PSI - Issue 44

Andrea Meoni et al. / Procedia Structural Integrity 44 (2023) 1632–1639 Andrea Meoni et al. / Structural Integrity Procedia 00 (2022) 000–000

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3.2. Sensing tests on beam samples

Electromechanical tests were performed on plain and doped beam samples to investigate their sensitivity to the applied loads. Fig. 2 exemplifies the testing setup. An impulsive compressive load varying from 0.5 kN to 3.0 kN was applied on the middle section of each beam, hence electrical measurements were performed consecutively at three different sections of the beam sample under testing by using a voltage square wave of 6Vpp (peak-to-peak), 1 Hz of frequency, and 50% of duty cycle, s signal input, while recording the current output (Downey et al. 2016). The electrical resistance, ( ) 1 , corresponding to the i -th section of the sample was computed as follows: ( ) 1 = / ( ) 1 (1) where is the applied constant voltage in the positive part of the square wave input signal, and ( ) is the electrical current sampled at the time instant t at the i -th section of the beam sample. Electrical measurements were also performed during the execution of the three-point bending tests described in Section 3.1 to investigate the effectiveness of plain and doped beam samples in detecting crack formation. Specifically, in this case, electrical measurements were performed by using a voltage square wave of 6Vpp (peak-to-peak), 1 Hz of frequency, and 50% of duty cycle, as signal input, while recording simultaneously the voltage drop at the reference resistor, placed in series with each beam sample, and at three consecutive sections of the beam sample itself. Once determined the current flowing through each tested sample, ( ) , the electrical resistance, ( ) 1 , corresponding to the i -th section of the sample was computed as follows: ( ) 1 = ( ) 1 / ( ) (2) where ( ) 1 is the voltage drop measured at the i -th section of the material at the time instant t . 3.3. DIC The DIC stereo-mode monitoring was performed by using the digital 3D image correlation system Q-400 of the Dantec Dynamic. The calibration of the cameras was such to ensure an accuracy of ±10 µ m. An acquisition frequency of 1 Hz was adopted together whit a facet size and grid spacing of 11 × 11 Pixels. The displacement field of the frontal surface of the specimen was acquired during the bending test using the setup of Fig. 1. The displacement acquired during DIC monitoring was associated to the reference system reported in Fig. 3. The 3D crack opening plot against the time and the spatial coordinate x (at y = 0) was achieved by post-processing the displacement component u provided by the DIC cameras. Once the displacement component along the x -axis is obtained, the difference between the component u i of adjacent point x i has provided the crack opening Δ u = u i - u i-1 . The crack opening plots associated with both dropped and plain samples were reported in Section 4.3.

Fig. 2. Setup adopted to test beam samples under compression loads.