Improving Tank Car Safety
By Joseph W. Cardinal
Technology long used in
military and commercial
aviation applications finds
new life in assessing the
structural integrity of
railroad tank cars.
12
pproximately 240,000 of the
railroad cars in operation in
North America today are tank
cars. Roughly half of these
tank cars carry materials
regulated by the U.S.
Department of
Transportation (DOT) and Transport
Canada because they contain materials
that are flammable, corrosive, poisonous,
Technology Today · Spring 2000
or otherwise hazardous. To ensure the
safety of these shipments, the railroad
tank car industry and U.S. and Canadian
regulators have undertaken considerable
efforts over the years to enhance the
structural integrity of tank cars and to
better understand the diverse environments in which they operate.
Since 1995, engineers in the
Structural Engineering and Materials
Engineering Departments at Southwest
Research Institute (SwRI) have been
working with the tank car industry to
apply damage tolerance analysis (DTA)
methods to analyze metal fatigue crack
growth in the welded stub sills (underframes) of tank cars. The problem of
fatigue crack growth in welded structures
is certainly not unique to the tank car
industry and has always been a concern
to the offshore structures and maritime
industries, among others. In fact, SwRI's
prior experience in these industries was
key to enabling the Institute to successfully compete for and accomplish the
goals of this program.
Damage tolerance analysis is a multidisciplinary, proactive approach to structural integrity that has its historical
origins in the military and commercial
aviation industries. Implementation of a
damage tolerance approach to structural
integrity ensures that flaws or cracks that
may exist in the structure will not grow to
a critical size and cause catastrophic failure within a specified period, such as a
safe inspection interval. Thus, by detecting structural problems prior to their
achieving a critical condition, the life and
safety of the structure can be managed
proactively instead of responding after a
costly accident occurs.
The four-year stub sill DTA program
was completed in late 1998 under the
auspices of the Tank Car Safety Project of
the Railway Progress Institute (RPI) and
the Association of American Railroads
(AAR). The use of a damage
tolerance analysis approach
for stub sills was motivated
when cracking was detected
in a number of the welds
joining stub sills to tank cars.
These cracks were detected
as part of an inspection program implemented by the
tank car industry working in
concert with U.S. and
Canadian regulatory bodies.
If these cracks remain undetected or go unrepaired, they
could lead to sill separation,
leaking of cargo, or a sudden
failure of a tank car during
transportation.
Joseph W. Cardinal is manager of the
Structural Integrity and Reliability
Section in SwRl"s Mechanical and
Materials Engineering Division. A
specialist in structural integrity analysis,
Cardinal has applied his expertise to
solving problems for the aerospace,
petroleum, power generation, and
transportation industries, as well as
government agencies such as the
U.S. Air Force and NASA.
In a report addressing a number of
tank car accidents, highly visible to the
public and identified to be caused by the
propagation of fatigue cracks, the
National Transportation Safety Board
(NTSB) reviewed the adequacy of the
DOT regulations for the inspection and
testing of tank cars. In this report, the
NTSB recommended establishing safe
inspection intervals for tank cars using a
damage tolerance approach. Subsequently, regulations have been enacted to further the use of DTA methods for the
substantiation of railroad tank car structural integrity. As a result, the tank car
industry and regulatory bodies have
taken the initiative to investigate a DTA
approach to est~blish inspection intervals for tank car stub sills.
Although this program constituted
the initial application of damage tolerance analysis methods to the tank car
industry, DTA technology has been used
for more than 30 years. Subsequent to its
origins in the aerospace industry, DTA
methods have been employed in the offshore structures, pressure vessels and
piping, and power generation (public
utility) industries to address structural
integrity issues and establish safe inspection intervals. Therefore, the objective of
the stub sill DTA program was to transfer
these established technologies to the tank
car industry to analyze cracks that have
been detected in welds.
Under supervision of the Stub Sill
Working Group of the RPI-AAR Tank Car
Safety Project, SwRI was contracted to
serve as an independent, third-party
Ta
,,
Stub Sill
Undetected fatigue cracks
in the welds that join stub
sills to railroad tank cars
may lead to a number of
undesired events, such as
leaking of cargo or sill
separation. Analytical
approaches developed at
SwRI and applied by tank
car builders and owners will
help assess the integrity of
stub sill designs.
Head Brace
Technology Today. Spring 2000
13
SwRI engineers are applying damage tolerance analysis, a
multidisciplinary approach to structural integrity, developed
originally for the military and commercial aviation industry, to
railroad tank cars to help establish safe inspection intervals.
program manager and to provide guidance and instruction in damage tolerance
analysis methods. The Working Group
included representatives from the RPImember tank car companies (ACF
Industries, Trinity Industries, Union Tank
Car Co., GATX Rail Corporation, and GE
Capital Railcar Services Corporation), the
AAR, the Federal Railroad Administration
(FRA), Transport Canada, the Chemical
Manufacturers Association, and the
Transportation Technology Center, Inc.
(TTCI), a wholly-owned subsidiary of the
AAR, located in Pueblo, Colorado. In a
parallel effort, the TTCI was funded by
the FRA to perform a full-scale damage
tolerance test on a stub sill tank car using
their Simuloader Test Facility. This TTCI
facility is a computer-controlled, servohydraulic test machine built especially for
the full-scale multiaxial fatigue and vibration testing of railcars.
The goal of this joint program was to
develop and demonstrate a DTA
approach applicable to tank car stub sill
cracking. SwRI engineers used their many
years of experience performing damage
tolerance analyses for the aerospace
industry, along with expertise in steel
structures and materials, to provide oversight and guidance for application of DTA
methodologies. Established DTA technologies were transferred to the tank car
builders and owners at a DTA tutorial,
hosted and taught by SwRI staff.
Specially tailored analysis methods
were developed by Sw RI for the tank car
manufacturers. These included the development of a generic set of material properties suitable for a range of stub sill
designs; the development of a load spectrum representing 10,000 miles of
over-the-road operation based on test
14
data collected by
the TTCI; and a
detailed set of
guidelines for the
performance of
stub sill damage
tolerance analyses. SwRI also collaborated with engineers at the TTCI to conduct a full-scale fatigue crack growth test,
the results of which were used to validate
the analysis methodologies. Crack growth
analyses of selected cracks that occurred
during the TTCI full-scale fatigue test
were performed by SwRI to validate the
DTA approach.
Considering the approximations
made in the analyses, good agreement
was demonstrated between tests and
analyses. While the objective of the overall DTA effort was to establish inspection
intervals to ensure the structural integrity
of a stub sill over its projected lifetime,
the results of the technology employed in
this program can be used by the builders
and owners to identify any tank cars or
stub sill designs that may be fracturecritical and to improve the quality of their
stub sill designs in general.
In late 1999, SwRI engineers began
work on a new program to investigate the
effects of low temperatures on the impact
resistance of tank cars. During the process
of assembling a train, cars of all kinds are
coupled using an energy absorbing
device called a draft gear that transmits
the dynamic (impact) longitudinal forces
to the car body. The couplers, the draft
gear, the stub sills, and the tank shell can
be subjected to severe impact conditions
and high dynamic loads during this
assembly process.
A combination of factors, such as low
temperatures that tend to reduce the ductility of structural steel and pre-existing
defects in the steel, may result in crack
propagation and, under some circumstances, sudden and brittle failure of the
tank itself. The principal goals of this
work are to determine the susceptibility
Technology Today . Spring 2000
of tank car steels to impact-induced failure, particularly under low temperature
conditions, and to determine the low temperature operational characteristics of
draft gear typically used on tank cars.
This 16-month effort is motivated in
part by incidents involving brittle fractures in tank cars in the U.S. and Canada
that occurred during cold weather.
Because this problem is of international
interest, the project was jointly funded
by the FRA and Transport Canada and is
administered by the Canadian government. SwRI has teamed with the Centre
for Surface Transportation Technology
of the National Research Council of
Canada in Ottawa (NRC/CSTT) to
perform this work.
SwRI is the prime contractor providing expertise in materials, fracture
mechanics, and tank car structural
integrity analysis, while the subcontract to
the NRC/CSTT provides expertise in rail
vehicle dynamics modeling, full-scale
impact testing at low temperatures, draft
gear behavior, and railroad operations.
Ultimately, the results of this program will
be aimed at achieving a better understanding of tank car impact phenomena
to reduce the incidence of failure caused
by impacts . •:.
Comments about this article? Contact
Cardinal at (210) 522-3323 or
jcardinal@swri.org.