Non-destructive testing will estimate the size, shape and state of defects without damaging the structures. Non-destructive testing methods include eddy current inspection (ECT), ultrasonography, X-ray inspection, etc. Ultrasonography shows good accuracy and reproducibility, but it needs direct contact with specimens and special liquids, which means complicated procedure and high cost. X-ray inspection has the disadvantage that operators can be exposed to irradiation. Although eddy current technology has the advantages of relatively simple structure, no contact with test specimens, low cost and fast testing, it still has a disadvantage. If sine wave is used as an excitation current, the increase in power can cause the excitation coil to be severely heated and damaged, and increasing frequency limits the depth to detect defects.
Pulsed eddy current (PEC) technique can overcome these limitations. PEC technique is widely used for contactless inspection of large areas of pressure body or tube made of ferromagnetic materials.
What is important in detecting internal defects of nonmagnetic materials by PEC technology is to increase the excitation power and the sensitivity. However, some parameters such as pulse amplitude and width cannot be increased constantly. Differential signals of defective and non-defective parts must be obtained to identify defects. The key to designing PEC probes is to optimize the magnetic field near defects to the maximum.
Jo Kwang Myong, a researcher at the Faculty of Mechanical Science and Technology, has built a finite element model of a concentric sensor and obtained the type of a relatively sensitive sensor, and, on this basis, he developed a sensor. In addition, he has made a differential sensor in the same way and compared it with the concentric sensor.
From the internal defects located 3mm below the surface in a stainless steel plate of 4mm in thickness, he determined the structural dimensions of a concentric sensor for maximum signals and manufactured it for a test. The concentric sensor determined the presence or absence of defects, but not their exact size or depth. In the meanwhile, the same test by the differential sensor showed that its signal strength is very high with a significant change in the depth of defects and it can fully process them without amplifying the signals.
His study demonstrates that differential sensors are effective in detecting the internal defects of nonmagnetic materials.
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