The design approach for bus body architecture has gone
through different phases—namely, chassis based, semi
integral, integral, and monocoque. Equally varied is the
choice of material for the bus “super structure,” the
predominant materials being mild steel with galvanization,
stainless steel (SS), and aluminum.
Researchers at Ashok Leyland make a case for choosing
stainless steel for the complete bus structure. With rapid
development in infrastructure and the public mass transit
system, particularly in the BRIC (Brazil, Russia, India, and
China) countries, a robust structure for buses that is
durable and crashworthy has become imperative.
Among the family of stainless steels, ferritic stainless steel
exhibits excellent mechanical properties with corrosion
resistance and better strength-to-weight ratio compared to
the galvanized mild steel. Stainless steel, by virtue of its
higher strength-to-weight ratio, brings down the unladen
weight of the bus and hence improves its fuel efficiency.
Although the initial material cost is higher for stainless
steel, it still scores better in other areas, namely lower
weight, less frequent replacement, lesser downtime, and
better recyclability. On the whole, the lower life cycle cost
(LCC) offsets the initial material cost and yields rich
dividends to the end customer.
Why stainless steel?
The salient features of SS that make the material suitable
for bus structures are superior mechanical and thermal
properties, corrosion resistance, low LCC, optimized
structure weight, and full recyclability.
The thermal properties play a significant role when the
structure is exposed to very high temperatures during
welding. The residual stresses in the structure thereby are
reduced, which is a direct benefit in terms of the durability
and longevity of the structure. Due to its superior thermal
stability, SS retains its structural integrity much longer than
that of carbon steel structure and even longer than that of
aluminum.
Stainless steel, by virtue of 10-12% chromium content, has
the intrinsic property to resist corrosion even in extreme
corrosive environments. The chromium forms a passive
layer of chromium oxide on the surface of the structure.
Even in the event of damage to this layer, it has “self-
repairing” properties that help form the passive layer.
Galvanized steel, on the other hand, loses its zinc coating in
three to four years, which leads to severe corrosion of the
structure. While corrosion leads to poor aesthetics, it also
adversely affects the reliability of the structure,
compromising crashworthiness.
With increasing awareness of the characteristics of
stainless steel and its long-term benefits, the developing
BRIC countries are embracing SS in a big way. In India, the
Indian railways, one of the largest rail networks in the
world, has already started migrating to stainless steel. Also
with the advent of metro rail transportation in India, these
coaches perhaps will have SS content from local
manufacturers.
Earlier SS applications in the BRIC regions required either
importing the complete end assembly or getting the
individual members and fabricating in-house. With such
extensive demand and usage of SS, the necessary raw
material availability is no longer a concern.
Another perceived challenge relates to painting SS because
of poor adhesion properties on a smooth surface. This will
not be a bottleneck in the case of a bus structure, which is
always covered by outer panels and hence no painting for
aesthetics is required. However, the underbody is given a
protective coat of wax oil to protect it from erosion and
corrosion.
Manufacturing SS in complex shapes could be costly, but
the type of profiles used in the bus body structure are
simple tubes, channels, and formed sheet metal parts. So
again, this is not a concern.
Looking ahead, new ultra-light grades of SS are being
formulated that can be readily adopted for bus structures.
These materials have to be judiciously used at critical
locations to enhance the strength-to-weight ratio without
drastically increasing the price.
The ‘super structure’
The bus super structure is often divided into the following
basic assemblies, each of which has further subassemblies:
roof structure, side structures, floor structure or under
frame, and add-on structure such as front and rear fascia
reinforcements, roof-mounted AC/CNG/hybrid supports, and
reinforcements for rollover compliance.
The floor structure differs significantly according to the
architecture of the bus. For instance, a traditional floor
structure has chassis C frames and outrigger, whereas
evolved structures have sturdy longitudinal hat sections
that are integrated with equally strong transverse box
sections.
The researchers from Ashok Leyland restricted their
analysis to a monocoque city bus structure that is 8 m (26
ft) in length, because city buses carry the maximum number
of passengers especially during peak hours and are subject
to variable loading during its service cycle.
When it comes to the type of structural members used in a
super structure, it is primarily square/rectangular tubes and
sheet metal parts. Of course, there are other profiles such
as L angles, hat sections, channel sections, Z profiles, and
other custom-formed profiles out of sheet metal.
Irrespective of the dimension of tubes or the profile of the
sheet metals, the costing is done based on weights. Hence,
the weights of all the tubular sections of the super structure
are grouped under one head, totaling 923 kg (2035 lb), and
all the sheet metal profiles under the other, totaling 717 kg
(1581 lb).
Life cycle costing
LCC is a widely used technique to account for various costs
incurred by the customer directly or indirectly starting from
raw material, processing or conversion, manufacturing,
assembly, logistics, maintenance, recyclability, and much
more. It is a very useful tool especially during the process
of material selection at the concept phase of design.
In the present context, this technique is used to compare the
LCC of a SS bus structure and a conventional galvanized
steel structure. The LCC is calculated for the desired life
cycle of a bus, which is 12 years according to the “Urban
Bus Specifications – II” report published by Ministry of
Urban Development, Government of India. The typical life of
a bus is expected to be in the range of 10-15 years.
The various components that contribute to the cost of the
super structure include raw material cost, fabrication cost,
maintenance/replacement cost, and recyclability benefits.
In the given scenario, three different materials are
considered: ferritic stainless steel of 320-MPa (46.4-ksi)
yield strength, galvanized steel also at 320 MPa, and a
conventional structural steel of 240 MPa (34.8 ksi). The raw
material and fabrication cost in itself is a sum total of
various components.
It becomes evident from the cost breakup that the mild steel
incurs additional cost for hot dip galvanizing. Also, this
process involves sophisticated infrastructure with stringent
safety requirements. Besides the cost, this process
increases the manufacturing cycle time by 15%, as reported
in Australian Stainless magazine.
Having worked out the raw material and fabrication cost per
kilogram, SS tube and sheet at this point result in higher
costs, followed by the equivalent-strength galvanized steel
and then the conventional steel.
Owing to the difference in yield strengths, the SS structure
and the equivalent EN10219 galvanized structure are lighter
than the conventional IS 4923 structure by 233 kg (514 lb).
This directly translates into better fuel economy and hence
reduced operating cost, as well as a reduced carbon
footprint.
Tire life is another critical factor. With reduced unladen
weight of the bus (the bus does not run at full capacity all
the time), the tires are loaded less and their life increases
significantly.
Regarding maintenance or replacement cost, the life of a
galvanized steel structure is not more than four years,
beyond which it starts to rust. The minimum expected life of
a SS structure is more than 10 years. Over a 12-year span,
the galvanized steel structure is expected to be replaced
twice before the complete vehicle is scrapped. The SS
structure is considered to be refurbished only once.
The comprehensive LCC comparison for a 12-year span
reveals that the SS structure is about 2.5 times less
expensive than the equivalent-strength galvanized
structure, and about four times less expensive than the
conventional galvanized steel structure.
Concept bus structure with mixed-material concept,
utilizing new ultra-light grades of stainless steel. (Grey = SS
1.4003; Orange = Duplex steel LDX 2304; Tan = Duplex
steel LDX 2305; Yellow = SS Nitronic 60 grade)
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