| Due to the high strength,light weight,corrosion resistance and good insulation properties,the fiber reinforced composites(FRP)are widely used as the metal reinforcement repair material in engineering.However,this type of reinforcement repair structure is vulnerable to impact or other damage during manufacture,service and maintenance,and they are difficult to be observed by using naked eyes,which would seriously affecting its performance,durability and even leading to structural failure.Therefore,it is of great significance to investigate non-destructive testing technology for detection of these composite reinforcement structures.In recent years,pulse eddy current thermography(ECPT)has been widely concerned and has shown its advantages in detection of material damage and structural defects.In this paper,the feasibility and applicability of ECPT for defect detection of FRP-steel hybrid structure are investigated.Based on the analysis of heat conduction theory and the electromagnetic coupling model of composite material and its reinforcement structure,the feasibility of ECPT for defect detection of CFRP reinforced metal hybrid structure and GFRP material is studied in theory.Firstly,the model of CFRP-metal hybrid structure excited by eddy current was established by COMSOL Multiphysics.The influence of crack and debonding defects on the induced current and thermal field distribution was investigated.Secondly,the electromagnetic induction heating model of GFRP-metal hybrid structure was established.The heating efficiency on GFRP material by induction heat generated on the metal GFRP was studied to obtain the surface temperature distribution on the GFRP specimen with impact damages.The effects of the material properties,thickness of the assistant heat-generating medium and the thermal conductivity of the filler on the detection results were studied to provide theoretical guidance for the actual experiments.The detection system of ECPT was established to detect various defects on CFRP reinforced sucker rods.According to the analysis of the IR images and temperature data,it can be found that the ECPT can quickly and efficiently detect all the defects existed in the CFRP reinforced sucker rods with a high detection sensitivity.The detected defects are mainly included internal steel matrix cracks,the exposure of the steel matrix,the debonding,and the excessive or insufficient adhesive on CFRP layer.Aiming at the problem that the ECPT can not be applied to the detection of the GFRP material,we proposed to realize the indirect pulse eddy current thermal excitation detection of GFRP by means of auxiliary heating metal medium.Based on the theoretical analysis,the ECPT experimental system for GFRP structure is established.And the optimal detection conditions for GFRP defect detection are obtained as follows: The assistant heat-generating medium should be made of ferromagnetic material which is easy to be heated and its thickness is as small as possible.Additionally,some sticky substance with a high thermal conductivity should be filled in the gap between the GFRP material and assistant heat-generating medium.Finally,the results show that the ECPT can realize the effective detection of impact damages and delamination defects on GFRP by means of GFRP-metal hybrid structure.Based on the blind signal separation algorithm and the frequency domain information reconstruction method,the IR image sequence is processed to enhance the image features and improve the detection sensitivity.The analysis of the principal component and the independent component based on the blind signal separation algorithm can be used to extract all the effective features that appear in the detection process and to show the characteristics of the defects,which can enhance the visual effects and achieve defect feature extraction automatically.In addition,the results of reconstructed images by frequency domain information show that the image reconstructed by amplitude at a certain frequency can effectively enhance the defect characteristics. |
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