Research On Debonding Detection Technology Of Absorbing Coatings Based On Microwave Thermography

With the rapid development of radar detection technology,the demand for radar stealth performance of aircraft is gradually increasing.In addition to the development of shape stealth technology,the development of material stealth technology for absorbing coatings has also become more important.The absorbing coatings with high absorption rate can greatly reduce the radar detection probability and improve the survivability and combat capability of aircraft.However,due to the problem of poor weather resistance and shedding,the daily maintenance of absorbing coatings on the aircraft has become extremely important.In addition to microwave performance of the absorbing coating,the debonding problem between coatings and aircraft fuselage cannot be ignored.In the actual flight,the absorbing coating is affected by environment,which will cause the coating debonding and further develop into the fracture and shedding of coating.On one hand,the shedding coating will increase the possibility of aircraft being detected by radar.On the other hand,if the shedding coating accidentally falls into the engine,the safe flight of aircraft will be seriously affected.Therefore,the development of on-site detection technology for debonding defects of absorbing coatings is an urgent problem to be solved.In the on-site detection of debonding defects of absorbing coatings,the existing nondestructive testing(NDT)technology has problems such as low testing efficiency and insufficient defect resolution.Therefore,a NDT technology combining microwave heating and infrared imaging,which is called microwave thermography(MT),is proposed in this dissertation.Compared to other NDT methods,microwave thermography is heating the coating by microwave,which will produce significant reflection and scattering at the discontinuous debonding interface inside the coating,thus enhancing the defect detection effect of the coatings.In addition,microwave thermography is based on infrared NDT technology,which is suitable for sub-surface testing of materials and has the characteristics of wide detection range,fast response and non-contact.So microwave thermography has huge potential in NDT of coatings.In this dissertation,a debonding detection technology of absorbing coatings based on microwave thermography is studied.Aimed at three aspects of theory,system and test methods,the electromagnetic heat problem of microwave heating of coatings,the system design problem and the research problem of detection methods are emphatically solved.As follows :1.The interaction mechanism between microwave heating and heat transfer of coating materials has been studied.The coating material is subjected to stratification and partition analysis,and the analysis method of electromagnetic field and the numerical method of temperature field are used to establish the heat transfer model of coating materials which is heated by microwave.First,for transverse uniform coating material,the power density distribution of microwave heating and longitudinal temperature field are analyzed,and the characteristic model of the coating surface temperature based on microwave heating in longitudinal 1D space is established,so the correlation between the surface temperature of the transverse uniform coating and heating power,electromagnetic parameters,heat conduction parameters and thickness is obtained.Then,aiming at the transverse non-uniform coating material,a pattern matching algorithm for analyzing the electromagnetic field inside the coating and a 2D finite difference method for calculating the temperature field are used to synergistically analyze the temperature distribution on the coating surface.Therefore,the characteristic model of the coating surface temperature based on microwave heating in transverse and longitudinal 2D space is established,and the relationship between the coating surface temperature and defect aperture,coating thickness is obtained.Finally,the 3D simulation model of the coating heat transfer is established,and the relationship between antenna near-field heating model and plane wave heating model was explored.The above research provides a theoretical basis and support for the defect detection of coating material by microwave thermography.2.According to the design requirements of the antenna for engineering applications and defect detection model,a C-band horn antenna with symmetrical beam is developed.According to the practical requirements of the antenna excitation source for engineering applications,a small microwave output system with output power adjustment,output power self-test and communication control is developed,and its operating frequency is5.8 GHz.According to the above devices,a hardware system that detect the coating defect based on microwave thermography was established.3.Based on the defect detection system,the continuous microwave thermography is used to realize the rapid debonding detection of the absorbing coatings.First,by establishing the test model of coating surface temperature,reflectivity and antenna input power,the effective heating time and cooling detection time of the absorbing coating in continuous microwave thermography are studied by defect signal-to-noise ratio(DSNR),which provides theoretical basis for the rapid and effective detection of the coating.Then,by establishing a test model of coating surface temperature and defect aperture,the defect detection ability of continuous microwave thermography was studied.Finally,aiming at the problem of insufficient detection ability of small aperture defects and adjacent linear defects,a signal reconstruction algorithm based on continuous microwave thermography is further proposed.In sum,the results show that the detection system can detect the debonding defect with a minimum size of 2 mm × 2 mm in the absorbing coating with a thickness of 1.8 mm by continuous microwave thermography.For the absorbing coating with a thickness of 1.3 mm,the minimum debonding defect of 1 mm × 1 mm can be detected.And the test time of continuous microwave thermography is not more than 60 s.4.Aiming at the influence of antenna uneven heating on the imaging results of continuous microwave thermography,the coating debonding detection based on modulated microwave thermography is studied.First,by establishing the characteristic model of the surface temperature of transverse non-uniform coating under modulated microwave heating,the defect detection mechanism of modulated microwave thermography is studied.Then,a modulated microwave thermography based on principal component filtering(PCF)is proposed to improve the defect clarity in the imaging results.Finally,according to the output results of the modulated microwave thermography based on PCF,a morphological edge detection algorithm based on double threshold segmentation is proposed to extract the defect features in the image.In sum,the results show that the detection system can detect the debonding defects with a minimum size of2 mm × 2 mm in the absorbing coating with a thickness of 1.3 mm or 1.8 mm by modulated microwave thermography.Although the test time of the modulated microwave thermography is more than 120 s,the defect clarity has been greatly improved.In a word,the debonding detection technology of absorbing coating based on microwave thermography proposed in this dissertation can realize the high-resolution and high-efficiency on-site detection of debonding defect,which provides an important technical guarantee for the daily maintenance of the absorbing coating.