When discussing trucks, spike heels, etc., it's important to remember
that force and pressure are different. Most materials respond to stress
(pressure) rather than simple brute force. In the case of spike heels,
for instance, the force exerted on the floor by a woman simply standing on
her heel will just be equal to her weight. However, the compressive stress
on the floor will be equal to the pressure under the heel, which could be
on the order of a thousand psi for a small heel area. As the woman moves
around, the instantaneous forces she exerts on the floor will often be
greater than her weight, and the pressures could become large enough to
exceed the elastic limit of the floor tiles, causing dents.
Even very tiny forces can lead to damaging pressures. How many of us are
old enough to remember phonograph cartridges? A cartridge had a stylus
that pressed on the vinyl surface of a record with a force on the order
of 1000 dynes (a tracking "force" of about 1 gram), but the pressure was
close to the elastic limit of the vinyl, causing the record to be deformed
every time it was played.
In the case of cars, trucks, and other things on the roads, tire pressure
is important. Concrete is strong under compression but is weak under tension
or shear stress. As long as the forces exerted by tires remain perpendicular
to the road, the compressive stress is (I think) about the same as the
instantaneous tire pressure. Since truck tires operate at much higher
pressures than passenger car tires, they are inherently more capable of
damaging the road than the car tires. Again, brute force isn't as important
as pressure. I remember riding my bicycle along an asphalt road one summer
and leaving a permanent rut in my wake, even though passing cars left no
deformation of the road surface. The cars outweighed me and my bike by an
order of magnitude, the 90 psi pressure in my tires was doing more damage
to the road than the 30 psi pressure in the car tires.
If the forces exerted by the tires aren't perpendicular to the road
surface, damage can easily be created by any kind of tires. How many of us
have noticed the wavy deformation of asphalt roads at stop lights and
stop signs? These are caused by hard stops of vehicles moving at high
speeds. The road must exert forces parallel to its surface to stop moving
vehicles, leading to shear stress and possible permanent deformation. On a
concrete road, the concrete will probably break under shear stress rather than
becoming deformed, and the resulting edges then become the focus for even
larger shear stresses.
I'll end this long discussion with a question. I've often thought about
_energy_ delivered to the road by the tires, since a certain amount of
energy is required to cause deformation. Anybody have thoughts on energy
vs pressure for the phenomenon of road damage?
Steve Luzader
Frostburg State University
sluzader@fre.fsu.umd.edu