Railroad tank car safety performance has improved during the past two decades, due to both improvements in railroad operating environment and the introduction of such safety features as head protection, double-shelf couplers, better protected fittings, thermal protection, as well as other improvements in tank car designs. These safety features, based upon past experiences involving a series of severe accidents occurred during the late 1960s and 1970s, have dramatically improved the tank car performance in severe accidents (collisions and derailments) by significantly reducing tank punctures and ruptures.
With the aging of the U.S. railroad tank car fleet, a series of tank car fractures under normal service conditions has demonstrated that fatigue cracking is a primary safety concern. To maintain tank car structural integrity, regulations (HM-201 Final Rule) were promulgated in 1995. Among other things, the regulations promote the use of a damage tolerance analysis (DTA) approach to determine inspection intervals and ensure that existing cracks will be detected and repaired before they grow to a critical size and cause tank car failure. Since then, cooperative efforts involving the tank car industry, research institutions and government agencies have been undertaken to address tank car stub sill cracking.
Representing an extension of previous work by the Federal Railroad Administration, the current study applies the DTA methodology to the entire tank shell of steel tank cars. The primary objective is to develop and demonstrate an appropriate general methodology and techniques for tank shell DTA, not to evaluate a specific tank car design. To achieve this goal, DTA is performed on a DOT 111A100W1 general purpose tank car shown below. Stress distributions and potential fatigue critical locations in the tank are determined using global and local finite element models. Welding residual stresses in an unconstrained TC128-B steel butt weld are obtained using neutron diffraction. Fatigue crack growth analysis using a tank car service load spectrum is carried out and inspection intervals are determined for various postulated initial flaws.