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Stainless steel could be a solution for the lightweighting of buses.

Constructing lightweight bus structures with stainless steel

Outokumpu and collaborators show a possible weight reduction of up to 35% by using high-strength stainless steel in place of carbon steel.

The weight of a typical bus could be reduced by up to 35% – more than 1,000 kg (2,205 lbs.) – by using high-strength stainless steel to replace tubular bus-frame elements traditionally manufactured in carbon steel. That is the conclusion of a first-of-its-kind project carried out by stainless-steel manufacturer Outokumpu, together with CAD/CAE solution specialist FCMS, the Munich University of Applied Sciences and RotherCONSULT. 

Corrosion-resistant stainless steel could offer sustainability combined with reduced maintenance time and costs. In addition, high-strength stainless steel grades have become commercially available that offer significant weight savings. The aim of this project was to examine what that could mean in terms of lower weight and reduced material costs. 

Figure 1 shows the suitable candidate materials. The usual material is low-cost carbon steel, normally grade S355 or the higher strength S460 – the designations indicating their minimum yield strength. Next on the list are stainless steels of low-strength levels. Finally, there is the high-strength range of stainless steels including lean duplex Forta LDX 2101 that has high corrosion resistance. The graph also shows Forta H800, a grade developed for its high-strength capabilities, which is why its corrosion resistance appears low for a stainless steel. 

To provide the most challenging comparison, this project focused on assessing the weight and cost benefits of Forta H800 against S460 carbon steel. The recently developed grade S700 also could be considered as it has a comparable yield strength to Forta H800, but with much lower elongation and crash absorption/impact resistance potential. 

Forta H800 is a fully-austenitic stainless steel developed for safety-critical structural vehicle components. In addition to ultra-high strength, its key characteristic is the TWIP (Twinning Induced Plasticity) strengthening mechanism that causes the material structure to harden continuously, enabling very high energy absorption when subjected to a crash impact (Figure 2). It also is nickel-free, removing the price volatility associated with grades that rely on nickel as an alloying element. 

Simulation of optimized structures 

FCMS, the Munich University of Applied Sciences, RotherCONSULT and Outokumpu collaborated to investigate how Forta H800 could be best used in bus structures. The project aims were to examine how it could: minimize structural weight; minimize material and manufacturing costs; ensure sufficient strength for dynamic (quasistatic) and cyclic loading; ensure sufficient static and dynamic stiffness; and ensure safety during rollover scenarios. 

A combination of tools was employed to simulate the performance of the bus structure, including parametric geometry modeling as well as fast and robust analysis methods. AI (artificial intelligence) was used to automatically generate design alternatives and their associated simulation models. Because the work required the evaluation and iteration of thousands of designs, it would not have been feasible to undertake this work manually. 

The Fast Concept Modeler (FCM) was at the heart of the project. This is a tool embedded in CATIA (computer-aided three-dimensional interactive application), a product design and development software package. FCM makes it possible to evaluate the impact of changes in profiles, wall thicknesses and materials, as well as changes in topology such as the position of frame segments and junctions. Topology changes were not considered at this stage since they would ultimately result in a redesign of the bus frame. 

Figure 3 shows the generic reference bus model used for the simulations. It was intended to be “typical,” rather than representing a bus from any particular manufacturer. It is around 14 meters (46 ft) in length with three axles, capable of carrying 63 passengers and two drivers, resulting in a total mass of 20,800 kg (45,860 lbs.). 

To restrict the number of design variables, the frame members were placed in 13 groups as indicated by the different colors in Figure 3. The structural members in each group were analyzed simultaneously for each design alternative. 

The model was analyzed under the following four load cases, with the two alternative structural materials of carbon steel and stainless steel: quasistatic loading; cyclic loading; torsional stiffness and NVH (noise, vibration and harshness) – an important factor for passenger comfort and driving dynamics; and rollover for compartment intrusion during a crash. 

Results prove weight savings 

Figure 4 shows the results of the simulations from a large number (thousands) of experiments. For the reference case, a pure frame structure was made entirely of carbon steel (S460) with a weight of 3,600 kg (7,940 lbs.). Even without material substitution, optimization enabled a useful weight saving of 116 kg (256 lbs.). When Forta H800 was used for the entire structure, the mass of the structure was reduced by one-third, saving 1,193 kg (2,630 lbs.). The rollover simulation also showed that the wall thickness of the tubing could be reduced from 4 mm to 2 mm (0.157 to 0.079 inches), accounting for a large part of the weight saving. 

The next stage was to examine a hybrid structure featuring both types of steel. Two alternatives were examined, with S460 and then Forta H800 used for the top mainframes. In the first case, the weight reduction was 25%. When most of the upper section is created in Forta H800, a 35% reduction was achieved. That is a potential weight reduction of almost 1,300 kg (2,866 lbs.). Furthermore, the volume of welding required to create the structure is reduced by more than 50%. There also is a small lowering in the center of gravity, which is beneficial for driving dynamics. 

Cost impact 

The cost impact of changing the frame material is shown in Figure 5. The relative costs of different steels can vary over time and according to market conditions, so three factors were used to scale the cost of Forta H800 relative to S460 – 2, 2.5 and 3 times. 

Even with a factor of 3, the weight reduction means that the hybrid structure is comparable in cost to the S460-only structure, while at a factor of 2 the hybrid design represents an 18% improvement in material costs. In this analysis, only pure material costs were compared and additional aspects such as energy consumption during production were not considered. 

Next steps

Further work is being undertaken to refine the simulation process. More variables also could be investigated such as cross-sections, wall thicknesses and junction locations that would effectively be a complete re-design of the bus structure to optimize the use of stainless steel. 

Future possibilities will be to evaluate the effect of manufacturing techniques. For example, the performance of welded joints under fatigue loading can be improved considerably by refining the joining process and displacing the welded seam away from the most stressed areas. This may even offer the possibility to further reduce the cross-section or wall thickness. If this project was repeated to include refined joining processes, there is a strong possibility that the difference between the carbon steel and stainless-steel structures would be even more significant. 

For further information see the Outokumpu Buses page.

Stefan Schuberth, sales manager for Automotive Business Line, Advanced Materials, Outokumpu; Prof. Dr.-Ing. Klemens Rother, University of Applied Sciences, Munich, Germany; and Dr. Werner Pohl, FCMS GmbH, wrote this article for SAE Media. 

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