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ReseaRch
134 www.railwaystrategies.co.uk
Rail networks across Europe are
getting busier with trains travelling
at higher speeds and carrying
more passengers and heavier axle
loads than ever before. The combination of
these factors has put considerable pressure
on the existing infrastructure, leading
to increased demands in inspection and
maintenance of rail assets due to the higher
risk of catastrophic failure. The expenditure
for inspection and maintenance has thus
grown steadily over the last few years.
Although, severe rail accidents are
relatively rare within the EU, their frequency
of occurrence is still at an intolerable level.
Classification of rail accidents depends on
their causative factor, which can be either
a human error, infrastructure defect or
train equipment failure. A number of all rail
accidents are infrastructure-related, while a
certain proportion of these take place due to
rail failure. The continuous increase in train
operating speeds means that catastrophic
failure of a rail may result in very serious
derailments, such as the one that took place
in Hatfield, UK in October 2000, causing loss
of life, critical injuries, severe disruption in
the operation of the network, unnecessary
costs, and loss of confidence in rail transport
by the public. In the Hatfield accident, a
rail section fractured under an Intercity 225
train travelling at 185 km/h on route from
London to Leeds causing it to derail. The
Hatfield derailment led to the death of four
passengers and the severe injury of seventy
more, while normal network operation was
disrupted for several weeks after the accident.
In-service rail tracks are subjected
to intense bending and shear stresses,
UltrasonicsUltrasonic inspection is carried out by a
variety of different instruments ranging from
hand-held devices, through dual-purpose
road/track vehicles to test fixtures that are
towed or carried by dedicated rail cars.
Unfortunately, the performance of existing
conventional ultrasonic probes in detecting
small surface defects such as head checks
and gauge corner cracking is inadequate
during high-speed inspection generating a
number of false alarms and hence resulting
in higher inspection times and associated
costs. This is also one of the reasons that
the current international practice is to
combine non-destructive evaluation of the
rail network with preventive maintenance
procedures, such as rail head grinding, in
order to optimise the trade-off between
maintenance cost and structural reliability.
Furthermore, the quality of ultrasonic
inspection can be adversely affected by rail
corrugation.
Eddy currents
Eddy current testing is applicable for the
inspection of the surface and near-surface
areas of rail head. However, the operation
of eddy current probes is sensitive to lift-off
variations. For that reason, the probes need
to be positioned at a constant distance (no
more than 2mm away) from the surface of
the rail head and particular attention needs
to be given to any lift-off variations that may
occur during inspection. The performance
of eddy current sensors can therefore
be adversely affected by the presence of
grinding marks on the rail.
Rail inspection
plastic deformation and wear, leading to
degradation of their structural integrity
with time. Defects in rails can develop in
the rail head, web or foot. The rail head
is the part of the rail where the defects
occur more often. Rail head defects can be
distinguished as those having internal origin,
such as progressive transverse cracking or
kidney-shaped fatigue cracks, and those
having surface origin, such as Rolling
Contact Fatigue (RCF) damage (including
gauge corner cracking, head checks, squats,
shelling, and corrugation), wheelburns, and
indentures. Rail web and rail foot defects
include longitudinal and vertical cracking,
cracking occurring at fishplate bolt holes or
other holes found in the web (star-cracking),
transverse fatigue cracking, and rail foot
corrosion.
To increase the reliability of the railway
network and improve the efficiency of
maintenance procedures rail tracks are
inspected at regular intervals for internal
and surface defects, as well as rail profile
irregularities and wear, missing fastenings,
failed sleepers, and abnormal variations in rail
gauge. Rail tracks are inspected either visually
by appropriately trained personnel walking
along the tracks and noting down defects
(a relatively subjective procedure which
may occasionally involve errors and misses
and does not provide any information with
regards to the presence of internal defects
or failed sleepers), or using a number of
common rail inspection techniques, including
ultrasonics, magnetic induction (or magnetic
flux leakage), eddy current sensing and
automated visual inspection.
MAYORKINOS PAPAELIAS, CLIVE ROBERTS and CLAIRE DAVIS review the range of inspection techniques under investigation at the University of Birmingham Centre for Rail Research and Education
technology
ReseaRch
www.railwaystrategies.co.uk 135
Magnetic Flux LeakageThe application of Magnetic Flux Leakage
(MFL) sensors is mainly focused on the
detection of near-surface or surface-breaking
transverse defects, such as RCF cracking.
However, transverse fissures are not the only
types of defects found in rails, which can
include deep internal cracks and rail foot
corrosion. These defects are not detectable
with the MFL method either because the
fissures run parallel to the magnetic flux
lines and hence they do not cause sufficient
flux leakage, or they are too far away from
the sensing coils to detect (i.e. the rail web
and foot). MFL is also adversely affected by
increasing inspection speed. With increasing
speed the magnetic flux density in the rail
head decreases and as a result, the signal
becomes too weak for the detection of
defects at speeds that exceed 35km/h.
Inspection systems based on the
simultaneous use of conventional ultrasonic
transducers with MFL sensors have a higher
probability of detecting smaller near-surface
and surface-breaking defects in the rail head.
However, as inspection speed increases,
the performance of MFL sensors tends to
deteriorate rapidly due to a reduction in
the magnetic flux density. More recently,
Pulsed Eddy Current (PEC) probes have been
added on certain ultrasonic test trains to
offer increased sensitivity in the detection
of surface defects at high inspection speed.
PEC probes perform better than MFL sensors
at higher inspection speeds but, as it was
mentioned earlier, they are affected more by
lift-off variations.
Alternating Current Field MeasurementAlternating Current Field Measurement
(ACFM) is an electromagnetic inspection
method capable of both detecting and sizing
(length and depth) surface breaking cracks
in metals. The basis of the technique is that
an alternating current can be induced to
flow in a thin skin near the surface of any
conductor. By introducing a remote uniform
current into an area of the component under
test, when there are no defects present
the electrical current will be undisturbed.
If a crack is present, the uniform current is
disturbed and the current flows around the
ends and down the faces of the crack, thus
allowing its detection and sizing as shown
in Figure 1. ACFM sensors are less affected
by lift-off variation than eddy current probes
and can operate even at 5mm away from
the rail surface.
Automated vision systemsAutomated vision systems can operate at
very high velocities (speeds up to 320km/h
are possible depending on the nature of
the inspection) and are typically used to
measure the rail profile and percentage of
wear of the rail head, rail gauge, corrugation
and missing bolts. Certain advanced vision
systems can be used for the detection of
RCF and other types of surface damage
such as wheelburns at slower inspection
speeds (<10km/h). Despite the usefulness of
automated vision systems, their applicability
is restricted to the detection of surface
features only and therefore the inspection
needs to be repeated using ultrasonics
sensors if internal defects are to be detected.
Radiographic inspectionRadiographic inspection of rails can be
carried out using either gamma or X-ray
sources. In the past radiography was carried
out more often using a gamma-ray source
and film to obtain a radiograph of the
inspected area of a rail. With the advent of
portable digital X-ray detectors, the use of
X-ray sources became more commonplace.
Radiography, although a particularly
efficient non-destructive evaluation (NDE)
method for inspecting rails for internal
flaws, inherently involves health and safety
drawbacks. Furthermore, the inspection is
time-consuming and for that reason, it is
only applicable as a means of verification
in places where defects have already
been detected using other non-destructive
evaluation techniques or in rail areas, such
as aluminothermic welds, and switches and
crossings, where inspection with other NDE
methods is unreliable and not very efficient
in detecting transverse rail defects. Figure 2
shows a three-dimensional reconstruction
image of a small cracked rail sample generated
using Computed Tomography.
Other methodsOther rail inspection techniques, which
are currently under different stages of
development, include the use of long-
range ultrasonics, electromagnetic acoustic
transducers (EMATs) and laser ultrasonics. Each
of these techniques has proved to have its
own technical problems and limitations and
for that reason they have so far found limited
application out in the field. There is also a lack
of standards, which needs to be addressed
before these techniques can be legitimately
adopted by the rail industry. However, for the
time being the main problems restricting the
use of these techniques are of technical nature.
Furthermore, a fundamental shift to inspection
techniques where new inspection standards
may be required and on which existing
maintenance staff personnel has limited
expertise would be undesirable. Therefore
a prolonged period of trials is likely to be
required before these new techniques can be
approved for widespread application.
Research workThe Centre for Rail Research and Education
in the University of Birmingham is extensively
involved in research focusing on the
development of advanced rail inspection
methods carried out in collaboration
Figure 1: Schematic showing the ACFM
principle
Figure 2: 3D reconstruction of a small rail
sample containing RCF cracks
136 www.railwaystrategies.co.uk
ReseaRch
with partners from both industry and
academia. The techniques currently under
investigation include, Alternating Current
Field Measurement (ACFM), eddy current,
conventional ultrasonics, non-contact
ultrasonics based on EMATs, acoustic
emission, ultrasonic phased arrays and
digital radiography. High-speed inspection
experiments are carried out using a
3.6m diameter spinning rail rig capable of
rotating at speeds between 1-80km/h. The rig,
shown in figure 3, is unique in the world and
provides a means of evaluating various NDT
technologies in a laboratory environment with
‘close to real world’ conditions.
The Centre is currently collaborating in
the field of advanced rail inspection with
several research and industrial organisations
throughout Europe, such as TWI Ltd., the
University of Warwick, the University of Bristol,
Network Rail, CEA, Physical Acoustics, NDT
Consultants, SNCF, Portuguese Railways, TSC
Inspection Systems, Envirocoustics, Feldman
Enterprises, EMEF, VTG, ISQ, Tecnogamma,
MERMEC Group, NKUA, APT Rail, STIB and Die
Lijn, to name some. The research carried out
at the Centre is funded directly by the industry,
the UK’s Engineering and Physical Sciences
Research Council and the European Research
Agency.
INTERAILThe Birmingham Rail Research Centre is
currently involved in INTERAIL (contract
number SPC8-GA-2009-23040), a major
collaborative research project on the
development of an integrated high speed rail
inspection system which is supported by the
European Research Agency. This five million
euro project officially launched in October
2009 in Lisbon, Portugal and is jointly led by
the Instituto de Soldadura and Qualidade and
the Birmingham Rail Research Centre with
the involvement of 12 more partners across
134 www.railwaystrategies.co.uk
l Dr Mayorkinos
Papaelias is a senior
research fellow at the
Birmingham Centre
for Railway Research
and Education at
the University of
Birmingham. He
specialises in the development of
novel non-destructive testing techniques
for the rail industry.
l Dr Clive Roberts
is a senior lecturer
in railway systems
at the University of
Birmingham. He is
director of research for
the Birmingham Centre
for Railway Research
and Education. His research interests
lie in the field of railway condition
monitoring and non-destructive testing,
systems engineering, energy simulation
and traffic management.
l Prof. Claire Davis is
a professor of ferrous
metallurgy working in
the School of Metallurgy
and Materials and the
Birmingham Centre
for Railway Research
and Education at the
University of Birmingham. She specialises
in relating material microstructure and
cracks to mechanical properties and NDT
output.
Web: www.railway.bham.ac.uk
Europe. The successful implementation of the
key deliverables of the INTERAIL project will
have a profound importance in the efficiency
of railway inspection technology available to
infrastructure managers. More information
about the INTERAIL project can be found on
www.interailproject.eu. Figure 4 shows
the overall concept of the INTERAIL approach
to rail inspection. INTERAIL is expected
to conclude in September 2012 with a
demonstration of the integrated inspection
technology developed by the consortium
members.
Figure 3: Spinning test rig. The rig was purchased by the University of Birmingham as part of an Advantage West Midlands (AWM) equipment grant
Figure 4: Simplified schematic showing the overall concept of the INTERAIL high-speed rail inspection and defect verification platform
n