## 7 Conclusions  
The linear and nonlinear vibrational response of defects in CFRP coupons is investigated. The investigation is performed using experimental data obtained from two CFRP components: (i) a cross-ply CFRP plate with both a shallow and a deep artificial delamination; and (ii) a quasi-isotropic CFRP plate with side-delaminations. Broadband vibrations are introduced using low-power piezoelectric actuators while the full field velocity response is recorded with an infrared scanning laser Doppler vibrometer.  
Time-frequency filtering, relying on (inverse) short-time Fourier transformations, is proposed to extracted linear and nonlinear velocity components (i.e., higher harmonics) out of the out-of-plane velocity measurement signal. This procedure enables the separation of the different harmonic components within a broadband response signal.  
The linear and the nonlinear vibrational components are analyzed in a function of frequency by calculation of the defect-to-background ratio. For the shallow delamination, the linear velocity component shows local defect resonance behavior at specific frequencies. The high vibrational amplitude at local defect resonance results in the efficient generation of nonlinear higher harmonic components. For the deep artificial delamination and for the side-delaminations, no local defect resonance behavior is observed in the linear velocity component. These defects are located relatively close to the backside which causes the local defect resonances to be present only at the (invisible) backside. However, the nonlinear vibrational components, generated by the defect, radiate through the thickness and can be detected from the inspection side.  
Broadband band power maps for both linear and nonlinear response are introduced. These broadband band power maps significantly improve the defect detectability. The nonlinear broadband band power maps, in particular, provide a high-contrast imaging of shallow as well as deep damage.  
Finally, the developed procedures are applied for baseline-free detection and localization of out-of-sight damage. This is illustrated for the detection of side-delaminations. The vibrations are measured only at a few small areas of the component. For each scan area, the direction of the waves are determined. The derived directions point correctly to the side-delaminations.  
Conceptualization, J.S., M.K. and S.H.; methodology, J.S.; software, J.S.; validation, S.H.; resources, M.K. and W.V.P; writing--original draft preparation, J.S.; writing--review and editing, S.H., G.P., M.K. and W.V.P; supervision, M.K. and W.V.P; funding acquisition, M.K. and W.V.P. All authors have read and agreed to the published version of the manuscript.  
The authors acknowledge the funding of through Fonds voor Wetenschappelijk Onderzoek FWO (fellowships 1148018N, 1511520N and 12T5418N) and the SBO project DETECT-IV (Grant no. 160455), which fits in the SIM research program MacroModelMat (M3) coordinated by Siemens (Siemens Digital Industries Software, Belgium) and funded by SIM (Strategic Initiative Materials in Flanders) and VLAIO (Flemish government agency Flanders Innovation and Entrepreneurship).  
The authors declare no conflict of interest.