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This technical paper deals with the development of a body shell mainly made of GFRP materials for small buses (19 to 34 passengers). The main goal was to substitute the monocoque design with a more efficient “modular panels” design. Fundamental aspects such as design, fabrication and the assembly of different parts of the body shell are discussed. The main advantages of the modular design are also discussed.
J. M. MEZA, GRUPO DE CIENCIA Y TECNOLOGÍA DE LOS MATERIALES, ESCUELA DE INGENIERÍA DE MATERIALES UNIVERSIDAD NACIONAL DE COLOMBIA
J. A. RESTREPO, ICOLFIBRAMANAGER
J. CRUZ, GRUPO DE INVESTIGACIÓN EN NUEVOS MATERIALES, UNIVERSIDAD PONTIFICIA BOLIVARIANA
Introduced in April 2009, Evolution-S is the result of a joint research project led by the Colombian government, the Pontifical Bolivarian University (UPB) and Icolfibra, a private company. The project was launched in May 2007 with the aim of finding a more efficient way to produce a body shell for small buses with a capacity for 19 to 34 passengers. For 20 years, Icolfibra have produced monocoque body shells with GFRP materials using two moulds and a combination of different techniques such as conventional spray lay-up, open-mould laminating techniques and polyurethane injection.
However, the manufacturing process was inefficient because a different mould was necessary for each chassis size. Moreover, residence times of the GFRP materials inside the mould were long. For these reasons, a new manufacturing process and a new body shell design were required. The final product was called Evolution-S. This paper describes the main design steps and discusses the main advantages of the final product.
The new product
Figure 1 shows the new design concept called modular panels. The principle is based on producing different sizes of floor, roof and side panels using the same moulds, while obviously being limited by the maximum size of the mould. These parts are assembled using mechanical fasteners and adhesives.
Building on knowledge gained through the construction of earlier bus projects, most of the bus body was designed using sandwich-type structures composed of GFRP skins and a polyurethane foam core.
The first design step for the modular solution consisted in drawing a 3D prototype using the Solid Edge software . Critical parts such as girders (see Figure 1) and joining parts were modelled using the ANSYS finite-element software . Several prototypes were constructed and mechanically tested based on the results of these simulations.
Based on the experimental results, the parts were redesigned and the results were incorporated in the 3D prototype. This 3D prototype was used to manually construct the models, which were mostly made of wood. These models were used to produce the GRPF moulds.
Figure 2 shows a schematic representation of a side wall section, a detailed view of the system used to join the side wall with the floor, and a section of the real part. The “omega” is made of laminated GFRP and has a metallic sheet insert to help better distribute the load transferred by the fasteners. An epoxy resin is also used to improve the joining of the different parts.
< /p> This new modular technology with minimum mechanical and chemical bonding results in great production flexibility. Each modular panel requires lower holding times in the mould, allowing the production of different body sizes (from 19 to 34 passengers) without requiring individual moulds for each of them. This new technology has increased production by almost 200% while reducing production costs by about 20%.
Passenger safety is a key requirement in public transportation systems. The Colombian Technical Standards (NTC) require a static load test (compression of the superstructure) . In our case, the test was carried out at a maximum compression load of 4.5 metric tons. Structural deflection was lower than 5 cm, resulting in a safety factor of about 2 which complies with the standard. The Colombian government will soon release the necessary facilities for rollover and corner jacking tests which will be required by the NTC standard in the near future. The final product is made of about 98 wt% GFRP. It is more than 30% lighter than metallic bodies produced by the nearest competitor. This results in an increased load capacity and promotes the use of alternative fuels such as gas and electricity.
So far, 18 vehicles have been commissioned, carrying an average of 170,000 monthly passengers in the cities of Bogotá, Medellín and Bello.
The statistics provided by the companies report a 35% reduction in fuel consumption and 20% in maintenance costs. Other obvious benefits include reduced environmental emissions, highway network protection, reduced noise emissions compared with metallic buses, and increased resistance to corrosion resulting in longer body life, which is especially useful in tropical weather.
A flexible method to facilitate the production of a bus body has been developed. This body is made almost entirely of GFRP materials and is much lighter than its metallic counterparts. The body not only saves production and operation costs, but also meets the NTC standard requirements.
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