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A New Method to Evaluate Surface Defects with an Electromagnetic Acoustic Transducer.

Zhang K, Yi P, Li Y, Hui B, Zhang X - Sensors (Basel) (2015)

Bottom Line: Moreover, it can also reduce the influence of other factors, such as the lift-off, during the testing.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.Meanwhile, it is not essential to measure the signal without defects.

View Article: PubMed Central - PubMed

Affiliation: School of Mechanical Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, China. zhangkang@hust.edu.cn.

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.

No MeSH data available.


Related in: MedlinePlus

The schematic diagram of the EMAT detection.
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sensors-15-17420-f001: The schematic diagram of the EMAT detection.

Mentions: The detection process of EMAT can be simplified as in Figure 1. The EMAT works in a pitch-catch mode. The transmitting probe and the receiving probe are located at different sides of the crack. In the transmitting module, the high alternating current pulsed through the transmitting coil induces eddy current within the sample skin depth. With the interaction of the eddy current and the external static field provided by the permanent magnet, the Lorentz force is yielded, as shown in Equation (1). According to Equation (2), the force generates elastic waves, which propagate in the opposite directions through the sample. The wave interacts with the crack, a part of the wave transmits through the crack and a part of the wave reflects from the crack. In this paper, we only pay attention to the transmitting wave. The receiving probe is far away from the crack to avoid the near-field enhancement. In the receiving module, the particle velocity induced by the ultrasonic motion interacts with the bias field to yield current density in the sample. In turn, a voltage induced in the receiving coil is produced by the time varying magnetic field generated by the current density. When the defect is changed, the voltage signal will change. By analyzing the relationship between the voltage and the defect, we can characterize the defect with the features extracted from the signal.(1)F=J×B(2)ρ∂2u∂t2=∇•T+Fwhere B is the magnetic flux intensity, J is current density, σ is the electrical conductivity, ρ is the mass density, u is the elastic deformation, F is the force per volume and T is the elastic stress tensor.


A New Method to Evaluate Surface Defects with an Electromagnetic Acoustic Transducer.

Zhang K, Yi P, Li Y, Hui B, Zhang X - Sensors (Basel) (2015)

The schematic diagram of the EMAT detection.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4541941&req=5

sensors-15-17420-f001: The schematic diagram of the EMAT detection.
Mentions: The detection process of EMAT can be simplified as in Figure 1. The EMAT works in a pitch-catch mode. The transmitting probe and the receiving probe are located at different sides of the crack. In the transmitting module, the high alternating current pulsed through the transmitting coil induces eddy current within the sample skin depth. With the interaction of the eddy current and the external static field provided by the permanent magnet, the Lorentz force is yielded, as shown in Equation (1). According to Equation (2), the force generates elastic waves, which propagate in the opposite directions through the sample. The wave interacts with the crack, a part of the wave transmits through the crack and a part of the wave reflects from the crack. In this paper, we only pay attention to the transmitting wave. The receiving probe is far away from the crack to avoid the near-field enhancement. In the receiving module, the particle velocity induced by the ultrasonic motion interacts with the bias field to yield current density in the sample. In turn, a voltage induced in the receiving coil is produced by the time varying magnetic field generated by the current density. When the defect is changed, the voltage signal will change. By analyzing the relationship between the voltage and the defect, we can characterize the defect with the features extracted from the signal.(1)F=J×B(2)ρ∂2u∂t2=∇•T+Fwhere B is the magnetic flux intensity, J is current density, σ is the electrical conductivity, ρ is the mass density, u is the elastic deformation, F is the force per volume and T is the elastic stress tensor.

Bottom Line: Moreover, it can also reduce the influence of other factors, such as the lift-off, during the testing.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.Meanwhile, it is not essential to measure the signal without defects.

View Article: PubMed Central - PubMed

Affiliation: School of Mechanical Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, China. zhangkang@hust.edu.cn.

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.

No MeSH data available.


Related in: MedlinePlus