# Delamination Assessment in Composite Laminates through Local Impulse Excitation Technique (IET)
## 2 Materials and Methods
### Local-IET Setup and Test  
The local-IET adopts a specific device which clamps the boundaries of the region of interest through a vacuum and confines the mechanical vibrations in the region comprised within the device. The vibrational response is thus only a function of the material properties of the investigated region, which strongly enhances the sensitivity of the vibrational test in the presence of local damage. The device has a frame shape with an external seal and an internal seal delimiting the zone vacuumed through a pump. The vibrational response of the region comprised within the internal seal is detected. At the end of the test, a shut-off valve facilitates the removal of the clamped plate. Figure 2 shows the four devices adopted in this work. The devices are characterized by different dimensions of the investigated region, namely 50 \(\times\) 50, 75 \(\times\) 75, 100 \(\times\) 100 and 150 \(\times\) 150 mm. The first three devices are made of 3D-printed polyethylene terephthalate, while the 150 \(\times\) 150 mm device is made of an aluminum alloy and produced by CNC machining, as the dimensions are incompatible with the 3D-printer. No influence of the device material can be expected with the laminates being in contact with the rubber seals. The devices are fixed to a working table, that is, one side of the composite plate is clamped with the device and, on the opposite side, the vibrational response is acquired. However, the setup can be easily converted to adopt the technique on industrial components, where usually only one side of the component can be accessed for non-destructive investigation. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Property [Unit]** & **Unidirectional** & **Plain Weave** & **Twill 2 \(\times\) 2** & **Twill 2 \(\times\) 2** & **Resin Infused** \\
**Carbon Fiber** & **Carbon Fiber** & **Pre-Preg Carbon** & **Pre-Preg Glass** & **Twill 2 \(\times\) 2 Glass** \\
**Plate—MAT@1** & **Plate—MAT@2** & **Fiber—MAT@3** & **Fiber—MAT@4** & **Fiber—MAT@5** \\ \hline Density [g/mm\({}^{3}\)] & 1.57 & 1.46 & 1.43 & 1.82 & 1.65 \\ Thickness [mm] & 0.96 & 0.96 & 1.8 & 2.1 & 1.2 \\ E\({}_{1}\) [GPa] & 153.2 & 46.5 & 51.1 & 24.0 & 19.0 \\ E\({}_{2}\) [GPa] & 9.2 & 46.5 & 51.1 & 24.0 & 18.6 \\ G\({}_{12}\) [GPa] & 5.2 & 4.0 & 3.9 & 5.2 & 3.6 \\ V\({}_{12}\) [-] & 0.35 & 0.08 & 0.08 & 0.2 & 0.17 \\ Tsai modulus & 174.0 & 101.6 & 110.7 & 60.4 & 45.9 \\
[GPa] & 12.83 & 7.49 & 54.92 & 47.28 & 6.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material properties of = investigated composite plates assessed through IET tests. According to [20], the confinement of mechanical vibrations to a subregion of the component enhances the sensitivity in the presence of local damage, whose severity has been characterized in [20] in terms of the degradation of the first resonant frequency. In order to replicate the local-IET test in the FE environment, the vibrational response of the first mode, i.e., the first resonant frequency and the modal displacement of the first mode, was investigated on the undamaged plates. A KEYENCE LK-H022K laser system was employed, which ensures a precise measurement of the out-of-plane displacement. From the displacement data, the first resonance was determined through a Fast Fourier Transform of the signal. Figure 3 shows the experimental setup. Figure 2: Clamping devices of the local-IET: (**a**) 50 \(\times\) 50 mm; (**b**) 75 \(\times\) 75 mm; (**c**) 100 \(\times\) 100 mm; (**d**) 150 \(\times\) 150 mm. The impulse was provided through a Maul-Theet Impact HaC-3 modal hammer, which guarantees the same input energy, thus allowing a reliable acquisition of the modal displacement to be obtained. For the determination of the first resonant frequency, a dedicated MATLAB script was developed, which first filters the acquired signal with a high-pass filter to eliminate the spurious vibrations induced by the vacuum pump, and then transforms the discrete signal in the frequency domain through Fast Fourier Transform.