## 3 Results and Discussion
This suggests that neural line defects with local hardening at the edge increase bending stiffness and also contribute to increased peak frequency due to the local reinforcement that can be created when defect edges touch. Additionally, a minor shift in frequency is seen at 5, 7, and 10 mm defects. Since the thickness of defects remains constant during bending, increasing their size has a limited impact on the vibration peak frequency. In addition, the error bars overlap between 3 and 10 mm flaws, indicating that increasing the defect size beyond 3 mm has little effect on peak frequency. The damping effect on the defect size with the vibration sensor is shown in Figure 14. As demonstrated, as the defect size rises from no defect to 5 mm of defect, the damping decreases. However, a small rise is seen from 5 to 7 mm. As observed, when the defect size increases from a fully dense state (no defect) to 5 mm, the damping decreases. This decline suggests that the position of the defect on the neutral bending line and minimal defect volume change diminish damping. For defect sizes like 7 and 10 mm, the presence of delamination (Figure 7d,e) generates minor internal friction, resulting in minimal energy loss, which moderately improves damping. Similarly to peak frequency, damping varies very little beyond 3 mm of defect. Damping values are therefore directly influenced by the observed frequency changes (see Figure 13), with the damping having been calculated using Equation (1). Hence, the vibration detector is highly sensitive to structural changes, such as delamination and defects positioned at the neutral line, due to a distinct variation in modal properties. Figure 15 presents the mean FFT curves for each defect size. As presented in Figure 15, Peak 2 for the vibration detector is also observed at a consistent frequency of 73.27 Hz across the no-defect sample and all defect sizes. As stated above, Peak 2 can be attributed to the defect location on the neutral bending line, which justifies the lack of frequency variation. However, a similar decreasing trend in the amplitude of Peak 2 is observed as the defect size grows from no defect to 7 mm followed by a slight increase to 56.02 mV for a 10 mm defect. In this case, the central fault causes a dispersion of vibratory energy, which reduces the energy transmitted to the vibration sensor, resulting in lower amplitude. A slight rise in amplitude from 7 to 10 mm may be linked to delamination, which can lead to  
Figure 13: Frequency peak as function of defect size. more efficient vibration of the part, resulting in a slight gain in amplitude. In addition, the amplitude values obtained with the vibration sensor are greater than those obtained with the acoustic sensor. This can be elucidated by the fact that the vibration sensor measures displacement directly and this detector is more sensitive to small movements than an acoustic sensor. Since the vibration sensor follows the local movement of the structure, there is a richer presence of numbers of vibrational harmonics in 0 to 1000 HZ than the acoustic sensor, which filters more. The sum of peak frequencies and integrated amplitude of these two peaks offers insights into the influence of the defect size on the FFT response, as summarized in Table 9. In addition, for the samples free of defects, the values for Peak 1 and Peak 2 readings show a lower combined peak frequency of 1463.9 Hz and a greater integrated amplitude of 739.954 \(\times\) 10\({}^{3}\) Hz.mV. This is due to a coherent vibration structure with no modal loss. In addition, the 3 mm defect samples display an enhanced sum peak frequency of 1507.47 Hz. For a 3 mm defect, the high sum peak frequencies are linked to localized reinforcement, which enhances the part's rigidity. Nevertheless, samples with a 10 mm defect exhibit the lesser integrated amplitude of 589.041 \(\times\) 10\({}^{3}\) Hz.mV. In this regard, delamination creates local decoupling, introducing a loss of vibratory energy in the damaged zone and leading to low amplitude.