###### Abstract  
This paper presents ultrasonic guided wave (UGW) propagation-based nondestructive testing (NDT) of an adhesively bonded composite structure (ACS). In the process, a series of scanning laser Doppler vibrometry (SLDV)-based laboratory experiments and time-domain spectral element method (SEM)-based numerical simulations were carried out on an ACS with barely visible impact damage (BVID) and a hole. A good agreement was observed between the numerical and experimental UGW signals in the cases studied. Finally, a full-field and elliptical signal processing method-based NDT strategy was proposed that uses differential damage features of the registered UGW signals to identify different types of BVIDs in the ACS.  
ultrasonic guided signal; adhesively bonded composite structure; barely visible impact damage; laser-vibrometry; damage detection +
Footnote †: journal: Physics Letters B  
00212212440/02/12/021221240/02/12/0  
debonding [7]. Other research refers to SLDV studies on aluminum honeycomb sandwich structures [8]. The impact test on the 3D composite honeycomb structure was studied with the use of SLDV and numerical methods [9]. The magnitude of the wave depends on the core of the honeycomb structures [10]. The evaluation of BVIDs in sandwich honeycomb plate using PZT excited guided waves [11] was presented using time-reversal methods. Analysis of honeycomb structures with debonding at varying temperature conditions was undertaken using guided waves, and identification of damage using damage index maps was shown [12]. Numerical and experimental analysis [13] of debonding in composite structures was carried out using guide waves.  
The spectral element method (SEM) developed for the numerical solution of the laminar fluid flow by [PERSON] [14] combines the flexibility of the finite element method (FEM) and fast convergence of spectral methods in the frequency domain. This SEM technique was successfully implemented in the field of modelling the phenomena of elastic wave propagation [14; 15; 16; 17; 18]. In the current research paper, we used SLDV to analyze the BVID in a semi-sandwich panel with a Nomex core and aluminum skin. The damage caused by impacts of 4 and 10 J was studied. In one region, a small portion of Nomex core was completely removed manually to make an indentation and, in another region, an indentation with a Nomex core protrusion was studied. The results obtained from SLDV were compared with numerical studies to visualize the damage region. The numerical approach was performed using the spectral element method (SEM) code. The gathered data was processed with the elliptical signal processing method for damage localization with various sensor network arrangements.  
The paper is structured as follows. The experimental and numerical process is explained in Section 2. Section 3 presents results obtained from the SLDV, SEM, and the elliptical-based code. The paper is concluded in the fourth section.  
The novelties in this paper are as follows:  
* A sandwich plate with Nomex core and single-sided adhesively bonded aluminum facet is studied experimentally, numerically, and analytically.
* SEM study with PZT SONOX P502 parameters is studied with a single-sided adhesively bonded composite structure (ACS).
* The SLDV full wavefield result is compared with the SEM result.
* The elliptical method is applied to the data obtained from SLDV and SEM.
* The elliptical signal method is not limited to the first time of arrival and does not use the differential signals.