Research On The Non-destuctive Testing Technologies Of Multi-layer Heterogeneous Metal Structures Based On Lock-in Thermography

The multi-layer metal-metal bonded structure is widely used in aviation and nuclear industry due to its high specific strength and fatigue resistance.However,in the stage of manufacturing,assembly and service,affected by the production process,environment,fatigue loading and other factors,the defects,e.g.,debonding,under gluing,and cracks at the bonding point between laminates and inside of the plate will occur.This will lead to deterioration of the thermal and mechanical properties of the structure and negative impact on the reliability and safety.Without damaging its performance,the nondestructive testing technology can be adopted to detect the defects of the tested object and evaluate its integrity,reliability and safety according to the test results.However,because of the particularity of its structure,the traditional nondestructive testing methods,such as eddy current,ultrasonic and X-ray have limitations and deficiencies on the defect detection of multilayer metal-metal bonded structure with thickness.Therefore,the accurate and efficient detection,identification and quantification of the defects in that bonded structure pose a great challenge to the nondestructive testing technology.In recent years,a variety of new nondestructive testing technologies based on infrared thermal imaging have been paid extensive attention in defect detection,failure analysis,life prediction and other aspects,because of their advantages such as large single detection area,fast detection speed,non-contact,pollution-free and intuitive imaging.In order to meet the requirements of nondestructive,fast and accurate detection of defects in multi-layer metal-metal bonded structure,the optical and eddy current lock-in thermography detection methods have been applied,and the relevant theoretical research,simulation analysis,system construction,algorithm processing and key technology research have also been carried out aiming to provide a new method and reference for the defect detection of multi-layer metal-metal bonded structure.The main research contents and innovations of this paper are as follows:1)The optical and eddy current lock-in thermography detection methods are innovatively applied to the defect detection of multi-layer metal-metal bonded structure.Based on the basic theories and thermal imaging principles of eddy current and optical lock-in thermography detection method,the mechanisms of the influence on detection results and some solutions are analyzed.According to the Maxwell equations,the electromagnetic field governing equation of the eddy current lock-in thermography is obtained.Then,combining with the heat conduction equation in the space-time domain,the coupling relationship between the electromagnetic field and the thermal field is established.The influence of thermal diffusion length and skin depth on defect detection depth is evaluated and the application characteristics between eddy current lock-in thermography and the optical lock-in thermography are compared and the data fusion based on the two methods is proposed.2)For the characteristics of multi-layer metal-metal bonded structure,the heat conduction process of multi-layer medium is studied,and the temperature field distribution model is established.The influence of excitation parameters,defect location and defect size on the detection results is simulated and analyzed by using the finite element software,which provides a reference for the optimization of detection parameters.In order to verify the validity of the detection methods,the artificial samples with simulated defects are implemented and the experiments are carried out by using the two methods of lock-in thermography.Based on the characteristics of the lock-in thermograhpy image sequence,the image preprocessing algorithms and four feature extraction algorithms are studied,and the results calculated from these algorithms on improving the sensitivity of defect detection are compared and analyzed.3)In order to build the optical and eddy current lock-in thermography detection systems,this dissertation mainly focuses on the design of the excitation power supply which rarely studied by domestic and foreign scholars.Based on the sufficient investigation and analysis of the electrical parameters and functional requirements of the excitation power supply in lock-in thermography system,several works are finished.Firstly,a novel design scheme of multi-mode excitation power supply suitable for optical excitation infrared thermal imaging is proposed innovatively.Then,the asscioated circuit topology,operation principle,design process and experimental verification are described.Secondly,a critical technique of the proposed scheme,i.e.,a multilevel gate driving circuit of SiC MOSFETs for mitigating coupling noise is proposed and the working principle,parameters calculation and experimental verifications of this circuit are described as well.Finally,due to the load of the eddy current lock-in thermography is inductive,a design of linear excitation power supply is proposed.The scheme of multimode excitation power supply and the gate driving circuit proposed in this dissertation can not only be applied to the lock-in thermography,but alse to other excitation modes of thermography system.The scheme provides a valuable and meaningful reference when the automation design,portable design or customized design for special application scenarios are required.It has good economic value and social value.The above researches will contribute to meet the demands of the nondestructive,fast and accurate detection of multi-layer metal-metal bonded structure in the manufacturing,assembly and service stages.These will be very helpful to expand the application fields and improve the detection quality of infrared thermography technology.