In North America, railcar axles are classified
based on the approximate weights of the cars as
defined in the Association of American Railroads
(AAR) Manual of Standards and Recommended
Practices (MSRP) Section G: Wheels and Axles (2022).
Table 1 shows the standardized AAR axle designs
and recommended load ratings for freight and pas-
senger car axles.
Failure of railway axles is typically attributed to
fatigue crack initiation and propagation phenomena.
In addition, nonmetallic inclusions from manufac-
turing and surface damage incurred during handling
or while in service can accelerate axle fatigue. The
effect of surface damage (e.g., dents, gouges, ballast
impacts) in the axle body, and fretting from corrosion
pits under the backing ring or near the wheel seat,
create stress risers that provide sites for fatigue initia-
tion. The mileage, tonnage, and environmental con-
ditions in which railcars operate all have cumulative
effects on fatigue resistance and crack propagation.
Several previously published reports and literature
have detailed the premature failure of axles attributed
to slow fatigue crack growths across the body and
journal fillet areas of the axle (Hoddinott 2004
Lonsdale and Stone 2004 TSB 2001, 2004, 2007, 2018,
2019). Figure 2 shows fatigue-initiated broken axles.
Timely detection and characterization of axle
indications are vital to ensuring the structural integ-
rity of wheelsets used in rolling stock. This structural
integrity is an essential aspect of safe and economi-
cal railroad operation, with implications for both the
railroads and their customers. Therefore, conducting
NDE inspections on moving trains in an in-service
environment in order to detect fatigue cracks on
railway axles would be least disruptive to operations.
Railcar Axle Inspection Technologies
Railcar axles undergo various inspections to ensure
their structural integrity and safe operation. The
specific inspection methods will vary depending
on the type of inspection being conducted and the
FEATURE
|
RAILROADS
Wheel
Bearing
Backing ring Axle body
Press seats
Rail
Wheel seat
Journal seat
Tapered end
Journal fillet
Transition radius
Dust guard seat
Figure 1. Schematic of a railway solid axle.
Figure 2.
Fatigue-
initiated solid
axle failures:
(a) axle body
(b–c) journal
fillet radius.
T A B L E 1
AAR axle designs and load ratings for freight and passenger car axles
Freight cars
AAR axle
designation Journal size (mm) Load carried by each
axle (metric ton)
Maximum weight on
railcar (metric ton)
E 152 × 279 24 100
F 165 × 305 31 130
G 178 × 305 34 143
K 165 × 229 31 130
L 152 × 203 24 100
M 178 × 229 34 143
Passenger cars
AAR axle
designation Journal size (mm)
Capacity for axles (metric ton) for normal
maximum operating speeds of:
Up to and including
137 kph 138 to 161 kph
D 140 × 10 16 15
E 152 × 279 20 19
F 165 × 305 24 23
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requirements of the rail industry. Internationally,
the rail industry follows specific guidelines and
regulations for axle inspections, and inspection fre-
quencies will depend on factors such as axle usage
and operating conditions. Qualified inspectors and
other nondestructive testing professionals regularly
conduct inspections to ensure compliance with
safety standards and to identify any issues that may
require maintenance or replacement. One example is
a full axle body inspection that requires the removal
of wheels, bearings, backing rings, and bearing caps
before the inspection. In North America, axle fatigue
cracks in the journal fillet radius are not detectable
during routine safety inspections because that part
of the axle is concealed by the roller-bearing backing
ring. Therefore, visual inspection in the journal
fillet radius is possible only during wheel and roller
bearing replacement or axle reconditioning.
The most common NDE methods currently used
for axle inspection include electromagnetic induction–
based techniques, such as magnetic particle testing
(MT) and alternating current field measurement
(ACFM), as well as a variety of ultrasonic testing (UT)
techniques. Researchers have also been exploring
other novel NDE approaches, which are described in
this paper. However, these novel approaches are not
yet commercially available and are currently in differ-
ent stages of research and development.
Magnetic Particle Testing
MT is a basic NDE method that uses magnetic fields
to detect surface and subsurface defects in ferromag-
netic materials. The underlying physics behind MT
involve the concept that a discontinuity in the test
piece will interrupt the flow of the magnetic lines of
force, thus forming opposite magnetic poles. When
fine magnetic particles are sprayed onto the surface
of the magnetized specimen, the particles will be
attracted by the new magnetic poles and provide a
visual representation of the surface discontinuity.
MT is generally more effective in detecting surface
defects compared to subsurface defects. For deeper
subsurface defects, UT may be more suitable.
In North America, when railcar axles undergo
inspection for surface cracks, the AAR standards
recommend using the MT NDE method during
wheel and roller bearing replacement or during axle
reconditioning. During the inspection, the surface
is cleaned, and the axle is placed on a fixture to be
rotated for testing. An encircling coil magnetizes
the axle to detect indications in the axial direction.
Similarly, indications along the radial direction are
detected by moving the motor-driven coil (with
the front side shower ring activated) along the part
(Maass et al. 2014). Figure 3 shows wet fluorescent
MT systems used to detect longitudinal and trans-
verse cracks in axles and wheelsets.
Figure 3. Wet fluorescent
MT of railcar axle inspection:
(a) axle inspection system
(b) wheelset inspection system
(c) longitudinal crack in axle
body (d) fretting fatigue cracks
in journal fillet radius.
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