This conference paper was submitted for presentation at the NAFEMS World Congress 2025, held in Salzburg, Austria from May 19–22, 2025.

Abstract

Mass transit (metro) rail vehicles must be lightweight to optimize payload and energy efficiency. Mass reduction measures must be implemented in the typically used welded aluminum carbody to meet cost targets and development schedule constraints. To achieve a comprehensive mass reduction of a carbody design, we follow a rigorous CAE-driven design approach starting with topology optimization covering the entire carbody structure. Knowing that the body is a design with a structural frame and a thin, load-bearing skin, both are implemented in a combined model for the topology optimization definition. This approach has a significant impact on the results for the optimized structure and offers a higher potential for mass reduction compared to classical volume-based topology optimization approaches. The second key ingredient for an appropriate optimization result is the selection of the acting load cases and their respective mass impact on the overall result. A representative selection of design relevant loads must be derived from the wide range of service loads on the real carbody. However, the main step from topology optimization to a feasible carbody design is critical and the approach must be based on profound experience in the field of mechanical engineering also considering the present manufacturing capabilities and total cost. Manufacturing costs, infrastructure and development timeline dictate the goal of using a classic aluminum chassis and proven widely accepted welding technology as the basis. Aluminum integral design using large extrusion profiles is well established in the rail industry because it provides high degree of automated manufacturing. The disadvantage of this technology is the relatively high mass of the carbody compared to sheet metal and frame designs, the so-called differential design. However, differential design is typically not as cost effective due to the high labor cost of manual welding required. The goal is to reduce the mass of the integral design without uneconomically increasing the cost. This requires highly automated manufacturing techniques such as milling, which can be combined with the automated welding of the extrusions. The process implemented is called the quasi-differential design approach. The carbody based on the quasi-differential design must withstand all static and fatigue design loads resulting from rail standards and/or multi-body system simulation results. The car behavior is simulated by Finite Element Analysis with load case superposition, which provides the amplitudes of the acting stress components for each location in the structure. The weld definition in the FEA model provides the allowable notch case in each direction relative to the local weld direction. These post-processing steps provides the utilization for each part of the carbody structure. To verify the concept, a metro carbody currently in production is used as the basis for the new design approach. The resulting structure, using the same design space and loads, results in a structure with twenty percent less mass. This considerable mass reduction is reached by consequent (CAE-based) structural lightweight design only - without any change of material or manufacturing technology usually applied for modern aluminum carbody production. A carbody prototype was manufactured to verify the technical concept and the cost predictions. Finally, the static type test program was successfully completed, fully confirming the huge potential of this innovative design approach for aluminum carbody structures.