Tank Car Structural Integrity
Rail transportation of hazardous materials in the United States is recognized as the safest method of moving large quantities of chemicals over long distances. Recent statistics show that the rail industry's safety performance, as a whole, is improving. In particular, the vast majority of hazardous materials shipped by rail tank car arrive safely and without incident. In general, the railroads have an outstanding record in moving shipments of hazardous materials safely.
The continued safe transport of hazardous materials is a key concern of the Federal Railroad Administration. As shown in the figure below, the number of accidents per year with at least one car releasing hazardous materials has decreased significantly over the past 25 years. The data in this figure were obtained from the FRA's Railroad Accident and Incident Reporting System (RAIRS). The decrease is attributed to improvements in tank car designs and to federal regulations that instituted requirements for head shields, thermal protection, and double self couplers, each of which reduces the likelihood of tank car puncture in derailment conditions.
Number of accidents with at least one car releasing hazardous material from FRA RAIRS database (1975-2005)
The Volpe Center provides technical support to the Federal Railroad Administration, Office of Research and Development, in safety matters regarding the transportation of hazardous materials by railroad tank cars. Research is conducted to support FRA/industry efforts in resolving problems related to metal fatigue and fracture in the current rail tank car fleet, the structural behavior of rail tank cars under potential collision and derailment scenarios, and improving the standards and procedures for future rail car designs.
Some of the most recent activities are summarized as follows.
Behavior Under Normal Conditions
Damage Tolerance Analysis
The aim of damage tolerance analysis is to accurately predict the growth of potential flaws in a structure, so that parts are inspected and, if necessary, repaired or replaced before failure can occur. The goals within this research program for railroad tank cars are:
- to determine the loads experienced by tank cars under normal service conditions, which may be characterized as low-amplitude, high-frequency loads,
- to determine the loads experienced by tank cars under severe loading conditions, which may be characterized as high-amplitude, low-frequency loads (e.g., coupling at excessive speeds),
- to examine fatigue crack growth behavior of tank car steel under laboratory test conditions, and
- to use engineering fracture mechanics to estimate the growth rate of potential flaws in critical areas of railroad tank cars, which ultimately will be used to develop guidelines for appropriate inspection intervals and procedures.
The implementation of damage tolerance analysis requires the synthesis of structural analysis, fracture-mechanics-based crack growth predictions, and probability of detection curves for candidate inspection techniques. Moreover, damage tolerance analysis is the aspect of an overall reliability program that ensures the continued integrity of structural components against damage accumulated during normal tank car service.
Residual Stresses and Welding Practices
A significant number of cracks that occur in tank car shells are near welds. The formation and growth of these cracks are affected by residual stresses that are induced by the welding process. Understanding these changes will help to develop recommendations for improved welding practices, inspection strategies, and inspection frequencies to avoid catastrophic failure or leaks in tank cars.
An X-ray showing cracks near the welds
Behavior Under Extreme Conditions
The structural integrity of a tank car shell must be maintained during extreme conditions such as impacts, collisions, and derailments. Historically, punctures from impacts to tank car heads have caused the release of many hazardous materials. Results from impact tests conducted in the 1970s and early 1980s led to regulations for head shields for certain tank cars carrying hazardous materials. Further work was conducted in the mid-1980s on tank cars for transporting chlorine and for aluminum tank cars.
Actual collision testing on a tank car
Recent train derailments that led to the release of hazardous materials and resulted in loss of life have renewed interest in the structural integrity of railroad tank cars under accident loading conditions. The Volpe Center has developed and initiated a multi-phase approach to assess the consequences of tank cars involved in accidents. Each phase involves the development of computational models with different objectives which provide inputs to the next phase.
- Derailment Dynamics
The first phase calls for the development of physics-based mathematical models to analyze the gross motions of rail cars in a derailment. Moreover, the objective of the derailment dynamics model is to estimate the closing velocities between an impactor (e.g., the coupler of another car) and the tank car head or shell.
A mathematical schematic of a car during a derailment
- Structural Finite Element Analysis
The second phase applies dynamic, elastic-plastic finite element analysis to simulate the structural response of the tank car head or shell to an assumed scenario (i.e., penetrator shape, initial closing velocity, and effective collision mass). The finite element analyses are carried out using commercial finite element software.
A software-rendered graphic of tank collision analysis
- Damage Assessment
The third phase is an assessment of damage created by the impacting loads. Moreover, analytical tools will be developed to calculate puncture force (i.e., the force at which the tank car head or shell is expected to occur). In addition, a test program is being conducted to examine high-rate fracture toughness of tank car steels.
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(a) Denting of tank car head
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| (b) Puncture of tank car head |
Credibility of the computational models is achieved through verification and validations studies. In this context, verification means assessing the accuracy of the solution from computational models by comparison with known solutions. Validation means assessing the accuracy of a computational simulation by comparison with test data.
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