# Delamination Assessment in Composite Laminates through Local Impulse Excitation Technique (IET)
## 5 Conclusions  
In this work, an innovative nondestructive methodology (local-IET) which allows for damage detection and quantification in composite plates was developed. The technique is based on the Impulse Excitation Technique (IET) and assesses the vibrational response of a local region of a component by clamping its boundaries through a specifically developed  
Figure 12: A map of the \(\frac{\beta_{dim}}{\beta_{random}}\) ratio for the delamination size of 50 mm and an impulse provided at 10 mm from the center: (**a**) delamination in the center; (**b**) delamination at 10 mm from the center; (**c**) delamination at 25 mm from the center; (**d**) delamination at 37 mm from the center of the device. device which exploits a vacuum to reversibly apply the constraint. From the vibrational response of the inspected region, the local material properties can be evaluated, and the technique was therefore referred to as local-IET. In this work, we aimed to identify the Finite Element (FE) model of the nondestructive local-IET test to predict the first resonant frequency and the modal shape response of the composite plates subjected to the nondestructive test. By investigating four different composite plates with devices with dimensions of 50 \(\times\) 50 mm, 75 \(\times\) 75 mm, 100 \(\times\) 100 mm and 150 \(\times\) 150 mm, it was shown that the boundary conditions approximate the condition of perfect clamping, being between the simply supporting and the clamping cases. By investigating the modal displacement with a laser displacement sensor, we found that as the flexural stiffness increases or the device size decreases, the plate decreasingly vibrates over time, which, in turn, affects the computation of the resonances and of the Frequency Response Functions (FRFs). Accordingly, the inspected region size cannot be indefinitely reduced, and a compromise with the flexural stiffness is necessary. The constraint provided by the device was modeled in the FE environment with translational springs and with rotational springs at the extremities. The comparison with the experimental measurements of the modal displacement on the undamaged plates showed that the rotational springs model better captures the vibrational response. The FE model was then validated on an eight-layer glass fiber plate with a circular delamination with a 50 mm diameter. The experimental investigations with a 100 \(\times\) 100 mm device showed that the local-IET can successfully assess the presence of delamination, as the second-order nonlinear coefficient \(\beta\), defined as the ratio between the power amplitude of the second resonance frequency and the squared power amplitude of the first resonance frequency doubles its value between the undamaged and the delaminated conditions. As a result of the nonlinearities due to the presence of damage, the second mode contribution increases. An excellent agreement was observed between the experimental and numerical data, proving the effectiveness of the FE model. Finally, by exploiting the validated FE model, a sensitivity analysis on the detection of delamination was carried out by varying the size of the delamination, which was assumed to be of a circular shape, its position within the device and the position of the impulse. The sensitivity analysis was addressed for an industrial composite material, indicated by _Leonardo S.p.A_, with the 150 \(\times\) 150 mm device. The results show that the local-IET has a good sensitivity in the presence of delamination, with a 5% increase in the \(\beta\) even for a delamination size of 15 mm, i.e., 1/10 of the device. Also, we found the maximum sensitivity of the local-IET for the delamination positioned between the maxima of the first and second modes' modal displacements, although not with a remarkable difference with respect to the other investigated positions. In order to reliably identify the presence of delamination with the local-IET, we suggest mapping the \(\beta\) factor within the whole inspected region.