Structural optimisation of a standard alternating current field measurement excitation probe for concave and convex surfaces
The use of the alternating current field measurement (ACFM) method can produce inaccurate results when an excitation probe is positioned on an uneven surface. In this paper, the effect of a standard excitation probe on a measurement signal is studied with the aim of improving conventional ACFM crack size measurements. A U-shaped core structure is selected to study four different geometrical parameters for leg height, core length, cross-section size and lift-off height. The characteristic distribution of the electromagnetic field signals is determined for different excitation parameters and the optimal geometric parameter of the magnetic core for defect detection is identified. The results indicate that the signal sensitivity decreases with increasing core leg and lift-off height within a typical range. When the leg height (h) is 87.5 mm, the defect location process is more sensitive and the measured defect size is more accurate. A change in the length (W) of the upper core has a very small effect on the accuracy of defect detection. In other words, the core length is practically irrelevant for the detection. The error is small for a core width of 60 mm, which means both detection accuracy and sensitivity are high. In addition, after analysing the simulation results, 0 mm is identified as the optimal lift-off height for the U-shaped ACFM probe. Considering the production of U-shaped ACFM probes and a ±1 mm lift-off variation, it is recommended that the initial lift-off height for practical engineering measurements is found to be 1 mm. Furthermore, a standard excitation probe, which can detect the uneven surface of a workpiece, is designed. The results serve as a theoretical basis for optimising the design of standard excitation probes in the future.
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Document Type: Research Article
Publication date: September 1, 2019
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- Official Journal of The British Institute of Non-Destructive Testing - includes original research and devlopment papers, technical and scientific reviews and case studies in the fields of NDT and CM.
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