PSI - Issue 68

J.M. Parente et al. / Procedia Structural Integrity 68 (2025) 160–165

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J.M. Parente et al. / Structural Integrity Procedia 00 (2025) 000–000

The piezoelectric results obtained for 100 cycles are shown in Fig. 3 and are representative of the electrodes positioned at the top (Fig. 3a) and bottom (Fig. 3b), respectively. The results demonstrate a corresponding electrical response to the application of a cyclic mechanical load, exhibiting a decrease in electrical resistance with an increase in the applied load. This is attributed to the placement of the electrodes on the top of the sample. When the electrodes are situated at the bottom of the sample, the electrical resistance response is reversed as a consequence of the increased distance between the electrodes due to tensile strain. A comparison of the overall electrical resistance response between the electrodes placed on the top or on the bottom reveals that the former configuration yields a more discernible signal. The results of the average gauge factor are shown in Table 2, where it can be seen that the sample with 0.25 wt.% GNP and 0.5 wt.% CNT is the one with the highest gauge factor, both for top and bottom electrode placement, with a value between 3.56 and 5.22 and 1.18 and 2.82, respectively. The use of a single nano-reinforcement led to the lowest values. Furthermore, the gauge factor values throughout the cyclic test increase with the increase in the number of cycles. Therefore, the formation of a more extensive and interconnected conductive network, due to the different sizes and geometries of the GNPs and CNTs, enhances these phenomena (Bhandari et al., 2021; Wang et al., 2020; Wang et al., 2022).

Fig. 3. Representative relative variation of electrical resistance (blue) with the variation in flexural cyclic load (red): a) electrodes on top surface; b) electrodes on the bottom surface.

Table 2. Variation of the gauge factor with the number of cycles.

Electrodes at the top surface

Electrodes at the bottom surface

Type of laminate

Cycles

Cycles

% GNP

% CNT

0-5

45-50

95-100

0-5

45-50

95-100

0.25

0.5

3.56 (0.52) 0.66 (0.02) 0.46 (0.08) 0.65 (0.04)

2.92 (0.54) 0.93 (0.01) 1.18 (0.05) 0.78 (0.06)

5.22 (0.53) 1.92 (0.01) 1.87 (0.05) 1.65 (0.05)

1.18 (0.25) 0.91 (0.01) 0.86 (0.03) 0.85 (0.05)

2.43 (0.13) 1.03 (0.04) 0.85 (0.06) 0.78 (0.04)

2.82 (0.43) 1.15 (0.02) 0.93 (0.04) 0.95 (0.07)

0.375

0.375

0.5

0.25

0

0.5

Fig. 4 shows the surface resistivity for different GNP and CNT weight contents, where is notorious that the sample containing 0.25 wt.% of GNP and 0.5 wt.% of CNT has the lowest resistance, with a value of approximately 6 ´ 10⁴ Ω/sq and 7.74 ´ 10⁴ Ω/sq, respectively, for electrodes positioned on the top and bottom, respectively. These values were 33.13% and 26.99% lower than those of the sample containing only one type of nano-reinforcement. This is attributed to the formation of a more compact conductive path, which is provided by the differing geometries of the GNP and CNT leads, resulting in a reduction in electrical resistance value. This phenomenon was also observed by Bhandari et al. (2021). Finally, Fig. 5 shows representative mechanical and piezoresistive results for nanocomposites with increasing bending strength (Fig. 5a) and with increasing stabilization time between cycles (Fig. 5b). From Fig. 5a, it is noticed that the increase in the mechanical load from the first to the second cycle is accompanied by an increase in electrical