Remember that the stiffness and strength of a panel varies with the
*cube* of its thickness. An aluminum panel as strong as a steel panel
will still be much lighter than the steel panel. You can make the
aluminum panel a great deal stronger than the steel panel, and it is
still a lot lighter.
That's the whole principle behind replacing steel with aluminum. It's
not only lighter; it's also stiffer and stronger, in terms of plate
stiffness and strength. (Not to complicate this point, but the tensile
and compression strengths of aluminum alloys are nearly identical to
those of steel panels of equivalent weight. But we're talking here
about denting or bending a panel, which is where the cube rule
applies.)
Where it can get complicated is in things like dent resistance. This
can be a complex resolution of forces. When the aluminum panel is a
lot stiffer, that also means that the area surrounding a dent is
putting up a lot more resistance to being bent. So, instead of
oilcanning and bouncing back, as a thin steel panel might do, the same
blow to aluminum might cause a dent, because the surrounding aluminum
is resisting oilcanning and that can allow a concentration of the
denting force in one local spot.
A little thought about this makes it clear that you can't generalize
about the dent resistance of aluminum. It depends a lot on the shape
of the panel. That steel panel might resist oilcanning because it has
a curved shape; it might, therefore, dent more easily than an aluminum
panel. A completely flat steel panel, in contrast, might just spring
away, or "oilcan," when the same force is applied. But you'll notice
that there is more crowning of panels in vehicles today, which is done
to improve stiffness as high-strength steel panels keep getting
thinner. That's how they save weight with the high-strength steels
used in car bodies today. They have to recover the lost stiffness by
crowning and reinforcing the steel.
An aluminum truck can be stronger, stiffer, and lighter than a steel
one. But its ability to resist dents and dings depends on the panel
shape -- in particular, how much it is curved, or crowned.
--
Ed Huntress