Structural optimization of reinforced shell structures

Abstract

This thesis investigates numerical structural optimization utilizing the commercial code OptiStruct. The objective is to reduce the mass of a metro raw car body shell, which is crucial for satisfying maximum axle loads and realizing energy savings in operation. First, the differences of topology and free-size optimization are examined. Next, the global stress constraint available in OptiStruct is looked at in-depth. It is found that the control of local notch effects is not sufficient. In the main task, the effects on the results when considering or neglecting a load-bearing outer sheet are examined closely for a combination of nine important load cases of DIN EN 12663-1. Therefore, the installation space is meshed with three-dimensional continuum elements which are later overlaid with shell elements on selected surfaces. The suitability of the model is verified through a convergence study and reasonable optimization parameters are determined. A topology optimization using the SIMP approach and a combined (topology and free-size) optimization are then carried out for the former and latter mesh, respectively. Furthermore, a script is introduced to update the boundary conditions with respect to the current mass in between the iterations. The disequilibrium of the weighting of element stiffnesses between the two optimization types, caused by the penalization used in SIMP, is reduced by defining low upper bounds on the shell thicknesses. The outcome when neglecting a load-bearing outer sheet is a framework structure with a weight reduction of 7.36% compared to a reference carriage. Including a load-bearing outer sheet, however, yields a weight reduction of 31.78%. Concurrently, stiffness properties (compliances) are improved between 9.53% and 33.37% depending on the load case. A major part of the structure is formed as braced panels. The arrangement of the girders is different for the two approaches, thus it can be claimed that the consideration of the outer sheet during the optimization is essential. Future prospects include the translation of the results into a double wall aluminum structure with extensive milling grooves and a downstream parameter optimization. By choosing the cross sections of the extruded profiles in a smart way, the major part of the weight saving is to be preserved, which would mark a great success of the established optimization approach for railcar bodies.