## 1 Introduction  
In recent years, honeycomb sandwich composite materials have been widely used in aerospace and aviation industries owing to their excellent physical and mechanical properties, such as high specific strength, high specific stiffness, good sound, and heat insulation property [1; 2; 3]. In particular, honeycomb sandwich composite materials are used as a structural material in the helicopter rotor, aircraft fuselage, the thermal protective layer of spacecraft, etc. Their performance are directly related to the safety and reliability of an aircraft. Therefore, it is necessary to continue the rigorous quality inspection of honeycomb composite materials. However, owing to the influence of temperature, pressure, and other factors, honeycomb composite materials often suffer from delamination defects, debond defects, or inclusion defects in the manufacturing and service process [4; 5]. These defects significantly reduce the fatigue life and mechanical strength of honeycomb composite materials, and seriously affect the safety and reliability of components. At present, although the preparation process is strictly controlled, production defect cannot be avoided. Therefore, an effective and reliable non-destructive testing technique should be used for the detection of defects in effects in such materials and components to ensure that the physical and mechanical properties of the components meet the requirements. Currently, the main technologies used for testing honeycomb sandwich composite materials include X-ray, thermography and ultrasonic methods, etc. The detection performance of X-ray digital radiography methods, thermography and ultrasonic C scan for the characterization of typical defects in honeycomb composite has been studied by [PERSON] [6] and [PERSON] [7]. The detectability of core defect of honeycomb structure by ultrasonic C-scan method was studied by [PERSON] [8]. Another on detection of skin-to-core debonding defect in honeycomb structure using lock-in thermography was shown by [PERSON] [9]. However, X-ray technology is less sensitive to area-type defects of honeycomb composite materials, because of debond efects, inclusion defects and other area-type defects, which cause small changes in the energy, leading to low imaging contrast. Thermography technology is very effective for the election of superficial defects, however it still has great difficulty in detecting deep defects, such as, the debond defects of honeycomb sandwich composite materials with thick skin. The immersion ultrasonic technology is restricted by the coupling agent, which can cause pollution and damage to the surface of honeycomb composite materials. Therefore, it is need to explore a kind of testing technology needs to be explored that does not need a coupling agent, is not limited by the depth of defects in materials, and can detect area-type defects. Air-coupled ultrasonic testing (ACUT) technique is currently a suitable testing technique, which has very significant advantages, such as, non-destructive, completely non-contact, suitable for in itu testing, strong defect characterization, and can avoid the traditional coupling agent (water, gels etc.) pollution and damage to materials [10; 11; 12; 13]. Therefore, the quality evaluation of honeycomb sandwich composite materials has promising applications based on ACUT technique. However, it is difficult to apply this technique for the inspecting inspection of honeycomb sandwich composite. This is because ultrasound will suffer high attenuation [14; 15; 16] and various noises [17; 18] during transmission, resulting in weak transmitted signal and poor signal-to-noise ratio. Hence, using effective methods [19; 20; 21] to overcome these limiting factors is the key to applying the air-coupled ultrasonic testing technique to the quality inspection of honeycomb sandwich composite materials. In this paper, the application of air-coupled ultrasonic technology to the non-destructive testing of debonding defects of honeycomb sandwich composites is studied and demonstrated.