Abstract
The continuous magnetization is usually applied to the Magnetic Flux Leakage (MFL) testing for ferromagnetic specimens. In this case, the detectability of flaws may be strongly influenced by the stray field from magnetization equipment. This paper describes the MFL technique for surface flaws in ferromagnetic components by using the residual magnetization procedure without the stray field. The Magneto-Impedance (MI) devices having high sensitivity to magnetic fields was used as a magnetic sensor to measure residual magnetic leakage flux density distribution arising from flaws, which were considered to be very low field strength.
Firstly, the vertical components of the residual magnetic leakage flux density distribution from the parallelepiped flaws were measured by the MI sensor, and the measured results showed to be symmetric with respect to the center of parallelepiped flaws. A lot of noise was also measured because the MI sensor is sensitive. Therefore, the wavelet decomposition and reconstruction techniques were applied for the cancellation of the noise. Comparing these experimental results with ones by an ordinary Hall probe, excellent nature of this MI sensor having high sensitivity and high resolution in the proposed MFL system has been confirmed. And then, the quantitative evaluation method for surface flaws based on the MFL technique by combined use of residual magnetizing procedure and the MI sensor was presented.
Moreover, the test specimen with the butt weld and the coating was measured by the MI sensor. It was clear that flaws were detected by the cancellation of the weld signals and the signal processing.
