In general, in material science, you want to maximize one property while minimizing another; in the case of the chassis of a FIRST robot, the two important properties are weight and stiffness. Because of this, it is often meaningless to compare just the tensile strength, or just the density of two materials. Rather, it is more useful to compare the ratios of these two properties for a given material. In the case aluminum and steel, the ratio of stiffness/density is actually comparable in high strength alloys of both (though by no means insignificant). Because most FIRST robot designs are hardly very well optimized with respect to stiffness/density, other considerations become more important in deciding what material to use. Several important differences between steel and aluminum that make aluminum usually the better choice are
- it comes in much thinner walled tubing than steel (more efficient shape)
- easier to machine
- lower density (makes simple, inefficient designs made from solid stock less costly in weight)
I sort of explained all this in another post and gave some ratios for certain materials (
http://www.chiefdelphi.com/forums/sh...647#post307647). I realized now that I used ultimate tensile strength while Young's modulus would be the correct measure for stiffness, but the comparison is still somewhat valid.