 The high mechanical performances of stainless steels allow manufacturing light framework. In fact, though stainless steel density is greater than that of the light alloys commonly used in the automotive industry, this material is characterized by a coefficient of elasticity that allows it achieving a similar specific rigidity (ratio between coefficient of elasticity and material density). An accurate design and planning stage allows developing and producing structures ensuring the same performances as those provided by light alloys. At the same time, it is possible to obtain also considerable savings in weight, since ultimately it is not the material density that determines the weight of a structure, but rather an optimum exploitation of the material properties during the planning stage.
 In addition, stainless steel has the capacity to absorb a considerable amount of energy in terms of deformation work, and is characterized by a mechanical behaviour influenced by the cold forming and by the speed through which deformation is impressed. For these reasons stainless steel is particularly suggested for manufacturing parts destined to absorb energy in the event of impacts. In fact, the strain hardening resulting from forming operations determines a yield point increase; a higher deformation speed involves the consequent use of an increased stress for achieving the required deformation. In both cases ductility reduction is negligible, as it can be determined through the high percent ultimate elongation of these materials, which is an index of the amount of absorbed deformation strength.
The first experiences made by the four-wheel industry date back to about 15-20 years ago when, because of the high corrosion rates to which some parts under the “waist” of buses were subject, planning engineers began to consider stainless steel as an alternative material to be used for definitely solving this hoary problem. Some bus manufacturers, encouraged by the high corrosion resistance and life prospects of stainless steel parts, began producing vehicles designed with a self-bearing frame entirely made of square and rectangular tubes, and selected for this purpose AISI 304 stainless steel. Over the years, this production “philosophy” has been followed by other bus manufacturers, and nowadays stainless steel has become an important reference material for any bus manufacturer world wide.
At the beginning of 2004, basing on the favourable results achieved in bus manufacturing, some automakers specialized in the production of mini-cars (classified in Italy as “light four-wheel vehicles”), in order to fully meet the safety and lightness requirements they had set, decided to use AISI 304 stainless steel tubes in the construction of the chassis of those “microcars”. In 2004, again, “NIDO”, a prototype of a two-seater car produced by Pininfarina  (Cambiano – Turin) saw the light. Presented in a preview at the car show “Mondial de l’Automobile” of Paris and later on at the Motor Show of Bologna, this vehicle has been designed and planned to meet the ever-growing “safety” properties required in the event of head-on collision. An ideal alliance between a new construction principle specially conceived for absorbing impacts, and a material – stainless steel – presenting high energy absorption characteristics in the event of impacts, and an increasing mechanical strength proportioned to the deformation speed.
Along with these special applications, in which this material is used for the whole structural part of the chassis of a car, the major world automakers have also begun using stainless steel in the construction of parts that prove particularly “critical”, in terms of safety, in the event of collisions. This is the case, for example, of particular car models or complete lines, such as Porsche GT, AUDI, Saab 9-3 Sport Sedan, and Hyundai.
While the world of the four wheels was taking its first steps in the direction of a structural use of stainless steel, the world of the two wheels did not certainly remain  idle in the meantime. During the Milan “Motorbike Show” of the past year, the new supermotard STR 650 CRC was presented in a world preview. This innovative model provides for a synergic use of AISI 304 stainless steel and aluminium alloy. A special frame (AIF – Aluminium Inox Frame) was designed for this new single-cylinder motorcycle with a four-stroke and liquid-cooled engine. The visible parts of the stainless steel frame, in addition to a natural sensation of sturdiness, give also this vehicle an elegant look, thanks to a satin finishing that makes the overall design extremely captivating.
From a structural viewpoint, fatigue tests and those performed on test track and road have confirmed the unquestionable performance of stainless steel.
The ever-growing interest shown by the automotive and motorcycle industries in the use of stainless steel as a structural material is an evidence of the possibilities provided by this material in a wide range of applications in which corrosion resistance may even take a minor value in comparison with other priorities.
Didascalie
Pict. 1: Long-distance bus with stainless steel framework
Pict. 2: Stainless steel tubular bearing structure
Pict. 3: AISI 304 stainless steel tubular frame of a light four-wheel vehicle
Pict. 4: It is possible to drive a microcar since the age of 14 years. Maximum speed cannot exceed 45 km/h, maximum engine power 4 kW, and overall weight 350 kg
Pict. 5: The NIDO at the 2004 car show “Mondial de l’Automobile” of Paris
Pict. 6: The “undressed” version of the NIDO: without the body, the structural stainless steel frame is clearly visible
Pict. 7: The STR 650 CRC at the 2006 Motorbike Show of Milan
Pict. 8: A detail of the stainless-steel tubular structure of the frame
By P. Viganò V. Boneschi |
