ABSTR ACT
This paper discusses the development of an in situ
noncontact electromagnetic acoustic transducer
(EMAT) nondestructive evaluation technology to
determine rail neutral temperature and estimate
rail stress in continuous welded rail (CWR). Stresses
develop in CWR due to a lack of expansion joints to
accommodate thermal expansion and contraction of
the rail when ambient temperatures vary over time.
The novelty of the work presented is the usage of
ultrasonic birefringence properties using EMATs to
estimate thermally induced stresses in rails. EMATs
produce polarized shear waves propagating through
the rail web in the pulse-echo mode. Experimental
tests were performed on machined 136RE and
141RE rail material with applied compressive and
tensile stresses to explore the stress-birefringence
behavior. Two additional sets of experimental tests
were conducted on full-size rail sections with in situ
surface conditions to study variations in the in situ
birefringence and the acoustic stress constant in
different rail materials including 115RE rail, 119RE
rail, two different 136RE rails, and 141RE rail. The
results show a highly linear relationship between
the stresses applied and the measured acoustic
birefringence.
KEYWORDS: continuously welded rails, acoustic birefringence,
EMAT, rail buckling, thermal stresses
Introduction
Continuous welded rail (CWR) has been an invaluable
enhancement to the rail transportation system around the
world since the evolution of thermite welding in the late 19th
century and the employment of this welding technique in the
railway industry in 1924 by the German State Railway and in
1930 by the Central of Georgia Railway in the United States
(Lonsdale 1999). CWR offers excellent advantages over the tra-
ditional mechanically connected rails using joint bars. CWR
allows it to operate at higher speeds, provide a smoother ride,
and need less maintenance due to the absence of joint gaps.
However, CWRs are susceptible to buckling or fracture due
to thermal stresses that develop in the rail due to the lack of
expansion devices to accommodate thermal expansion and
contraction. The rail is susceptible to buckling at elevated tem-
peratures, and tensile thermal stresses can lead to cracking
and propagation of existing defects during cold weather. Rail
buckling and defects can eventually lead to derailment of the
train from the track, which can result in increased costs and
safety issues (Huang et al. 2023 Liu et al. 2012).
Rail neutral temperature (RNT) is the temperature at which
the rail is neither in tension nor compression and is often
linked with the temperature at which the track was installed
and fastened. Designated rail-laying temperatures are often
established by railroads and are based on geographic and
average yearly ambient temperature to provide a specific
desired rail neutral temperature (DRNT) to prevent track
buckling in hot weather and pull-aparts and broken rails in
cold weather. However, RNT is not a fixed value and varies
along the length of the track and throughout the track’s lifetime
for several reasons. Repair of the damaged rail sections or
destressing operations to adjust to a desired RNT following
repair or maintenance can also affect the RNT. Heavy mechan-
ical forces from braking and tractive forces at specific segments
of the track (Kish et al. 2013) and the variability of constraint
from rail anchoring and track fasteners can also impact the
RNT (Miri et al. 2021).
Longitudinal rail stresses must be monitored and evaluated
periodically to prevent such scenarios and provide safe train
operations. Investigating the state of having a zero longitudinal
force is interchangeable with finding the RNT and can be gen-
erally related to the thermal forces in the rail as a function of
temperature using Equation 1: ​ ​ ​
(​​1)​​ ​ P =EAα(​​ ​ ​ TR​​ ​ − ​ TN​​​)​​​​​
BIREFRINGENCE TECHNIQUE FOR EVALUATING
THERMAL STRESSES IN RAILROAD RAILS
AQEEL T. FADHIL*, GLENN WASHER†, AND ANISH POUDEL‡
*University of Missouri, Department of Civil and Environmental Engineering,
416 S. 6th St., Columbia, MO 65201 aqeel.fadhil@uobaghdad.edu.iq
† University of Missouri, Department of Civil and Environmental Engineering,
416 S. 6th St., Columbia, MO 65201 washerg@missouri.edu
‡ MxV Rail, Research and Development, 350 Keeler Parkway, Pueblo, CO 81001
1-719-696-1848 anish_poudel@aar.com
Materials Evaluation 82 (1): 79–87
https://doi.org/10.32548/2024.me-04382
©2024 American Society for Nondestructive Testing
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where
P is the longitudinal force in the rail,
E is the modulus,
A is the cross-sectional area,
α is the coefficient of thermal expansion,
TR is the rail temperature, and
TN is the rail neutral temperature.
However, inferring the longitudinal stress directly using
Equation 1 is generally not practical since the RNT may change
over time and is affected by the track constraints and boundary
conditions that are difficult to characterize.
The overall goal of the research reported herein is to develop
a technology for in situ in-motion measurement of longitudinal
stresses in rails. The research objectives reported in this paper
are to establish the stress-birefringence relationship in common
rail materials and assess the potential variation of fundamental
acoustoelastic properties between different weight rail sections.
This paper is organized to present readers with a brief background
on the RNT measurement conducted by other researchers, the
underlying theory for the ultrasonic stress measurement (USM)
approach, and results from a series of laboratory tests. Conclusions
and future outlook are discussed at the end of this paper.
Background
Several studies have addressed the issue of assessing the RNT
and controlling axial stresses in railroad rails. These studies have
investigated many destructive techniques, semi-destructive tech-
niques, and nondestructive evaluation (NDE) methods. Before
the progression of the semi-destructive and nondestructive
stress evaluation techniques, early tests relied on the destruc-
tive method of rail cutting for measuring the axial forces. These
tests included cutting the rail in two places and measuring the
cut length change to assess the axial forces and RNT (Johnson
2004). A semi-destructive approach for measuring the thermal
stresses is the rail uplift method, which is the basis of contem-
porary technologies such as the VERSE® technology by Vortok
International and AEA technology in the UK (Shrubsall and
Webber 2001). This method is considered semi-destructive
because it involves unclipping part of the rail to perform the
measurement. The method is based on the beam-column
deflection theory by applying a vertical load to the rail and mea-
suring the resulting deflection to calculate the axial force (Kish
et al. 1993 Kish and Samavedam 1987). The rail-cutting method
and the semi-destructive procedure of rail uplift are consid-
ered the most reliable methods to monitor rail buckling force
with sufficient accuracy and reliability (Kish et al. 2013). Strain
gages have also been implemented to evaluate longitudinal
rail stresses in rails but the installation of many strain gages is
required if a long rail path is to be monitored (Kish et al. 1982
Liu et al. 2018 Weaver 2006). For tracks in service, strain gage
usage requires either the knowledge of the longitudinal forces in
the rail by other means before installing the strain gages, or the
zero-force condition has to be established by cutting the rails
before installing the strain gages (Enshaeian and Rizzo 2021).
Several NDE methods have been investigated to estimate
longitudinal rail stresses and RNT. Some of these include
magnetic-based methods that rely on the ferromagnetic prop-
erties of rail steel, such as the magnetic Barkhausen noise
(MBN) effect (Posgay and Molnár 2011 Wang et al. 2013),
acousto-magnetic interactions (Burkhardt and Kwun 1988
Kwun et al. 1990), and variations in magnetic properties of the
steel (Utrata et al. 1995). However, the outcome of these efforts
reported that this technique is confined to assessing surface
stresses and is also affected by localized variation in metal-
lurgy. X-ray, or the Bragg diffraction technique, has similar
limitations of assessing only a small volume of the rail on a
clean surface, and it is usually performed with bulky measur-
ing equipment that makes it inconvenient for field application
(Kelleher et al. 2003 Ruud 1979 Turan et al. 2019).
Vibrational methods have also been examined where a
small part of the rail is excited laterally by a shaker source and
the generated wave is recorded and analyzed. The frequency
and modal parameter changes are related to the axial forces in
the rails. Different forms of this technique have been explored,
including the use of mechanical shakers like a hammer (Béliveau
1997 Zhang et al. 2021) or the use of constant-frequency or
variable-frequency piezoelectric excitation (Kjell and Johnson
2009 Read and Shust 2007). Those studies have reported that the
major challenge with this technique is that the lateral stiffness
from railroad crossties significantly impacts the wave properties
and the resonant frequency more than the axial forces in the rail.
Another study extensively investigated the use of non-
linear coefficients related to the nonlinear guided waves
(Phillips et al. 2014). The method showed promising results,
but field applications were challenging, and the approach
requires sophisticated, nonlinear semi-analytical finite element
modeling for the rail being tested to yield results.
Acoustoelasticity using different ultrasonic wave modes,
including longitudinal and reflected waves (Szelaz˙ek ˛ 1992),
Rayleigh waves (Gokhale and Hurlebaus 2008 Hurlebaus 2011),
transverse shear waves (Szelaz˙ek ˛ 1998), and surface-skimming
shear horizontal waves (Alers and Manzanares 1990) have also
been explored by various researchers. In some experiments, good
linear agreement has been reported for the relation between the
rail stress and the wave speed change in the laboratory. However,
the challenges included acoustic coupling variations and surface
effects when piezoelectric sensors were used, effects of surface
conditions and surface stresses, and plastic deformation in the
head of the rails affecting wave properties. Additionally, varia-
tions in texture, metallurgy, and composition in the materials
being tested affected the acoustoelastic properties of the material
and reduced the accuracy of results. Temperature effects that
change the velocity of the ultrasonic waves also present a chal-
lenge for the practical implementation of the technologies.
Recent approaches that are still under development and labo-
ratory investigation include the use of nonlinear solitary waves
(Nasrollahi and Rizzo 2019), the photoluminescence piezo-
spectroscopy method (Yun et al. 2019), and the deformation-
based method using StereoDIC technology (Knopf et al. 2021).
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