## 1 Introduction  
Composite materials such as carbon fiber-reinforced polymers (CFRP) are currently used in a wide range of industries. Because of their design flexibility and high specific stiffness and strength, these composite materials are often used in load-bearing components where they replace the traditionally used metals. However, a concern in the use of composites is that their layered structure is susceptible to internal damage which can be introduced during the manufacturing process as well as the operational life. One example is low-velocity impact damage which is typically referred to as barely visible impact damage because the impact introduces delaminations in between the layers while the impacted surface remains (almost) intact. Internal damage leads to a strong local decrease of the strength (and stiffness) of the component which can eventually result in unexpected failure. In this study, a baseline-free vibrometric non-destructive testing (NDT) approach is proposed for localizing shallow and deep internal damage in CFRP components. The method uses low-power (<10 W) piezoelectric excitation combined with non-contact scanning laser Doppler vibrometer (SLDV)measurements. Using SLDV measurements, it was shown earlier that defected plates can show local resonances at the location of the defects [1; 2; 3]. The frequencies at which these local resonances occur are referred to as 'local defect resonance (LDR) frequencies'. Depending on the size, shape and depth of the defects, these LDR frequencies are typically in the 1 to 100 kHz range. LDR behavior has been investigated for different defect types: flat bottom holes [3; 4; 5], artificial delaminations [4; 5] (e.g., Teflon delaminations), disbonds [6] and barely visible impact damage [3; 7]. If these defects are relatively shallow (depth up to 50% of the sample's thickness), they can be detected by a manual or automated [8] search for LDR behavior in the broadband response of the component. However, if the defects are located deeper than the mid-thickness of the component, no local resonances occur as the reduction in the local bending stiffness at the defect becomes too limited [9]. Apart from local defect resonance, other wave-defect interactions can be exploited for NDT using full wavefield measurements. For instance, the defect-related shift in local wavenumber is exploited in local wavenumber estimation methods [10; 11; 12; 13]. Alternatively, the typical increase in vibrational amplitude at damage is exploited using the (weighted) root mean square energy calculation [14]. An increased sensitivity to damage is found when performing wavenumber filtering prior to the calculation of the energy maps [15; 16]. In this way, both the shift in wavenumber as well as the shift in energy are exploited. These vibrometric methods show promising results for NDT but also have their limits in terms of minimal defect size and maximum defect depth [17; 18]. In order to further increase the sensitivity to small and deep damage, nonlinear defect imaging techniques were developed, such as nonlinear elastic wave spectroscopy (NEWS) [19; 20; 21; 22; 23; 24; 25] and nonlinear elastic wave modulation spectroscopy (NEWMS) [26; 27; 28; 29]. These methods focus on the detection of nonlinear frequency components, for example higher harmonics, in the output signal which are then correlated to defects. The nonlinear response of the defect is caused by classical material nonlinearity combined with multiple complex contact mechanisms which are triggered at relatively high vibrational amplitudes [30]. In order to achieve the required level of vibrational amplitude at the defect when using low-power piezoelectric actuators, the defect can be excited at LDR frequency [31; 32; 33]. The current authors showed recently that the nonlinear LDR response of a deep backside-delamination can be observed experimentally [34]. When using nonlinear defect imaging techniques, the nonlinear components have to be extracted from the output response of the sample.