In both high speed and freight rail systems, the modern construction method is Continuous Welded Rail (CWR). The
purpose of the CWR method is to eliminate joints in order to reduce the maintenance costs for both the rails and the
rolling stock. However the elimination of the joints increases the risk of rail breakage in cold weather and buckling in hot
weather. In order to predict the temperature at which the rail will break or buckle, it is critical to have knowledge of the
temperature at which the rail is stress free, namely, the Rail Neutral Temperature (Rail-NT).The University of California
at San Diego has developed an innovative technique based on non-linear ultrasonic guided waves, under FRA research
and development grants for the non-destructive measurement of the neutral temperature of railroad tracks. Through the
licensing of this technology from the UCSD and under the sponsorship of the FRA Office of Research and Development,
a field deployable prototype system has been developed and recently field tested at cooperating railroad properties.
Three prototype systems have been deployed to the Union Pacific (UP), Burlington Northern Santa Fe (BNSF), and
AMTRAK railroads for field testing and related data acquisition for a comprehensive evaluation of the system, with
respect to both performance and economy of operation. The results from these tests have been very encouraging. Based
on the lessons learned from these field tests and the feedback from the railroads, it is planned develop a compact 2nd
generation Rail-NT system to foster deployment and furtherance of FRA R&D grant purpose of potential contribution to
the agency mission of US railroad safety. In this paper, the results of the field tests with the railroads in summer of 2013
are reported.
Propagation of nonlinear guided waves is a field that has received an ever increasing interest in the last few decades.
They are excellent candidates for nondestructively interrogating long waveguide like structures since they conveniently
combine high sensitivity to structural conditions (typical of nonlinear parameters), with large inspection ranges
(characteristic of wave propagation in bounded media). Nonlinear wave propagation in solids has been classically
studied using finite strains theory. According to this framework a system of nonlinear PDEs is required to
mathematically describe nonlinear phenomena such as acoustoelasticity (wave speed dependency on state of stress),
wave interaction, wave distortion, higher harmonics generation, and so on. This work introduces a novel physical model
aimed at predicting nonlinearity in constrained waveguides characterized by infinitesimal (ideally zero) strains subjected
to thermal variations. Interatomic potentials are employed to explain the origin of nonlinear effects under constrained
temperature changes. These potentials highlight at least a cubic dependence on strain of the residual strain energy that
is stored in the material due to the prevented thermal expansion. The cubic relationship between strain energy and
strain produces second-harmonic generation of propagating elastic waves. This principle is validated experimentally for
longitudinal bulk waves propagating in a steel block under constrained thermal excursions.
Continuous Welded Rail (CWR) is used in modern rail construction including high-speed rail transportation. The
absence of expansion joints in these structures brings about the risk of breakage in cold weather and of buckling in warm
weather due to the resulting thermal stresses. The University of California at San Diego (UCSD), under a Federal
Railroad Administration (FRA) Office of Research and Development (RandD) grant, is developing a system for in-situ
measurement of the rail Neutral Temperature in CWR. Currently, there is no well-established technique able to
efficiently monitor the rail thermal stress, or the rail Neutral Temperature (rail temperature with zero thermal stress), to
properly schedule slow-order mandates and prevent derailments. UCSD has developed a prototype (Rail-NT) for
wayside rail Neutral Temperature measurement that is based on non-linear ultrasonic guided waves. Numerical models
were first developed to identify proper guided wave modes and frequencies for maximum sensitivity to the thermal
stresses in the rail web, with little influence of the rail head and rail foot. Experiments conducted at the UCSD Largescale
Rail NT Test-bed indicated a rail Neutral Temperature measurement accuracy of a few degrees. The first field tests
of the Rail-NT prototype were performed in June 2012 at the Transportation Technology Center (TTC) in Pueblo, CO in
collaboration with the Burlington Northern Santa Fe (BNSF) Railway. The results of these field tests were very
encouraging, indicating an accuracy for Neutral Temperature measurement of 5°F at worst, on both wood ties and
concrete ties.
Most modern railways use Continuous Welded Rail (CWR). A major problem is the almost total absence of expansion
joints that can create buckling in hot weather and breakage in cold weather due to the rail thermal stresses. In June 2008
the University of California, San Diego (UCSD), under the sponsorship of a Federal Railroad Administration (FRA)
Office of Research and Development (R&D) grant, began work to develop a technique for in-situ measurement of stress
and detection of incipient buckling in CWR. The method under investigation is based on ultrasonic guided waves, and
the ultimate goal is to develop a prototype that can be used in motion. A large-scale full rail track (70 feet in length) has
been constructed at UCSD's Powell Structural Laboratories, the largest laboratories in the country for structural testing,
to validate the CWR stress measurement and buckling detection technique under rail heating conditions well controlled
in the laboratory. This paper will report on the results obtained from this unique large-scale test track to date. These
results will pave the road for the future development of the rail stress measurement & buckling detection prototype.
Currently 90% of bridges built in California are post-tensioned box-girder. In such structures the steel tendons are
the main load-carrying components. The loss of prestress, as well as the presence of defects or the tendon breakage, can
be catastrophic for the entire structure. Unfortunately, today there is no well-established method for the monitoring of
prestressing (PS) tendons that can provide simultaneous information related to the presence of defects and the level of
prestress in a continuous, real time manner. If such a monitoring system were available, considerable savings would be
achieved in bridge maintenance since repairs would be implemented in a timely manner without traffic disruptions. This
paper presents a health monitoring system for PS tendons in post-tensioned structures of interest to Caltrans. Such a
system uses ultrasonic guided waves and embedded sensors to provide simultaneously and in real time, (a)
measurements of the level of applied prestress, and (b) defect detection at early grow stages. The proposed PS
measurement technique exploits the sensitivity of ultrasonic waves to the inter-wire contact developing in a multi-wire
strand as a function of prestress level. In particular the nonlinear ultrasonic behavior of the tendon under changing levels
of prestress is monitored by tracking higher-order harmonics at (nω) arising under a fundamental guided-wave excitation
at (ω). Moreover this paper also present real-time damage detection and location in post-tensioned bridge joints using
Acoustic Emission techniques. Experimental tests on large-scale single-tendon PT joint specimens, subjected to multiple
load cycles, will be presented to validate the monitoring of PS loads (through nonlinear ultrasonic probing) and the
monitoring of damage progression and location (through acoustic emission techniques). Issues and potential for the use
of such techniques to monitor post-tensioned bridges in the field will be discussed.
The University of California at San Diego (UCSD), under a Federal Railroad Administration (FRA) Office of Research
and Development (R&D) grant, is developing a system for high-speed and non-contact rail defect detection. A prototype
has been designed and field tested with the support of Volpe National Transportation Systems Center and ENSCO, Inc.
The goal of this project is to develop a rail defect detection system that provides (a) better defect detection reliability
(including internal transverse head defects under shelling and vertical split head defects), and (b) higher inspection speed
than achievable by current rail inspection systems. This effort is also in direct response to Safety Recommendations
issued by the National Transportation Safety Board (NTSB) following the disastrous train derailments at Superior, WI in
1992 and Oneida, NY in 2007 among others. The UCSD prototype uses non-contact ultrasonic probing of the rail head
(laser and air-coupled), ultrasonic guided waves, and a proprietary real-time statistical analysis algorithm that maximizes
the sensitivity to defects while minimizing false positives. The current design allows potential inspection speeds up to 40
mph, although all field tests have been conducted up to 15 mph so far. This paper summarizes (a) the latest technology
development test conducted at the rail defect farm of Herzog, Inc. in St Joseph, MO in June 2010, and (b) the completion
of the new Rail Defect Farm facility at the UCSD Camp Elliott Field Station with partial in-kind donations from the
Burlington Northern Santa Fe (BNSF) Railway.
KEYWORDS: Ultrasonics, Transducers, Wave propagation, Waveguides, Interfaces, Finite element methods, Chemical elements, 3D modeling, Bridges, Signal detection
Monitoring load levels in multi-wire steel strands is crucial to ensuring the proper structural performance of post-tensioned
concrete structures, suspension bridges and cable-stayed bridges. The post-tensioned box-girder structural
scheme is widely used in several bridges, including 90% of the California inventory. In this structural typology,
prestressing tendons are the main load-carrying components. Therefore loss of prestress as well as the presence of
structural defects (e.g. corrosion and broken wires) affecting these elements are critical for the performance of the entire
structure and may conduct to catastrophic failures. Unfortunately, at present there is no well-established methodology for
the monitoring of prestressing (PS) tendons able to provide simultaneous and continuous information about the presence
of defects as well as prestress levels. In this paper the authors develop a methodology to assess the level of load applied
to the strands through the use of ultrasonic nonlinearity. Since an axial load on a multi-wire strand generates proportional
contact stresses between adjacent wires, ultrasonic nonlinearity from the inter-wire contact must be related to the level of
axial load. The work presented shows that the higher-harmonic generation of ultrasonic guided waves propagating in
individual wires of the strand varies monotonically with the applied load, with smaller higher-harmonic amplitudes with
increasing load levels. This trend is consistent with previous studies on higher-harmonic generation from ultrasonic plane
waves incident on a contact interface under a changing contact pressure. The paper presents the results of experimental
researches on free strands and embedded strands, and numerical studies (nonlinear Finite Element Analysis) on free
strands.
Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete structures.
Prestressing tendons are the main load-carrying components of these and other post-tensioned structures. Despite their
criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level
of prestress in order to detect dangerous stress losses. In collaboration with Caltrans, UCSD is investigating the
combination of ultrasonic guided waves and embedded sensors to provide both prestress level monitoring and defect
detection capabilities in concrete-embedded PS tendons.
This paper presents a technique based on nonlinear ultrasonic guided waves in the 100 kHz - 2 MHz range for
monitoring prestress levels in 7-wire PS tendons. The technique relies on the fact that an axial stress on the tendon
generates a proportional radial stress between adjacent wires (interwire stress). In turn, the interwire stress modulates
nonlinear effects in ultrasonic wave propagation through both the presence of finite strains and the interwire contact. The
nonlinear ultrasonic behavior of the tendon under changing levels of prestress is monitored by tracking higher-order
harmonics at (nω) arising under a fundamental guided-wave excitation at (ω). Experimental results will be presented to
identify (a) ranges of fundamental excitations at (ω) producing maximum nonlinear response, and (b) optimum lay-out of
the transmitting and the receiving transducers within the test tendons. Compared to alternative methods based on linear
ultrasonic features, the proposed nonlinear ultrasonic technique appears more sensitive to prestress levels and more
robust against changing excitation power at the transmitting transducer or changing transducer/tendon bond conditions.
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