In the automotive industry, many external forces influence a particular design concept and corresponding material selection for a prospective assembly or component; including such things as gas prices, government mandates (e.g. CAFE standards), or company design philosophies (e.g. the aluminum Audi A8).   

But fundamentally, in selecting a material, either steel or aluminum, for a specific structural or closure application, the gauge and/or the component cross-section required is governed by eight key parameters:

1. Shape Factor (f) - Refers to the degree of part shape optimization

2. Yield Strength - The onset of permanent deformation 

3. Ultimate Tensile Strength - Point where on plastic region added strain reduces force 

4. Young Modulus - The inherent material stiffness (the slope of the elastic region)

5. Poisson’s Ratio - The ratio of lateral and axial strains

6. Density - How many times heavier than water (= 1)?

7. Cost - Includes material cost and the cost of development and production

8. Manufacturability - Is the process at both the material supplier and OEM robust (formability, weldability, handling, mill specs)?

For steel or aluminum, only yield strength is influenced by chemical composition and temper. Young’s Modulus, Poisson’s Ratio, and Density are essentially constant.  Figure 1 shows a breakdown of steel grades applied to a body structure.

Figure 1:  Shown is the ULSAB - AVC isometric breakdown, illustrates the magnitude of various steel grades with varying strength levels (YS of 210 to 1000 MPa).  Each of these grades, with tailored material properties are efficiently applied to either strength sensitive or stiffness governed members (source: American Iron and Steel Institute).

 Back | Top | Next