Abstract
Characterizing a surface defect is very crucial in non-destructive testing (NDT). We employ an electromagnetic acoustic transducer (EMAT) to detect the surface defect of a nonmagnetic material. An appropriate feature that can avoid the interference of the human factor is vital for evaluating the crack quantitatively. Moreover, it can also reduce the influence of other factors, such as the lift-off, during the testing. In this paper, we conduct experiments at various depths of surface cracks in an aluminum plate, and a new feature, lift-off slope (LOS), is put forward for the theoretical and experimental analyses of the lift-off effect on the receiving signals. Besides, by changing the lift-off between the receiving probe and the sample for testing, a new method is adopted to evaluate surface defects with the EMAT. Compared with other features, the theoretical and experimental results show that the feature lift-off slope has many advantages prior to the other features for evaluating the surface defect with the EMAT. This can reduce the lift-off effect of one probe. Meanwhile, it is not essential to measure the signal without defects.
Highlights
Through the electromagnetic acoustic transduction principle, electromagnetic acoustic transducers are widely used for thickness measurement and defect characterization without making physical contact with the test object [1,2]
In view of the interactions of the signals in the electromagnetic acoustic transducer (EMAT) testing, we proposed the expressions of the features of the peak-to-peak amplitude and the transmission coefficient
We proposed a theoretical model to investigate the feasibility of this feature to evaluate the surface defects, which was verified by the experiments
Summary
Through the electromagnetic acoustic transduction principle, electromagnetic acoustic transducers are widely used for thickness measurement and defect characterization without making physical contact with the test object [1,2]. Based on the analysis of the lift-off effect on the receiving signal, a new feature, the “lift-off slope” (LOS), is put forward in this paper, and a new method is proposed for this feature in the EMAT testing. For the PEC, the relationship between the receiving signal and the crack is linear, while this differs for the EMAT. With this method, the lift-off effect of one probe can be reduced, and a signal without a defect is not required during the characterization process. On the basis of the principle of the EMAT detection, we propose the feature “lift-off slope”. We give a discussion of the proposed new evaluation method for surface defects
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