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.
This article is based on SAE International technical paper 2013-01-2418 by Sreedhar Reddy and Vignesh T Shekar of Ashok Leyland, Ltd.