Nondestructive evaluation of defects in welded sections of rails via unsaturated AC magnetic flux leakage measurements with a gradiometer

Tsukada, Keiji; Okayama University; Japan

Tsukada, K.; Okayama University; Japan
Majima, Y.; Okayama University; Japan
Nakamura, Y.; Okayama University; Japan
Hayashi, M.; Okayama University; Japan
Sakai, K.; Okayama University; Japan
Kiwa, T.; Okayama University; Japan
Sakashita, S.; Futaba Railways Industries Co., Ltd; Japan
Seki, M.; Futaba Railways Industries Co., Ltd; Japan
Furukawa, T.; Japan Power Engineering and Inspection Corporation; Japan

ID: ECNDT-0274-2018
Download: PDF
Session: Eddy Current-Techniques 2
Room: H2
Date: 2018-06-14
Time: 13:30 - 13:50

Railway has an extensive history. Continuous welded rail is currently widely used because it affords increased speed and allows heavy haul transportation. Consequently, the demand for more precise and convenient inspection methods to prevent serious risks has progressively increased. Eddy current testing and magnetic flux leakage (MFL) are noncontact and convenient methods; however, they are limited in their ability to perform surface and subsurface detection and are easily affected by lift-off variation and characteristic magnetization changes of the rail near the welding. In this study, we have evaluated the performance of crack detection of the welded segments of the rail and confirmed inner crack configuration via phased-array ultrasonic testing. Test rail samples with artificial surface and internal cracks at the centers of welded joints were prepared. An unsaturated AC (USAC-) MFL measurement is dissimilar to conventional MFL measurement in that the magnetic sensor can detect weak leaked AC magnetic fields when a weak magnetic field is applied in the unsaturated region of the rail. A gradiometer applying a pair of magnetic resistance (MR) sensors was employed to measure the magnetic field gradient. By analyzing each intensity and phase of the detected magnetic field, the respective differential signal intensity and phase were calculated. Surface cracks were easily detected, with scanning data yielding a peak at the crack position. In contrast to the single MR sensor data, the paired MR sensors enabled signal change detection as measured via the gradiometer; additionally, magnetic fluctuation caused the welding to deteriorate. Inner cracks of the rail were also detected, and deeper cracks were detected via frequency decrement. On comparing differential intensity and phase data, the differential phase data yielded increased clarity of signal change and reduced lift-off influence.