Truck & Off-Highway Engineering - August 2023 - 24

Constructing
bus structures with
stainless steel
Stainless steel
could offer reduced
maintenance time and costs.
Outokumpu and collaborators show a possible weight reduction of up to 35% by
using high-strength stainless steel in place of carbon steel.
by Stefan Schuberth, Klemens Rother and Werner Pohl
T
he 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 safetycritical
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.
24 August 2023
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 re-design of the bus frame.
Figure 3 shows the generic reference bus model used
for the simulations. It was intended to be " typical, " rather
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