|Finite Element Analysis - Global Model|
To properly analyze critical crack size of potential defects in tank cars, detailed knowledge of the stress distributions within the tank car structure is necessary. The finite element analysis (FEA) of tank car structures has two objectives: (a) to identify potential fatigue critical locations (PFCLs), and (b) to provide a detailed stress distribution at each of the identified PFCLs for use in fatigue crack growth analysis. To achieve these objectives in an efficient and accurate manner, the FEA has been performed using both global analysis and submodel analysis. As part of this analysis process, the substructuring (super-element) technique is employed for efficiency. ANSYS, a commercial FEA computer code, is used for the analysis.
The following model details have been included to improve stress predictions in the tank. (1) Two groups of contact elements have been introduced between (a) the front sill pad and the tank shell, and (b) the body bolster pad and the tank shell. These contact elements are important to accurately simulate the load transfer between the major pads and the tank shell under various loading conditions. (2) Top and bottom openings are introduced. Specifically, the openings included in the FE model are a manway nozzle, a multi-housing nozzle, two safety valve nozzles and a bottom outlet flange and skid. (3) With the introduction of the tank openings, the geometric symmetry at the tank longitudinal center plane is lost. Therefore, a half tank car model is required. The half tank car model also provides the ability to apply the vertical coupler force (VCF) non-symmetrically (i.e. apply VCF at one end only), which is believed to be more realistic. (4) To simulate the truck spring effect, which proved to be significant for the VCF load cases, spring elements are added to support the center plates. (5) A range of loads and boundary conditions are applied. Of special interest is a simulation of the draft gear effect on applying longitudinal coupler force (LCF). Spring elements and loading blocks are introduced for this simulation. (6) For computational efficiency, the substructuring (super-element) technique is also used. Since detailed stress results in the stub sill are often not needed for tank shell DTA, and the stub sills do not have non-linear behavior to be modeled, they are well suited for treatment as super-elements. Therefore, super-elements representing the stub sills at each end of the tank car have been developed.