Finite Element Analysis - Submodel

Based on the results from the global model analysis, four general observations can be made. (1) For the same loading mode, lading conditions usually do not change the locations of local high stress areas, i.e., loaded and unloaded tank will have the same high stress areas for a given loading mode. (2) High stress areas caused by VCF loads are limited to the section from the tank head to the stub sill extension, and along the bottom of the tank. (3) Also along the bottom of the tank, LCF loads cause high stress areas both in the body bolster area and in the bottom outlet hole area. (4) Although local high stress fields exist around the top opening, such as the area between the manway nozzle and the multi-housing nozzle, the stress values are much lower than those at the bottom outlet or in the stub sill section. Therefore, top opening areas are excluded from consideration as PFCLs for this non-pressure tank car. The above results from the global model helped to identify the PFCLs in the tank. Based on the results, a refined submodel is developed. Since most of the high stress areas observed are along the weld beads between major pads and the tank, the submodel includes the weld filler metal in order to achieve accurate stress solutions.

The submodel covers all the observed high stress areas in the tank except for the bottom outlet hole area. The lower-left figure shows an overview of the submodel. The model consists of 47,270 elements and 57,036 nodes. Of the 47,270 elements, 9,230 are general surface-to-surface contact elements. Eight-node brick elements with extra mode shapes comprise the remainder of the elements. The lower-right figure shows a comparison of meshes for both the submodel and the global model in the tank head area. Gravity and lading loads on the submodel are applied the same way as in the global model. Symmetry conditions are imposed on the tank’s transverse center plane, as was done for the global model. The unique boundary conditions for the submodel are applied on the cut boundaries, where displacements determined from the corresponding global model analysis are interpolated onto the cut boundary nodes. With significant mesh refinement and the inclusion of weld filler metal to represent the weld bead, the submodel analysis provides reasonably accurate stress results under assumed conditions. Local high stress areas have been identified and will be considered in the fatigue crack growth analysis.

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