|Fatigue Load Spectrum|
Railroad tank cars are subjected to various complicated service load conditions with multiple load transfer paths. These include coupler loads (vertical and horizontal), lading, rocking, truck hunting, etc. For the purpose of damage tolerance analysis (DTA), the available fatigue load spectra are for vertical and horizontal loads transferred by couplers, which are the most severe loads for tank car structures under normal operating conditions. The spectra used in the current fatigue crack growth analysis are a modified version of the spectra developed by the Southwest Research Institute (SwRI) in Tank Car Stub Sill DTA project, which in turn was based on the available tank car load spectrum data acquired from two over-the-road (OTR) test programs (Freight Equipment Environmental Sampling Test, or FEEST in short) performed by the Association of American Railroads (AAR). The spectra are now referred to as FEEST1 and FEEST2. It is noted that the measured load data was counted by rainflow cycle counting method on the fly. Thus, load phasing and load sequence information is not available. The spectrum developed by SwRI is for 10k miles of revenue service and has an unloaded-to-loaded mileage ratio of U/L=1.06.
A modification to the SwRI spectrum is carried out in order to perform tank shell DTA. The necessity for the modification is the fact that the upward and downward VCF loads induce quite different stress distributions and magnitudes in the area of head brace to tank connection. The downward load pulls the front sill pad away from the tank head, a much more severe stress concentration compared with the upward load. The original spectrum does not allow for consideration of such differences because the upward and downward loads are combined together and the same stress-scaling factor must be used for both upward and downward loads. To consider the difference between the upward and the down loads, the VCF spectrum is split into two spectra at the zero load level and rearranged. The figures below show, respectively, the loaded and unloaded spectrum for the downward VCF load. Each vertical line in these figures represent a distinct load step, i.e., the minimum load to the maximum load in a load cycle. The number of cycles at a given load step is not shown in these figures.