Several commercial axle inspection systems
are available for use in the shop and depot. They
require that either the wheelsets or bearings be dis-
mounted, and the bearing end caps be removed
from the axle. These automated systems use various
fixed phased array probes for full-volume coverage
to inspect for discontinuities along the length of the
axle. The wheelset is automatically positioned and
rotated in a gantry. At the same time, the vertical
arms deliver gimbaled holders that carry the ultra-
sonic probes. The inspection takes approximately 4
to 5 min per axle (Marty et al. 2012).
HOLLOW AXLES
Hollow axles are inspected similarly to solid axles—
that is, in the depot without disassembling the
wheelset or during overhaul, which requires dis-
assembling the wheelset. The PAUT technique, as
described for solid axles, is the primary technique
for inspecting hollow axles. Several automated
hollow axle inspection systems exist and are com-
mercially available, as shown in Figure 6.
In one of the automated systems (Marty et al.
2012), phased array probes are mounted on the end
of a shaft to inspect axles from the borehole. Phased
array ultrasonic probes are arranged in the internal
probe to inspect for circumferential and axial dis-
continuities in the axle. Simultaneous rotation
and axial movements generate a helical scan of
the entire axle, and the complete inspection and
handling time per axle using this system is approx-
imately 6 min. In addition, online roll-out C-scan
and B-scan images are displayed for reliable assess-
ment of the axle specimen. Another automated
system utilizes up to 10 angular and zero-degree
ultrasonic probes to maximize the probability of
detection in axles. For each set of angled probes,
one probe looks both forward and backward
(Gauna et al. 2018).
Emerging/Advanced NDE Technologies for
Axle Inspection
This section briefly highlights some of the initiatives
researchers around the world have conducted on
emerging/advanced NDE technologies, which are at
different phases of development and are not com-
mercially available.
INDUCED CURRENT FOCUSING POTENTIAL DROP
TECHNIQUE
The induced current focusing potential drop (ICFPD)
technique is similar to the ACFM technique in that it
works on the principle of electromagnetic induction.
The primary difference between these two tech-
niques is that the ICFPD technique uses induction
High-angle
scan
Zero-degree
scan
Far-end
scan
Solid axle
Near-end
scan
Figure 5. UT scan coverage in solid axles: (a) conventional UT (b) PAUT methods (Marty et al. 2012).
Figure 6. PAUT hollow
axle inspection system:
(a) production system
(Marty et al. 2012)
(b) PAUT results (Gauna
et al. 2018).
J A N U A R Y 2 0 2 4 • M A T E R I A L S E V A L U A T I O N 31
2401 ME January.indd 31 12/20/23 8:01 AM

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
M A T E R I A L S E V A L U A T I O N • J A N U A R Y 2 0 2 4
2401 ME January.indd 32 12/20/23 8:01 AM
40