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