On Fri, 13 May 2005 09:01:53 -0400, Guess who
wrote:

If still confused, think of expansion and contraction of a steel rod
due to heating/cooling.

That's a bad analogy. Steel is isotropic - it has the same behaviour in
all directions. Timber is anisotropic, the shrinkage is different on all
three axes. A "steel tree" wouldn't crack as it dried out.

Most carpenters know that timber shrinks "crosswise" but not
"lengthwise". That's still not enough to cause cracking. What many
carpenters don't realise is that the tangential shrinkage is twice the
radial shrinkage. It's this, coupled with the fact that trees are made
of nested cylinders, which causes the radial cracking.

The outer _may_ dry out faster than the inner (although this is much
overshadowed by the effects of the ends, and the much greater water
transport lengthwise). If you bake the outside of a log to dry it you
could even "case harden" the timber and cause checking - however this
would be the usual honeycomb checking of bad seasoning, not the
inevitable radial cracking of drying logs in the round.

A consideration of shrinkage and the geometry will show us that a dry
log cracks, no matter how slowly or carefully you do it. Thinking about
the strains rather than the stresses will show that all timbers will do
this, not just "weak" ones.

--
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