A Research Of Thermography Ndt For Inner Defect In Multi-layer Metal-to-metal Bonded Structure

Lead-steel bonded structure have characteristics of high strength,anti-fatigue,and anti-nuclear radiation,which has been widely applied in nuclear industrial fields.However the bonded structure is prone to exist poor bonding,voids,porosity,and disbond defects during the manufacture process as well as in service stage.These defects damage the integrity of the bonding structure,which can lead to the degradation of the load capacity and mechanical behaviors,even to radiation leakage.Therefore,it is essential to develop a reliable non-destructive test(NDT)method to assess the bonding quality and guarantee safety.The two main excitation modes of infrared thermography Nondestructive testing: eddy current thermography and optical thermography have been applied to inspection and feature extraction of the disbond defects of lead-steel bonded structure.Furthermore,this paper proposed periodic pulsed thermography nondestructive testing method,which increase the defect detectability of deeper defects and small defects without damaging the adhesive.The outline of the work is organized as follows:1)Based on the traditional eddy current pulsed thermography nondestructive testing technology,the internal disbond defects of the lead steel bonded structure are detected.The finite element software COMSOL Multiphysics is utilized for numerical simulation.The excitation parameters is analyzed and optimized based on sensitivity,which is verified through experiments.2)Aiming at the problem that traditional pulsed eddy current thermography Nondestructive testing technology is difficult to detect the micro defects and deep defects of lead-steel bonding structure,an improved periodic pulsed thermography Nondestructive testing method is proposed.In this paper,the theoretical derivation and data processing of the periodic pulse thermography detection method is completed.3)Due to the physical properties of the specimen,excessive instantaneous heat energy will cause thermal shock to lead steel specimen damage or quadratic disbonding.Therefore,the paper introduces the optical thermography method and optimizes the excitation mode.The method of periodic pulsed with the combination of optical thermography and eddy current thermography is to complement the lead steel multilayer adhesive components.The advantages and disadvantages of the application of optical thermography and eddy current thermography in the detection of multi-layer adhesive components are analyzed and compared.In this paper,the proposed method can detect the defect of minimum diameter 3mm and the deepest 5.4mm of lead-steel bonded structure ensuring the specimen safety.It is necessary to study the detection of deeper defects in future.