## 2 Materials and Measurements  
Two CFRP test specimens were investigated for damage (see Figure 2). The first plate (Figure 2a) measured \(290\times 140\times 2.1\) mm\({}^{3}\) and was manufactured by unidirectional CFRP prepreg according to cross-ply layup [(0/90)2]\({}_{\text{S}}\). When stacking the prepreg, two artificial delaminations were introduced using \(20\times 20\) mm\({}^{2}\) inserts made out of a double layer of 25 \(\upmu\)m thin brass foil. One insert was placed between the first and second ply (shallow defect), while the other insert was placed between the seventh and eighth ply (deep backside defect). The exact shape and size of the induced delamination was not fully controllable, because the epoxy resin tended to flow partially in-between the insert's layers during the autoclave curing cycle. The second test specimen (see Figure 2b) was a \(330\times 165\times 5.5\) mm\({}^{3}\) CFRP plate with quasi-isotropic layup [(+45/0/-45/90)3]\({}_{\text{S}}\). Side-delaminations were created at the top side of the plate using a fine razor blade. The exact spatial dimensions of the induced side-delaminations were unknown. The numbers of the plies in between which the blade was inserted are indicated on the figure. Note that the majority of the damage is located relatively deep into the 24 ply plate. This type of side-delaminations is representative of damage created by improper cutting and (rivet hole) drilling of multi-layer CFRP plates. Figure 1: Schematic overview of all steps in the damage detection procedure. Figure 2: Carbon fiber-reinforced polymer (CFRP) components with defects. (**a**) Cross-ply [(0/90)2]\({}_{\text{S}}\) plate with artificial shallow (ply 1–ply 2) and backside (ply 7–8) delamination. (**b**) Quasi-isotropic [(+45/0/-45/90)3]\({}_{\text{S}}\) plate with side-delaminations at the top side. The components were excited using piezoelectric patches (type EPZ-20 MS64W and EPZ-27 MS44W from Ekult, with a diameter of 12 mm and 20 mm respectively). The patches were bonded to the CFRP components (see Figure 2) using phenyl salicylate. As there was no prior knowledge on the damage and its corresponding LDR frequencies, a broadband chirp excitation was required to make sure that the LDRs were excited. In that way, it was assured that the nonlinear behavior of a defect will be triggered in an efficient way. The first coupon with delaminations was excited using a chirp signal with linearly increasing frequency from 10 to 60 kHz and length of 36 ms. The chirp signal was zero-padded at the start and at the end for 2 ms resulting in a total duration of 40 ms. The voltage of excitation was amplified to 100 V\({}_{\text{PP}}\) using a Falco System WMA-300 voltage amplifier. The coupon with side-delaminations was excited with a 100 V\({}_{\text{PP}}\) chirp of 16 ms and frequency from 5 to 100 kHz. The electrical power delivered by the amplifier to the piezoelectric patches was only around 5 Watts. The response of the CFRP specimens was recorded at a uniformly spaced grid of scan points with grid spacing 2 mm using a 3D infrared SLDV (Polytec PSV-500 3D Xtra). The sampling frequency was set to 0.625 MS/s. Only the out-of-plane velocity component was discussed in this study. However, note that similar observations can be made by analyzing the in-plane velocity component [3]. Although a SLDV with infrared lasers (wavelength \(\lambda=1550\) nm), showing high sensitivity on even black surfaces was employed, the signal-to-noise ratio (SNR) was further improved by additional measures. The inspected surfaces were covered with reflective tape (3M\({}^{\text{TM}}\) Scotchlite\({}^{\text{TM}}\) 580E-10). Using reflective tape, the resolution of the out-of-plane velocity component was around 0.15 \(\frac{\text{$\text{$\text{$\text{$\text{$\text{$\text{$\text{$\text{$ $\text{$\text{$\text{$\text{$\text{$\text{$\text{Figure 2:0}Figure 2:1}Figure 2:2}}$}}{/ \sqrt{\text{Hz}}}\). In addition, for the component with side-delaminations, 3 averages were made to further increase the SNR.