Research On Detection Method Of Metal Surface Crack Defect Based On Eddy Current Pulse Thermal Imaging

Crack is one of the common defects in metallic materials,the presence of cracks affects the performance of the entire mechanical system and may cause catastrophic accidents in the industry.The research on detection methods for crack defects on metal surface has great significance,improving the reliability of the mechanical equipment and preventing the occurrence of catastrophic accidents.Nondestructive testing technology is widely used in the detection of metal surface defects.Physical methods are used to detect defects in the material without breaking structure of the material.As an emerging nondestructive testing technology,eddy current pulsed thermography(ECPT)is applied to determine the internal state and structure of materials based on the surface temperature distribution.Currently,the defect detections based on ECPT focus on qualitative identification of cracks and the analysis of thermal image,the research on the influence of parameters and quantitative analysis methods is not comprehensive.Therefore,it is important for crack detection to study the influence of induction coil and excitation parameters on the detection effect and the quantitative analysis method of the cracks.In this paper,the characteristics of the thermal image sequence are analyzed.The crack profiles in the thermal image are determined based on the Canny edge detection algorithm,achieving the qualitative evaluation of the crack.Quantitative analysis methods for crack length,width and depth are proposed from the two aspects of crack geometry and depth.Secondly,the COMSOL Multiphysics finite element simulation software is used to build simulation models of different induction coil types,copper tube widths and copper tube spacing,the influence law of induction coils on the detection effect is obtained.The simulation model of different distances,excitation currents and excitation frequencies are built,and the influence law of excitation parameters on the detection effect is obtained.Then,the perturbation law of the cracks on the eddy current and temperature distribution is determined through simulation modeling and analysis.According to the result of the edge detection of the thermal image,the crack profile and position are identified.The crack boundaries are determined by the temperature distribution and the slope curve on one or several trajectory curves of the crack profile.Quantitative analysis of the crack geometry is achieved.Subsequently,quantitative analysis of the crack depth was achieved using the monotonic linear relationship between the crack depth and the maximum surface temperature of the samples.Finally,the experimental system was built.Experimental tests were carried out based on two sets of samples with different widths and depths.Quantitative analysis of crack width and depth was achieved,and the detection error of length and width was small.The influence law of the induction coil and excitation parameters is analyzed through the simulation model.Using the edge detection algorithm,the qualitative information such as crack position and contour is determined.The quantitative analysis method of crack length and width based on the temperature distribution of the trajectory curve through the crack profile and the quantitative analysis of crack depth based on maximum surface temperature are proposed.The feasibility of the methods are verified by simulation modeling and experimental analysis.