resolution of this system was not sufficient to detect
the stress concentration due to a notch cut into the
axle. Implementation issues, such as surface prepa-
ration, were not addressed in this work instead, this
was a preliminary investigation of the technique’s
sensitivity and whether it was sufficient for inspect-
ing an axle.
LASER SHEAROGRAPHY
In 2016, researchers at MxV Rail explored and
tested potential applications for using the laser
shearography NDE method for axle inspection. In
this approach, the surface is illuminated with laser
light, and a camera equipped with special optics
photographs the surface twice, once at a neutral
state and once at a stressed state. However, while
laser shearography could detect surface cracks and
near-surface defects in the axle body, the method
was susceptible to surface displacements caused by
near-surface defects. Figure 11 shows the test setup
in which a thermal load was manually applied
using a heat gun. Shearography revealed the local-
ized surface strains formed around the notch. The
interferograms from the unstressed (original) and
stressed states (after thermal loading) were used to
calculate the phase map (the first derivatives of out-
of-plane displacements). The notch was visible in the
shearography result. The sensitivity for near-surface
defects was excellent however, this technique did
not detect forging defects inside the axle body. Any
stress anomaly far enough away from the surface that
its strain localization effect was dissipated could not
be detected by shearography. In addition, this test
did not address implementation concerns related to
capturing images on a moving axle.
RESONANCE TESTING
The resonance testing method can be used to detect
shifts in the axle resonance frequencies induced
by cracks in the axle. All objects have resonance
frequencies that are fundamental to their physical
properties, and these resonances change along with
any changes in shape, size, mass, rigidity, and other
physical properties. These resonance frequencies
have multiple modes that describe how an object
will vibrate upon impulse excitation. These vibra-
tions dissipate rapidly but can be captured and
converted from the time domain to the frequency
FEATURE
|
RAILROADS
Figure 10. 3D
DIC test setup:
(a) application of
the speckle pattern
(b) experimental
setup in the FAST
pit (Witte and
Poudel 2016).
Axle
EDM notch
Camera
Tripod Laptop
Notch
White paint
marker
Heat gun
Axle
Laser shearography
camera and optics
Figure 11. Laser
shearography NDE
of cracked axle:
(a) test setup
(b) shearography
image result.
34
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domain, showing which resonance frequency (or
frequencies) exist for that object.
From 2005 to 2007 (Verhelst 2008), under
the European Commission and its consortium,
the WOLAXIM program, supported a project
titled “Wheelset Integrated Design and Effective
Maintenance (WIDEM)” for the development of a
compensated resonance inspection prototype for
wheelsets. This project aimed to improve inspection
techniques on axles and expand the lifetime of axles
in Europe. A set of fully decommissioned axles, both
good and cracked, were tested in a suspended static
test rig. The axles were excited by an impact source
from a tap hammer, and the resonance frequen-
cies were recorded and analyzed. The WIDEM data
showed that resonance frequency shifts on fatigued
axles were significant.
In 2017 and 2018, researchers at MxV Rail
expanded on the WIDEM work by conducting
research to determine the feasibility of using res-
onance testing to detect axle discontinuities on
moving trains (Poudel and Witte 2018 FRA 2020).
The objective of this work was to determine the
range, sensitivity, and repeatability of the measure-
ment that will be required to detect an axle dis-
continuity and also to assess whether the result is
feasible for in-motion detection. In the first step, the
measurement range and repeatability were deter-
mined using accelerometers mounted to the axle on
a wheelset rolling in a laboratory rig. It was demon-
strated that, with appropriate excitation, the axle
resonance exceeded rolling noise at high frequen-
cies (above 20 kHz) and was repeatedly measured
within 3 dB for a single impact (tap testing) on a
rolling wheelset. The measurements were shown to
be repeatable for each axle tested. The resonance
response of a given axle/wheel assembly is more like
a fingerprint than a characteristic, in that it is unique
to each assembly. However, the resonance differ-
ences between new axles of the same type were in
the same order as the resonance differences between
nondefective and artificially notched axles. This
finding implies that axle anomalies will only be iden-
tifiable if a baseline resonance test is stored for every
axle in the sample population. In other words, a
baseline resonance response must be taken for every
axle/wheelset that will be monitored by this method.
INFRARED THERMOGRAPHY
In 2017 and 2018, researchers from MxV Rail and
Boeing Research &Technology (BR&T) explored the
use of infrared thermography (IR) NDE methods
to develop an in-motion technology for detecting
cracked axles on moving trains (Poudel and Witte
2020). Several experiments were designed and
conducted to eliminate the most relevant unknowns
related to applying the flash IR NDE method to
moving axles and demonstrating flash IR inspec-
tion capabilities. The key findings from this work
were that the IR approach could detect fine surface
or near-surface cracks of varied shapes, sizes, and
orientations within the body of the axle in a moving
car using high frame rate IR cameras. In order to
measure 20% to 25% of the circumference, four or
five flash infrared camera systems must be spaced
to image the axle at angles 72° to 90° apart. A stand-
off distance of 1.5 m or more between the camera
and the axle, with off-line-of-sight surface angles of
more than 30°, are easily achieved, with minimal
reduction of crack detectability. Figure 12 shows
Magnetic sensor for triggering
Ramp for rolling the test sample
IR camera head
Giraffe system
Saturation of detectors
Figure 12. Flash IRT:
(a) in-motion laboratory test
setup (b) in-motion flash
IRT results on axle with EDM
notches (c) high speed (1000
frames/s) long-wavelength
flash IRT results for the test axle
rotated on the lathe at 200 rpm
(Poudel and Witte 2020).
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