Detection of Inner Cracks in Thick Steel Plates Using Unsaturated AC Magnetic Flux Leakage Testing with a Magnetic Resistance Gradiometer

In order to ensure the safety of infrastructure, it is important to be able to detect both surface and inner cracks in the thick steel plates used. However, it is difficult to use conventional magnetic measurement techniques for inspecting inner cracks in steel because of its high permeability as well as owing to the variations in it. To solve this problem, we developed a method called unsaturated ac magnetic flux leakage (MFL) testing based on a magnetic resistance gradiometer for analyzing the inner cracks in steel. The proposed method uses a sensor probe consisting of a semicircular yoke with induction coils at each end, and a gradiometer with two anisotropic magnetic resistance sensors for detecting the components perpendicular to the steel surface. The conventional MFL testing method requires a strong power source for observing the MFL in the magnetic saturation region. In contrast, in this paper, a gradiometer-based method for detecting the differential intensity and phase is used to detect low levels of magnetic leakage using a weak power source when the test steel sample is unsaturated. During the measurements, a deep inner crack could be detected by decreasing the frequency. The line-scanned differential signal exhibited a peak just above the crack position and was observed to be depth dependent. Based on these results, the method could be used to determine the position of the crack as well as its depth.