An orthogonal multi-frequency eddy current technique for simultaneous horizontal and vertical crack evaluation in steel components /
Nurul A’in Bt Nadzri
- xviii, 194 pages : illustrations ; 30 cm. + 1 CD-ROM
Faculty of Electrical & Electronics Engineering Technology
Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang – 2024
Includes bibliographical references
Cracks in steel components can lead to structural failure if left undetected and treated, and detecting cracks is a crucial step in preventing fatal accidents. Detecting defects using conventional Eddy current testing probes that utilizing a circular coil for magnetic field generation, faces challenges due to limited sensitivity, as the sensor placement compromises effectiveness in both vertical and horizontal crack detection. The design constraints of these probes hinder their ability to identify complex crack shapes, emphasizing the collective influence of induced Eddy current geometry, sensing configuration, and crack geometry on effective defect detection. Moreover, efficient Eddy current testing relies on the strategic placement and quantity of magnetic sensor elements, but increasing channels for multi-directional crack detection enlarges probe size and introduces sensitivity inconsistencies among channels. Apart from that, the characterization of cracks in steel currently relies on human inspectors interpreting complex signals from Eddy current testing (ECT), facing challenges due to factors like crack size, shape, depth, and orientation, as well as interference from noise and external sources. In a multi-channel sensing system, each channel responds differently, requiring skilled interpretation, and the complexity is heightened when detecting multi-orientation cracks. To address these issues, this research aims to examine the correlation between induced magnetic signal change (Vpp) and various Eddy current induction configurations in relation to slit orientations. Then, to enable simultaneous detection of vertical and horizontal cracks, this research studies the integration of horizontal and vertical cracks detection techniques using an orthogonal Eddy current excitation technique in an ECT probe, and assess their sensitivity (Vpp) in relation to slit direction. Furthermore, the reserach continues in the reconstruction of the magnetic response distribution induced by slits using a superimposed technique and finding the correlation between the slit parameters (shape, length, width, depth) and the induced magnetic signal's peak-to-peak voltage (Vpp) using orthogonal Eddy current excitation techniques. To assess the theoretical magnetic responses from the Eddy current inductions, a three-dimensional finite element method (3-D FEM) was employed to analyze the circular, single-direction, and orthogonal direction coil configurations. From the theoretical investigations, two types of probes were developed, each featuring the flux gate sensors with circular and orthogonal Eddy current excitation configurations. The performance and characteristics of the circular and orthogonal Eddy current probes were examined for detecting artificial horizontal and vertical slits on mild steel plate samples with varying depth (2 mm to 10 mm), width (0.3 mm to 1 mm), and length (5 mm to 180 mm). A superimposed technique was used to merge the multichannel probe output, which involved investigating two different frequency excitation modes:single-frequency mode and multi-frequency mode. The results showed that the orthogonal Eddy current probe outperformed the conventional circular Eddy current probe in detecting vertical and horizontal slits simultaneously. A similar sensitivity, evaluated using the normalized peak-to-peak Vpp change, in detecting vertical and horizontal slits was obtained using the multi-channel fluxgates and multi-frequency orthogonal Eddy current induction. The analysis of result data indicates a 50% increase in sensitivity for detecting cracks in the real component when transitioning from a circular probe to an orthogonal probe, while the detection of cracks in the imaginary component shows a 13% improvement. The depth and length of multi-orientation slits were able to be characterized from the differential real and imaginary magnetic responses. A significant improvement of the slit reconstruction for crack-shaped slit was observed in the orthogonal Eddy current probe using the multi frequency mode. It was found that the limitation of the multi-frequency orthogonal Eddy current probe mode was 5 mm in terms of slit length and better than 0.3 mm in terms of slit width. In the future, the developed system can be expected to help the development of a more sophisticated crack detection system for steel components.
THE0009871 (Local)
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