Authors: Yong Li, Gui Yun Tian *, Steve Ward
NDT&E International 39 (2006) 367373
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
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: email@example.com (G.Y. Tian).
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. .
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 . Recent progress in circumferential MFL includes
the improvement on probe velocity up to 4 m/s with a pipe wall thickness
of 25 mm .