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Tank Car Integrity

Critical Flaw Size

The AAR Manuals (referenced in US DOT hazmat regulations) set forth guidelines for tank car structural integrity determination and design parameters.  These guidelines are very general in nature and their use in tank car integrity evaluation, e.g., periodic inspection to determine remaining life is not well defined.  Gathering/setting up critical flaw size of certain defect types is critical to the application of damage tolerance analysis (DTA) in tank car remaining life determination.

Critical flaw sizes for a variety of tank car components must be identified to understand tank car behavior, acceptable manufacturing tolerances for tank car parts and components, as well as in defining nondestructive testing performance and acceptance criteria.  Critical flaw sizes must be defined for all fatigue critical locations of the tank car tanks and its structural members.  Important to defining critical flaw sizes is a better understanding of tank car material properties and fracture properties.  Notwithstanding the numerous studies that have been performed or supported by the FRA, recent work in the areas of structural integrity, including fracture mechanics-based DTA of tank car stub sills, exposes a need for a compilation of past testing to catalog the range of known material properties and additional work to document fracture toughness.

In recent years, investigators have shown that the fracture toughness of a material is a strong function of specimen geometry.  Attempts to use values for fracture toughness obtained from laboratory specimens to predict the direction and extent of crack growth in real materials have been only moderately successful.  If transferability were possible, then the ability to predict incipient, through-thickness crack growth for railroad equipment, including tank cars, would be greatly improved.  Recent advances have shown that two macroscopic near-tip parameters will be needed to accurately represent the effects of both geometry and loading on the crack tip stress and strain states that will eventually lead to crack growth.  These parameters can be obtained for a material through a series of controlled laboratory tests.  Using these test results, computational models of cracked structural components can be developed and both the direction and onset of crack growth predicted more accurately.  Clearly, the development of simple finite element models for implementation of a two-parameter fracture prediction methodology would be of great utility in assessing the severity of existing flaws in tank car steels.  This research is expected to continue for 2-3 years.

Shelf Coupler Redesign/Height Mismatch Effects

One safety feature found on all tank cars used in hazardous materials service is the double-shelf coupler.  These couplers are designed to remain engaged when subjected to forces that occur during switching operations and train accidents.

Before the use of double-shelf couplers, couplers would become uncoupled in accidents and during normal switching operations.  Couplers would become uncoupled and, riding up over the draft assembly of a tank car, puncture the head of that car.  To understand the transmission of forces through the train and the effects of these forces on tank cars, both in normal operation and in an accident environment, a study of the double-shelf coupler will be conducted.  This study, which will continue for several years, will include stress analysis to understand the load paths through the coupler and a look at the coupler/coupler interface and coupler/draft assembly interface as a dynamic system.  After such a study, appropriate design and material changes to the shelf couplers may be based upon the known system.

Tank Car Pressure Relief Devices

In the 1980's, an analytical program was developed by the FRA and the tank car industry for calculating the effects of fire on railroad tank cars for the purpose of selecting pressure relief device type, sizing, capacity, and pressure settings.  These procedures were used to predict various parameters, such as the start-to-discharge time to failure, the residual amount of lading and pressure in the tank at time of failure, the time to reach this pressure level, etc.  Further work to enhance the program was made in 1992.  Additional work is being conducted to further refine and expand the software.  This research should be completed in FY 2001.

Flow capacity and rating of tank car pressure relief devices, especially non-pressure or general-purpose cars, requires an understanding of the thermodynamic behavior of the product in fire conditions and the performance of the car.  This also is the case for poison by inhalation material service.  This project will identify parameters and rules to apply in the formulation of proper relief properties for this type of lading, similar to what has been done for pressure tank cars.

Tank Car Steels

Authorized materials of construction of tank cars are specified in the regulations. The objective of this project is to develop a performance-oriented specification for steel for tank car construction.  This project will develop a parametric model for the determination and selection of steel for tank car construction.  Past research showed that tank car steel properties, for tank car steels in similar service, were not consistent, in that they did not follow definite patterns.  A compendium report, to include as many pertinent prior and current works in this area, will be prepared.

Another concern is weldability.  Trying to understand and document tank car steel weldability is difficult and is dependent on the number of factors associated with weldability:  steel, parent metal, filler metal, welding conditions, and method of evaluation.  If it were a simple concept, there would not be the large number of welding tests that now exist.  However, several recent tank car failures were due in part to weld defects.

A three-dimensional finite element model examining residual stresses from welding was analyzed.  High tensile residual stresses were shown to foster the formation and growth of fatigue cracking.  Such cracking has been observed in tank cars near the end of the weld bead in the contiguous region of base plate on which the bead is laid.  Further work to identify residual stresses and tank car cracking will be conducted through FY 2003.  Advances in nanotechnology will be evaluated for application to tank car materials.

Tank Car Closures: Manways/Bottom Outlets/Flanges

Spills from tank car closures are significant safety concerns of tank car handlers, shippers, owners, and operators.  This is mostly a human factor’s problem, but should nevertheless, be researched to diminish its effect on tank car integrity.  Work will be initiated in FY 2001 to look at unintentional leaks and will continue for several years.

Intermodal Tank Integrity

Use of intermodal portable tanks for HAZMAT is increasing.  The containment integrity of such tanks will be reviewed for safety in railroad service.  Enhancements have been made to the integrity of intermodal tanks, but intermodal tank design criteria and performance need to be evaluated in terms of international transportation requirements and size and weight limitations.  This research is planned for FY 2002 and is expected to continue for 3-4 years.

Thermal Integrity of Tank Cars

As the tank car fleet ages, there are problems with deterioration of the tank car thermal protection systems and slumping of insulation materials.  Current regulations require qualification of thermal systems periodically.  However, research is necessary to ensure there is no negative effect on the overall tank car thermal integrity.  This research will begin in FY 2003 and continue for several years.