Improving Tank Car Safety
Technology long used in military and commercial aviation applications finds new life in assessing the structural integrity of railroad tank cars.
Approximately 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, 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.
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 establish 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 program manager and to provide guidance and instruction in damage tolerance analysis methods. The Working Group included representatives from the RPI-member 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 SwRI 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 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 fracture- critical 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 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.
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Published in the Spring 2000 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.