Detecting And Characterizing Of The Surface Defect With Electromagnetic Acoustic Testing Considering The Lift-off Effect

In the remanufacturing process of a used part, it is important to decide whether it’s worth remanufacturing. Thus, it is essential to detect the defect of the returned parts in a retired machine. There are many NDT methods to fulfill this target. Among these NDT methods, the electromagnetic acoustic transducer(EMAT) is an attractive one, which requires no coupling medium, and can operate without direct contact with the tested part. In this paper, we used the EMAT to excite surface wave to detect the surface defect of the non-ferromagnetic material, and study the lift off effect on the testing process.Firstly, based on the Lorentz force principle, we analyzed the equations of the EMAT testing process. With the aforementioned theory, the software COMSOL was adopted to model the EMAT. For the coil region, we analyzed the differences of three equations. Compared with the velocity and the normalized displacement in theory, the wave generated in the simulation is Rayleigh wave. Moreover, the experiment results demonstrated the correctness of the simulation.Secondly, based on the transmission of the signals during the testing process, we put forward a signal transmission model to describe the relationships of the lift-off of the transmitting probe, the lift-off of the receiving probe and the depth of the defect. The results of the simulation and the experiment illustrate that the above parameters are not relevant. What’s more, the lift-off functions of the transmitting and the receiving probe are both exponential, the relationships of the above functions and other functions are multiplier. The model is accurate and can be used to analyze the lift-off effect.Thirdly, with the signal transmission model, the paper analyzed the lift-off effects on the peak-to-peak amplitude, transmission coefficient and the cut-off frequency. It is found that the feature transmission coefficient is not sensitive to the lift-off. However, we need to measure the signal without defect at the same lift-off with this feature. The feature peal-to-peak is sensitive to the lift-off. For the feature, cut-off frequency, if we keep the lift-off unchanged during the testing process, the lift-off has a little influence on the cut-off frequency. Nevertheless, if the lift-off varies in the inspection process, the cut-off frequency is sensitive to the lift-off.Finally, in this paper, we put forward a new feature, lift-off slope, to evaluate the depth of the crack. It is demonstrated that we can calculate the depth of the crack with this feature. Compared with other features, the lift-off slope is not sensitive to the lift-off of one probe, and we don’t need to measure the signal without defect when we evaluate the depth of the crack.