Characterization of Small Delamination Defects by Multilayer Flexible EMAT

In this study, a miniaturized multilayer flexible electromagnetic acoustic transducer (EMAT) probe was designed for the detection of small-scale delamination defects, successfully identifying a 2 mm delamination defect. Additionally, an innovative characterization method was introduced, which involves analyzing the ratio of the defect echo to the bottom echo at the peak of the defect echo, thereby characterizing the defect features. This approach effectively analyzes delamination that are smaller in width than the diameter of the coil. Finite element simulations were carried out to model the ultrasonic reflections within a specimen containing a 2 mm delamination defect under EMAT excitation. Based on the simulation results, an EMAT with a diameter of 5.5 mm was designed to characterize the sample, and the center point of the EMAT at the peak of the defect echo during scanning was defined as the positioning point. The displacement size of the positioning point relative to the defect center was analyzed for a 5.5 mm coil sample across different widths of delamination defects, aiding in determining the center position of defects in experiments. Subsequently, the ratio of the defect echo to the bottom echo at the positioning point was examined, and segmented fitting was performed based on the variations in defect positioning points, to assist in judging the size of delamination defects in actual experiments. Finally, the feasibility of the theory was validated through practical experiments. This demonstrates the effectiveness of the EMAT design and the method for characterizing small-scale delamination defects proposed in this article.