Safe Rail Transport via Nondestructive Testing Inspection ◾ 53
3.4.3.1 Conventional Ultrasonic Rail Inspection
During the inspection of rails using conventional ultrasonic probes, a beam of
ultrasonic energy is transmitted into the rail. e reected or scattered energy of
the transmitted beam is then detected using a collection of transducers. e ampli-
tude of any reections together with when they occur in time can provide valuable
information about the integrity of the rail. Ultrasonic inspection is carried out by
a variety of dierent instruments ranging from handheld devices, through dual-
purpose road/track vehicles to test xtures that are towed or carried by dedicated
rail cars. A couple of diculties encountered by the ultrasonic methods are: very
cold weather, where ice interferes with testing by providing an intervening inter-
face; damaged rail where a sliver of rail can slice/puncture a tire; heavily applied
lubrication can aect the inspection results, producing an intervening interface.
3.4.3.2 Laser Ultrasonic Rail Inspection
Laser ultrasonic testing combines the sensitivity of ultrasonic inspection with the
exibility of optical systems in dealing with complex inspection problems. Its
remote nature allows the rapid inspection of curved surfaces on xed or moving
parts. It can measure parts in hostile environments or at temperatures well above
those that can be tolerated using existing techniques. Its accuracy and exibility
have made it an attractive new option in the NDT rail infrastructure eld.
Laser-based ultrasonics is a remote implementation of conventional ultrasonic
inspection systems that normally use contact transducers, squirter transducers,
or immersion systems. Laser ultrasonic systems operate by rst generating ultra-
sound in a sample using a pulsed laser. When the laser pulse strikes the sample,
ultrasonic waves are generated through a thermoelastic process or by ablation. e
full complement of waves (compressional, shear, surface, and plate) can be gener-
ated with lasers. When this ultrasonic wave reaches the surface of the sample, the
resulting surface displacement is measured with the laser ultrasonic receiver based
on an adaptive interferometer. In Reference [42], preliminary tests showed that
Not
detected
Initiation point
(a) (b) (c)
Figure3.6 Coverage of train-mounted and walking stick probes (a), geometrical
limitations on the current ultrasonic inspection (b), and example of a rail break
due to small rail foot defect (c). (Data from http://www.networkrail.co.uk/.)
54 ◾ Advances in Communications-Based Train Control Systems
the developed laser ultrasonic system can be used to inspect the entire rail section
including rail head, web, and base.
3.4.3.3 Phased Array Ultrasonic Rail Inspection
e main advantage of using arrays in NDT over conventional single-element
transducers is the ability to perform multiple inspections without the need for
reconguration and also the potential for improved sensitivity and coverage.
Aparticular array is able to undertake a range of dierent inspections from a
single location and so is more exible than a single-element transducer. Infact,
an array can generate ultrasonic elds of almost innite variety. However, they
are most commonly used to produce elds similar to those fromtraditional
single-elementtransducers, that is, plane, focused, and steered beams. Additionally,
most types of arrays (with the exception of annular arrays) can be used to produce
images at each test location. is allows rapid visualization of the internal struc-
ture of a component.
Electronic scanning permits very rapid coverage of the components, typically
an order of magnitude faster than a single transducer mechanical system. Beam
steering (usually called sectorial or azimuthal scanning) can be used for mapping
components at appropriate angles to optimize the probability of detection of dis-
continuities. Sectorial scanning is also useful when only a minimal footprint is
possible. Electronic focusing permits the optimization of the beam shape and size
at the expected discontinuity location, as well as the optimization of the prob-
ability of detection. Overall, the use of phased arrays permits optimizing discon-
tinuity detection while minimizing testing time. No practical systems involving
ultrasonic phased arrays have been developed for high-speed rail inspection so
far, due to the problems that arise from the large amount of data that need to be
analyzed.
3.4.3.4 Rail Inspection Using Long-Range Ultrasonics
(Guided Waves)
Guided wave testing method is one of the latest developed methods in the eld of
ultrasonic NDT. e guided wave method usually employs low-frequency waves,
compared to those used in conventional ultrasound testing, typically between
10 and 100 kHz. Higher frequencies can be used in some cases, but a detection
range is signicantly reduced. e method employs mechanical stress waves that
propagate along a structure while guided by its boundaries. is allows waves to
travel a long distance with little loss in energy compared with unguided waves
of the same frequency, and as such numerous and widespread applications of
guided wave technology can be found in literature [43–44]. Rails are natural
excellent wave guides due to their installation as continuous welded lengths.
Safe Rail Transport via Nondestructive Testing Inspection ◾ 55
Inthis light, guided wave variations have been developed for rail track inspections
[37,45] where typically three sensor setups are encountered: (1) xed sensors on
rail (Figure 3.7), (2) guided wave rail inspection vehicle, and (3) sensor-on-train
system [45]. In the rst case, the sensors are permanently mounted on a rail,
inspecting mainly the areas where the probability for defect detection is very
high and the access to carry out the conventional NDT techniques is limited,
such as level crossings, switches and crossings (cast crossings), and tunnels. In the
second case, ultrasonic transducers are mounted on both ends of the inspection
vehicle, whereby energy can be induced into the rail at one end and received at
the other end. In the last case, the sensors would be mounted on the train and the
ultrasonic energy and vibrational patterns would be propagated forward from the
moving train and reected back with a modied pattern recorded by the trans-
ducer if defects were encountered.
3.4.4 Magnetic Flux Leakage Rail Inspection
Magnetic ux leakage is a magnetic method of NDT that is mainly used to
detect corrosion and pitting in steel structures, most commonly pipelines and
storage tanks. e basic principle is that a powerful magnet is used to magne-
tize the steel. At areas where there is corrosion or missing metal, the magnetic
eld “leaks” from the steel. In a magnetic ux leakage tool, a magnetic detector
is placed between the poles of the magnet to detect the leakage eld. Analysts
interpret the chart recording of the leakage eld to identify damaged areas and
hopefully to estimate the depth of metal loss. In rail inspection using magnetic
ux leakage, search coils xed at a constant distance from the rail are used to
Figure3.7 Transducer mounting for rail foot inspection.
56 ◾ Advances in Communications-Based Train Control Systems
detect any changes in the magnetic eld that is generated by a direct current
electromagnet around the rail. In the areas where a near-surface or surface trans-
verse defect is present in the rail, ferromagnetic steel will not support magnetic
ux, and some of the ux is forced out of the part. e sensing coil detects a
change in the magnetic eld and the defect indication is recorded [46]. e
magnetic ux leakage can detect mainly transverse ssures because the aws run
parallel to the magnetic ux lines or the aws are too far away from the sensing
coils to detect.
3.4.5 Eddy Current Rail Inspection
Eddy current inspection techniques have originated from Michael Faraday’s dis-
covery of electromagnetic induction in 1831. e principle of eddy current is based
on the phenomenon that occurs when an alternating current ows within a coil,
causing a changing magnetic eld to be produced. If the excitation coils producing
the changing magnetic eld are brought near the surface of a conductor, regardless
of whether it is ferromagnetic or paramagnetic, it will cause electric currents or
eddy currents to be induced within the conductor. Depending on the frequency of
the excitation alternating current as well as the conductivity and relative perme-
ability of the conductor, the eddy current eect may be stronger or weaker. By low-
ering the frequency of the excitation, alternating current eddy currents will tend
to ow at higher depths from the surface of the conductor. If higher frequencies
are used (e.g., in the range of several hundreds of kilohertz and above), the depth
that eddy currents will ow will be restricted signicantly. Based on Lenz’s law, if
there is no defect present, the induced eddy currents owing inside the conductor
will generate a secondary magnetic eld, which will tend to oppose the primary
magnetic eld created by the excitation coil. In the presence of a defect, the ow
of the induced eddy currents will be disturbed and hence the secondary magnetic
eld will uctuate, giving rise to changes in the impedance of the sensing coil.
ese impedance changes can then be related to the size and nature of the defect
detected [47].
For several years, the application of eddy current technology was limited for
inspection of individual rail welds. More recently, eddy current systems have been
developed to perform inspections on rails at speeds of a few meters per minute in
order to detect cracks due to RCF. Signicant developments in inspection of rails
using eddy current technology have been reported in Refs. [38,40,46]. e sensor
is pushed by the operator along the rail head who looks for changes in the signal
caused by the presence of RCF cracks or wheel burns. It is very important to guide
the eddy current probes so that the signals are not inuenced and the sensitiv-
ity does not uctuate due to lifto from the test surface. e rail inspection test
situation is especially complex, because the probe has to be positioned at an angle
relative to the guiding surface.
Safe Rail Transport via Nondestructive Testing Inspection ◾ 57
3.4.6 Alternating Current Field Measurement Rail
Inspection
Alternating current eld measurement (ACFM) is an electromagnetic inspection
method which is now widely accepted as an alternative to magnetic particle inspec-
tion method [49]. Although developed initially for routine inspection of structural
welds, the technology has been improved further to cover broader applications across
a range of industries. e technique is based on the principle that an alternating
current can be induced to ow 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 ows around the ends and
down the faces of the crack. Because the current is an alternating current, it ows in
a thin skin close to the surface and is unaected by the overall geometry of the com-
ponent. In contrast to eddy current sensors that are required to be placed at a close
(<2 mm) and constant distance from the inspected surface, a maximum operating
lifto of 5mm is possible without signicant loss of signal when using ACFM probes.
e incorporated ACFM array has been shaped to conform to the shape of the head
of the rail. is allows the application of the ACFM system in both new and worn
rails. e inspection across the rail head is carried out by sequentially scanning across
the group of sensors enabling the uninterrupted inspection of the rail.
3.4.7 Rail Inspection Using Electromagnetic Transducers
Electromagnetic acoustic transducers (EMATs) may be used to generate and
detect ultrasound in an electrically conducting or a magnetic material [50]. is is
achieved by passing a large current pulse through an inductive coil in close proxim-
ity to a conducting surface in the presence of a strong static magnetic eld, often
provided by a permanent magnet. e orientation of the magnetic eld, geometry
of the coil, and physical and electrical properties of the material under investigation
have a strong inuence on the ultrasound generated within the sample. EMATs
have the advantage that they operate without the need for physical coupling or
acoustic matching as it is an electromagnetic coupling mechanism that generates
the ultrasound within the sample skin depth. is also means that the perturbation
that physical coupling causes is insignicant and operation at elevated temperatures
is possible. EMATs are therefore suitable for rail inspection.
3.5 Comparison of NDT Techniques
Comparison of the advantages and disadvantages of the NDT methods available to
rail and fastening parts discussed earlier is shown in Tables 3.1 and 3.2.
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