Research Of Surface Crack Detection On Thread Rod Part Using Pulsed Eddy Current Thermography

Pulsed Eddy Current thermography(PECT)is a new testing technology for metal surface crack detection.Compared with traditional methods like penetration and magnetic testing,it has the advantages of environmental protecting and simple operation.But its detection effect is closely related to the surface shape of parts.Currently,most of PECT detection researches were developed based on flat parts.A few articles target cylinder parts as a object,lacking research of more complex shape parts.Selecting the screw rod as the research object which has a wide range of applications,this paper intends to explore laws and problems when PECT technique is applied to undulating curved surface parts with concave and convex characteristic.Then causes of problems were analyzed.Solutions and improving methods were put forward.These researches aim to promote the application of PECT technology.The main research contents are as follows:(1)Existing magnetic excitation schemes were analyzed.Two kinds of magnetic exciting methods were designed for axial and circumferential magnetization demand of thread rod.The magnetic exciters possess the advantages of high coupling efficiency and without blocking of observing.Combined with homemade high frequency power supply,it can realize the excitation of 100 kHz、2 kW of pulsed eddy current.(2)This paper explored laws and problems when PECT tests the thread rod crack using COMSOL as the main method.To simplify modeling and improve the model quality,this paper simplified the thread rod to the cylinder with two parallel circumferential grooves.Simulation shows that the abnormal caused by thread rod cracks agrees with the abnormal caused by plane parts crack.The crack tip signal is strongest when crack is vertical to eddy current.But when testing thread rod,the stimulating uniformity is poor.Especially under axial magnetization,the heating power density is small at the bottom of thread rod where the crack detection sensitivity is 65 percent of the crack at the adjacent crest of thread rod.The crack at bottom of screw is easy to be missed.In the same condition,experiment shows the similar rule,proving the simulation to be reasonable.(3)The causes of exciting inhomogeneity were analyzed.Under axial magnetization,thread structure enhances the magnetic resistance between two poles.Magnetic lines spread to the back of the thread rod and the air.Under circumferential magnetization,the decrease of the perimeter at the thread root increases the circumferential line density of eddy current.The superposition of heat source at the top and the side of the thread edge makes the temperature at the edge higher.Decreasing the excitation frequency and time and changing the magnetic core shape can improve the detection effect.Processing the thermal imaging data with comparative method can effectively suppress the influence of the inhomogeneity.