Akustik emisyon esaslı hasar karakterizasyonu ve elektromanyetik iletim performansı kapsamında C-tipi kompozit sandviç radom panellerinin çok disiplinli araştırılması

This study aims to investigate the electromagnetic transmission performance of composite radome sandwich panel structures used in aviation and to cluster the damage mechanisms caused by barely visible impact damages within the panels with the Acoustic Emission (AE) method. Two different sandwich radome panel samples consisting of skin materials made of E-glass and aramid prepregs and Nomex® honeycomb as the core material are examined in the research. Flat sandwich panels with equal skin and core thicknesses are produced by the hot-press curing method. Measurements of the electromagnetic transmission and reflection coefficients are performed by the free space test method in the frequency range of 5-25 GHz. As a result, transmission coefficients including dielectric coefficient and loss tangent values of the panel and its constituents are obtained experimentally. Sandwich panels are numerically modeled as a multilayer substrate by using Hyperworks® FEKO software, where the material parameters that are obtained from the experimental study are used as the input for the model. Planar Green's function approach is used as the solver for the electromagnetic simulations and it is found that the results correspond well with the experiments. As a result, the aramid sandwich panel sample is showed better electromagnetic properties compared to the E-glass sandwich panel sample within the specified frequency range. Barely Visible Impact Damage (BVID) characteristics are investigated in the samples with the quasi-static indentation test approach and the data obtained by the acoustic emission sensors are subsequently clustered with the k-means algorithm to examine and categorize the damage mechanisms that occur in the structure. GAP function is used to specify the optimum initial clustering value of the k-means algorithm. Aramid sandwich panel sample is deformed under the indentation loading and several different damage mechanisms are observed throughout the sample like matrix cracking, fiber breakage, and core crushing. Unlike the aramid samples, debonding and delamination are observed at the interface of the prepreg and core structure within the E-glass sandwich panel sample. Different failure and damage mechanisms within the microstructure are also verified by SEM images. Finally, it has been found that the acoustic emission method can be a useful approach in the damage classification of radome sandwich panels under quasi-static indentation loading. Besides, aramid sandwich panel can be a more suitable material for radome applications in high frequency operating conditions due to its low transmission losses and high structural strength against indentation loads.