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Authors: Yong Li, Gui Yun Tian *, Steve Ward

School of Computing and Engineering, University of Huddersfield Queensgate, Huddersfield HD1 3DH, UK
Received 19 September 2005; received in revised form 17 October 2005; accepted 18 October 2005
Available online 28 November 2005

Abstract:

High-speed non-destructive inspection (NDI) systems using magnetic flux leakage method (MFL) is in great demand in online metal inspection and defect characterisation, especially in pipeline and rail track inspection. For MFL systems at high speed measurement, in addition to magnetic flux, eddy currents exist in metal specimen because of the relative movement between the probe and specimen. These currents alter the profile of electromagnetic field, which increases difficulty in signal interpretation and defect characterization. In this paper, eddy currents generated by high speed movement and their characterisation in high-speed MFL inspection systems were investigated by using numerical simulations. Besides, the MFL signals from high speed MFL measurement against defect depth were examined.

Keywords: Magnetic Flux leakage method; High Speed inspection system Eddycurrent; Numerical simulation

* Corresponding author. Tel.: C44 1484 472 323; fax: C44 1484 451 883.
E-mail address: g.y.tian@hud.ac.uk (G.Y. Tian).

1. Introduction
Magnetic flux leakage method (MFL) is broadly used for electromagnetic non-destructive evaluation (EM NDE) techniques and widely employed in the petrochemical, transportation, energy and metal industries, such as rail tracks, pipelines, tubes, etc. [1–3].
In MFL, permanent magnets or DC electromagnets are applied in generation of magnetic field in order to magnetize the ferromagnetic specimen under inspection to saturation. The magnetic flux lines are coupled into specimen using metal ‘brushes’ or air coupling. If there are any anomalies or inclusions, the magnetic flux lines will leak outside the specimen in proximity of the anomalies and the sensor or sensor array will detect the leakage magnetic field, which conceives information on anomalies or inclusions such as corrosions, cracks, groovings, etc. [4,5].
For online or real-time MFL inspections, the conventional static inspection systems are unsuitable for dynamic inspection, particularly for signal interpretation, because the distribution of magnetic field will be influenced by eddy currents due to the movement of the probe, which leads to variation of MFL signals from sensor/sensor array when the probe travels above the inspected specimen at different speed [6,7].
According to the distribution of magnetic flux lines coupled into the specimen, MFL systems that comprise magnetizer and sensors/sensor array are categorised into two methods [1,6,8]: (1) circumferential MFL excelling in detection and sizing of longitudinal defects; (2) axial MFL that is apt to volumetric or metal-loss defects with a significant circumferential extent or width. Both methods suffer from the probe velocity effect on MFL signals. It has been reported that velocity effects for circumferential MFL are more significant than for axial MFL and the speed at which probe velocity influences the circumferential MFL is much lower than that for axial MFL. Furthermore, from simulations and practical experiments, probe velocity effects vary with the thickness of pipeline [4,5]. In the light of previous research, the velocity effects for axial MFL dramatically influence the MFL signals at speeds over 2.5 m/s in pipes with 12-in. diameter, whilst those for circumferential MFL are noticeable at speeds of 1.0 m/s [8]. Recent progress in circumferential MFL includes the improvement on probe velocity up to 4 m/s with a pipe wall thickness of 25 mm [6].