For many years Wood was the most favourable choice for construction of Vehicle-bodies in the transportation sector.

Let us look at the reasons behind this choice:

• Traditionally wood was used in the transportation sector for building Chariots, Animal drawn Carts, Palanquins etc – hence it became the natural choice for building bodies of the automobiles, omnibuses etc. at the first phase of industrial revolution.
  
  
• Wood has impressive mechanical properties. The elastic modulus of wood is in the range of 8-20 GPa which is as good as materials like PMMA and GFRP. The density of wood is about 0.6-75 Mg/m³ – lighter than most of the polymers except polymeric foams. The strength of the wood is about 30 MPa which is again comparable to high-performance polymers.
  
  
• Other advantages of wood are recyclability, ease of machining and aesthetically pleasing quality.

With the advent of mass-scale production and automation in car-industry, it became necessary to replace wood by metals and metallic alloys. Typical metal shaping technologies like sheet forming which can handle large batch size (10⁵ to 10⁶ units per batch) became very much suitable for the massscale production of vehicles.

There were two-choices in terms of use of metals and metallic alloys: Steel and Aluminium Alloys. Why these materials became so popular for Car-design? Let us find the material indices most relevant from car-body construction point of view. It is observed that three most significant issues in car-body design are:

1. Stiffness of the sheets which is expressed as an objective to minimise mass against a specified deflection limit. Minimisation of mass directly implies the use of less amount of material and hence less cost per unit. Also, mass minimisation would increase fuel efficiency of the vehicle. For a flat panel of size (LxB), thickness t, modulus of elasticity E and density ρ, this would involve the search for a material having maximum value of an index
   (E^1/3/ρ). Later we will discuss about the origin of such indices.
   
2. Another important consideration is dent resistance. A similar study would indicate that this requires the maximisation of an index (σ_y t⁴/k), where σ_y is the yield strength and k is the stiffness of the panel.