Surface Crack Detection On Metal With Paint Coating Using Eddy Current Thermography

Eddy current thermography is a method combining eddy current testing and thermography technology.It has the advantages of non-contact,large detection area and high resolution imaging.Aiming at the problem of low signal-to-noise ratio in conventional eddy current thermography methods for surface crack detection on metal under anti-corrosion paint coating,a kind of eddy current thermography method based on direction modulation is proposed.The main research contents are as follows:(1)Experimental study of conventional eddy current thermography methods and analysis of factors affecting crack detection.Comparing the crack detection results of conventional pulse eddy current thermography,high-power pulse eddy current thermography and lock-in thermography,the results show that the lock-in thermography with modulation idea has high signal-to-noise ratio while other methods are not.But it can't detect cracks in all directions at once.Then,the influence of the thickness of paint coating,the angle between the crack and the probe,and the uneven excitation on the test results are analyzed by simulation.(2)Aiming at the problem of low signal-to-noise ratio in conventional eddy current thermography methods,a kind of directional modulation eddy current thermography method is proposed.The approximate analysis of single-point excitation synthesis modeling is carried out to study the excitation synthesis problem of two-channel induced eddy current,and the numerical solution method is used to analyze the synthesis results of two-channel induced eddy current under conditions of ideal,inconsistent amplitude and non-perpendicular direction.(3)According to the amplitude and direction synthesis of the two-channel excitation results,combined with electromagnetic thermal coupling analysis,the whole tested area is simulated.When the surface is free of defects,the changing trajectory of synthetic eddy current is analyzed.And the surface temperature signal is extracted to perform Fourier transform to analyze the surface amplitude and phase relationship.When there is crack on metal surface,the test results show that amplitude map results are better than the phase map,and it doesn't affected by the crack direction.(4)Experimental verification and analysis.Design circuit of two channel induction heating power supply and build an experimental platform.The experiment results of electric discharge machining crack sample under 0.85 mm paint layer and weld sample under 0.9mm paint layer are analyzed to verify validity and feasibility of the new method.The experimental results show that the direction modulation method can effectively enhance the signal-to-noise ratio of surface crack detection under paint layer and detect cracks in all directions at once.