## 3 Impact Damage Setup and Ultrasonic C-Scan  
A controlled laboratory experiment is performed to create point impact damage on the given sample plate as shown in Figure 3. The impact tests are carried out using an Imatek IM10 ITS drop tower with a capacity of 4000 Joules according to the ISO standard [ISO standard 6603-2, 2000]. The test is carried out using a falling mass in the form of a trolley with a mass of 8 kg fitted with a hemispherical impactor with a diameter of 20 mm. The composite plate is fixed inside a metallic part with an open area for impact. An anti-bouncing device is implemented to avoid a second shock, which could further damage the structure at a random energy. In order to obtain the desired energy, the drop height is changed. The specimen is placed in a compressed air clamping system on a circular support with an internal diameter of 40 mm and an external diameter of 60 mm in order to hold the specimen during the impact. The quantities measured, using sensors, are the impact force as well as the displacement of the falling mass. Then, the energies, initial (E\({}_{i}\)), absorbed (E\({}_{abs}\)) and restored (E\({}_{r}\)), are post-processed thanks to the displacement-time and effort-displacement graphs. Preliminary impact tests were carried out on similar composite plates in order to create a non-visible damage. We have found that 40 J of impact energy is sufficient to damage the plate without penetrating through, where damage is not visible on the impact side and barely visible on the other side.  
Figure 2: Optimized phase velocity dispersion curves corresponding to 6.2 mm-thick composite plate.  
Imversion c-scan ultrasonic images are obtained using 5 MHz longitudinal pulses propagating through the composite plate. The ultrasonic c-scan image is obtained using a pulse-echo configuration, where gates 1 and 3 correspond to the front and back wall echoes as presented in Figure 4a. The c-scan image shown in Figure 4b is performed on the basis of the change in the back-wall amplitude of the ultrasonic wave, which drops in the presence of damage. Indeed, the defect area appearing in blue color indicates that the 40-joules point-impact test creates a damaged area of \(\sim\)30 mm diameter around the impact point. In the literature, the conical-shape cavity has been found in different contributions during the impact tests performed on composites. Indeed, the overall damage pattern through the thickness in composite materials follows a conical shape as found by [35; 36; 37]. The size of the defect region as well as the involved damage mechanisms depend on the used impactors (hemispherical, conical, etc.) and on the composite characteristics as well. The use of hemispherical impactors was found to produce larger delamination areas compared to a conical impactor in laminates [23]. For instance, [PERSON] et al. [38] showed that the different impactor shapes produced different damage mechanisms, which directly affect the energy absorption characteristics of the material. This result was also confirmed by [PERSON] et al. [39]. Using a metallographic microscope, [PERSON] et al. found that the structure of the fibers affects the delamination pattern and size [40]. Since the composite studied is transverse-isotropic, we can reasonably suppose, on the basis of the presented ultrasonic c-scan imaging as well, that the structure of the fibers in our case affect the delamination in an isotropic way. The 3D finite elements (3D-FE) model will then reasonably consider a damage area with a conical geometry and the same diameter, the value of which is equal to 30 mm according to the c-scan image.  
Figure 3: Experiment setup for controlled impact damage of energy equal to 40 Joule.