wire. When an alternating current passes through
the induction wire, which is placed next to the
conducting material, the current is induced in the
material and flows through the surface layer due to
the skin effect. When a discontinuity is encountered
in the path of the current flow, it causes a potential
drop in the region of the axle with the discontinuity.
Detecting and measuring this drop is the main idea
behind the ICFPD technique, which was developed
to detect fretting fatigue cracks in the wheelset of a
high-speed train. Reportedly, the ICFPD technique
can detect cracks 1.5 and 2.0 mm deep in a press-fit
railway axle at a scanning line 5 and 10 mm away
from the pick-up pins without having to remove
the wheel from the axle (Figure 7) (Kwon and Shoji
2004).
ALTERNATING CURRENT INFRARED THERMOGRAPHY
The alternating current (AC) infrared thermogra-
phy technique applies infrared thermal imaging
technology to detect and analyze temperature vari-
ations resulting from the local heating that occurs
along the discontinuity when a high-frequency AC
is passed through a part or component of interest.
This approach was investigated by TWI, as shown
in Figure 8, under the support of the Whole Life Rail
Axle Assessment and Improvement (WOLAXIM)
consortium (Rudlin et al. 2012 Tian et al. 2005).
Although this technique looks promising, it suffered
technical difficulties during its implementation
phase and thus cannot be used to inspect the area
underneath the wheel seat and bearings.
LASER AIR-COUPLED ULTRASOUND
In 2004, researchers at MxV Rail (formerly TTCI)
developed an automated axle inspection system
that uses high-powered laser ultrasound (Morgan
et al. 2006) to detect the presence of fatigue
cracks in railcar axles. The goal of this research
was to develop a prototype wayside detection
system that automatically inspects railcar axles
for surface-breaking cracks. The prototype system
used a high-powered laser to generate ultrasonic
waves in the axle body and used air-coupled trans-
ducers to receive ultrasonic signals. Six axles were
tested during the early proof-of-concept (POC)
demonstrations in laboratory settings. The defects
in the test axles represented both in-service gen-
erated cracks and machined (EDM) notches. The
axles were positioned over the laser-illuminat-
ing zone and wheelsets were rolled by at walking
speeds. After the POC demonstration, 87.8% of the
defects were correctly identified, with only one false
positive. It was also determined that cracks farther
from the center of the axle body were more chal-
lenging to detect.
The initial design of the field inspection system
was installed at the Facility for Accelerated Service
Testing (FAST) in Pueblo, Colorado, and tested
through 2005. The system was designed to auto-
matically interrogate one-third of the axle with
prototype data acquisition and signal processing
software. This system again showed positive results
using the same test axles that were used in the POC
demonstration. Based on the test results from the
initial prototype, design efforts focused on improv-
ing the system’s capability and expanding the ability
of the system to inspect all axles of a passing railcar.
Figure 9 shows a photo of the installed system at
FAST, which can inspect the entire axle body during
a full-wheel revolution. Additional test axles were
provided for the testing of the improved prototype.
FEATURE
|
RAILROADS
3
40
Sensor
Axle Wheel
Potential
drop
Figure 7.
Schematic
of the ICFPD
measurement
system for railway
axles (Kwon and
Shoji 2004).
Figure 8. AC infrared
thermography
for railway axle
inspection: (a) test
setup (b) indication of
a warm wire on an axle
(Rudlin et al. 2012).
32
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40
During this testing, concerns over the system’s
capability were raised when the system did not
detect a 178-mm-long (measured circumferen-
tially) lab-generated defect. After encountering this
setback, additional laboratory work was completed
to determine the differences between laboratory
and in-service generated cracks. Both conventional
and laser ultrasonic methods were evaluated during
this study. Several defect types were studied and
compared for reflected energy, transmitted energy,
attenuation over the signal path, crack length,
depth, width, and fracture surface conditions.
This study revealed that the crack width influ-
enced the amount of energy reflected. It was also
determined that the minimum crack width nec-
essary to produce a detectable reflection was
approximately 20 µm. Axles collected from in
service ranged in crack width from 11.6 to 32.6 µm,
indicating that some in-service cracks are below
the minimum threshold. On the other hand,
machine-generated notches in an axle are over
250 µm wide. Additional laboratory work was com-
pleted to determine if other factors influenced the
identification of surface-breaking fatigue cracks in
railcar axles. Sources of variation (e.g., laser source
power, coupling efficiency, and ultrasonic attenu-
ation) were studied to determine if optimization
of the laser and air-coupled transducers could
improve the capability of the system to detect the
178-mm-long surface-breaking fatigue crack that
was not identified by the FAST prototype system.
Additionally, several other combinations of
ultrasonic technologies (e.g., water jets, thermi-
onics) were explored and tested in the lab envi-
ronment to determine if they could reliably detect
fatigue cracks in axles. The testing showed that the
prototype system provided inconsistent results and
therefore could not reliably be used to detect fatigue
cracks in axles. Specifically, the following chal-
lenges could not be resolved with the prototype as
designed:
Ñ Air-coupled transducers were extremely sensitive
to incident angle and weather fluctuations.
Ñ Reflected waves from in-service cracks
were severely attenuated compared to
machine-generated notches.
Ñ The “ringing” effect of air-coupled transducers
significantly impaired the ability to distinguish
reflections from cracks from baseline noise.
Ñ Additional wave modes were less repeatable in
the field than under lab conditions.
Ñ The large air gap (~406 mm) between the surface
of the axle and the air-coupled transducer signifi-
cantly attenuated the signal and reduced the
capability of the technology to distinguish defects.
In summary, although the technology provided
consistent results in a laboratory environment, the
field testing proved that the technology is too sen-
sitive to be used in the extreme operating environ-
ment of the North American railroads.
DIGITAL IMAGE CORRELATION
In 2016, researchers at MxV Rail explored the fea-
sibility of using a vision-based, 3D digital image
correlation (DIC) strain measurement technology
for cracked axle detection (Witte and Poudel 2016).
The technique uses high-resolution stereovision
cameras to track the motion of surface features
between different load states. Figure 10 shows the
3D DIC test setup. Experiments were conducted
on an axle with thin notches cut into the axle body
to simulate shallow cracks. The experiment was
designed to test the sensitivity of a commercially
available DIC system for detecting the strain local-
ization that occurs around a defect. The technique
measured bulk strain of about 300 microstrain
near the notch when the axle was subjected to the
maximum static load (the full weight of a loaded
car at 119 metric tons). It was determined that an
order-of-magnitude improvement in the sensitiv-
ity of the DIC system would be needed to detect
the strain localization around the flaw, because the
Figure 9. Laser-based automated in-track axle inspection
system.
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