" wrote:

Angy! Wow great reply!

Ok so why does it strain that way, that is the real question. Why does
it shrink on the outside more than the inside?

Because it reduces in direct proportion to the initial size as Andy
notes--ergo, larger diameter reduces more than smaller.

The reason I think is this. When I look at a round disk of wood, as its
drying, you can see the darkness change as it dries. It dries faster on
the outside of the disk, than the inside. That's why it splits.

Why does it dry faster on the outside of the disk? Because it has more
surfaces through which to evaporate (i.e. the outer surface, where the
bark may be). The inside part of the disk can only evaporate through
the front of back of the disk. Anyway, the disk does look like its
darker on the inside and dry on the edge, when I look at one, its
fairly obvious.

The moisture has to diffuse to a surface before it can evaporate and
diffusion is driven by concentration differential and controlled by the
material properties. In a uniformly porous material, diffusion is more
nearly homogenous in all directions where as in wood it is constrained
much more to go in the direction parallel to the growth rings. The more
porous-diffuse the wood, the more extreme the differential.