(e). 4.2 Electromechanical tests on mortar samples In Figure 2(f), the experimental setup for conducting electromechanical tests on the smart mortars is shown. Herein, electrical measurements are performed using two electrodes; simultaneously, the specimens are mechanically loaded (with a force of 4 kN) and unloaded (at 0 N) at intervals of 5 s, respectively. This cyclic compressive load produces changes in electrical resistance (see Figure 2(g)) and capacitance (see Figure 2(h)) as outputs. The fractional change in resistance (FCR) and the fractional change in capacitance (FCC) are thus obtained by subtracting and then dividing the physical quantity (resistance or capacitance) by its value at zero force. 5. Results 5.1 Electrical tests on mortar samples Figure 3(a) unveils how the inclusion of carbon fibers progressively decreases the electrical resistance of the smart mortars. Additionally, the ripple in the signal in samples without carbon fibers (0%) indicates a symmetry loss between the resistance obtained in the positive cycle of the biphasic signal compared to that obtained in the negative cycle. That means the material responds in steady state plenty differently to positive than negative voltages. Based on these observations, the specimens at 0.1% exhibit greater stability with a standard deviation of ±0.4 Ω for a mean value of 26.1 Ω , which depicts a coefficient of variation of the output of 1.5%. In contrast, the lowest stability (12.9%) was observed for specimens having a CMF concentration of 0.25%. Furthermore, a piezo-capacitive response is also observed which increases with the concentration of carbon fillers, as shown in Figure 3(b). In this case, the greatest stability was found for the 0.25% specimens with a standard deviation of ±1.4 from a mean value of 60.3 , which represents a coefficient of variation of 2.3%. The other mortars, with lower CMF concentrations, also exhibit good piezo-capacitive stabilities of 3.5% (plain mortar at 0%) and 2.5% (smart-mortars at 0.025% and 0.1%). In summary, comparing the resistive and capacitive responses, the former provides a slightly better stability over time but only at an optimal doping level. 5.2 Electromechanical tests on mortar samples A key property of smart-mortar is to exhibit piezoresistive (see Figure 4(a)) and piezocapacitive (see Figure 4(b)) responses, depending on its carbon fillers concentration. The highest mean value of fractional change in resistance (FCR = -1.4) occurred in the first cycle for specimens at 0.25%. Although the piezoresistive response shows slightly higher sensitivity than the capacitive response (FCC = -1.3), the capacitive response remains more stable when the samples are subjected to a cyclic load of 4 kN, which is confirmed for all specimens with different concentrations.