Recognition Of Deep Defects In Sandwich Panels Based On Local Defect Resonance

Composite sandwich structure has the advantages of high specific modulus and high specific strength,so it has been widely used in aircraft,naval vessels and other large military equipment.However,sandwich structures are vulnerable to vibration,impact,corrosion and other factors in the process of manufacture,transportation and service,resulting in delamination,cracks,debonding and other damage.Nondestructive damage detection methods for sandwich structures have been studied extensively.However,due to the high insulation and sound insulation of sandwich cores,it is still a challenge to identify the deep defects of sandwich structures,such as the far-plate defects.Recently,the Non-Destructive Testing method based on local defect resonance proposed internationally can theoretically excite the nonlinear vibration characteristics of defects with low power,thus providing identification features for defect identification,and providing a new idea for the research of defect identification of composite sandwich structures.In this paper,a far-panel defect detection method for composite sandwich structures based on local defect resonance is studied.1.Analytical Approach and Finite Element Method are used to analyze the local resonance frequency at the defect of sandwich structure,and the difference of resonance frequencies between the two methods is compared,which provides a reference for the selection of excitation frequency in subsequent experiments.2.A Non-Destructive Testing system based on local defect resonance is constructed with aluminum plates with blind holes and glass fiber composite plates with delamination damage as samples,which provides a reliable experimental platform for the further study of defect identification of composite sandwich panels.3.The light wood core sandwich panel and Nomex honeycomb core sandwich panel were used as the samples to prefabricate debonding defect between the surface of far-panel and sandwich core.The size of the debonding defect included 30×30mm~2and 20×20mm~2.The resonance frequency of the local defect and the corresponding nonlinear vibration characteristics were measured experimentally.The local defect resonance method was used to debond the far surface core of composite sandwich panel.The feasibility of local defect resonance method to identify the debonding defects of composite sandwich panels.4.The damage of sandwich panels with aramid honeycomb core was made by impact method.The resonance frequencies of local defects and the corresponding nonlinear harmonic characteristics were measured experimentally.The difference of nonlinear vibration characteristics between far panel excitation and near panel excitation was compared and analyzed.Feasibility of identifying local delamination defects of composite sandwich panels by local defect resonance method.5.The method of identifying faults of composite sandwich panels based on broadband sweep excitation is studied.Compared with the method of local defect resonance frequency excitation,it is easier to realize automatic detection in engineering practice.