Why does wood split radially?

[Guess who]

The cause is that the wood cells are very much longer in the vertical [alive and well] direction as compared to the horizontal. Think of a lot of thin tubes lightly glued [bonded] together. Suppose they all shrink equally in all directions. Now think of the structure of the tree, and simplify it. One cell in the center. More around that, more around that, more around that ...ever increasing circles of cells. Now let all shrink in all directions equally, so losing the same percentage per cell. The inner ones will each shrink by the same absolute amount as those individual cells around the outside, but since there are more around the outside, the total absolute amount will be greater there.

If this is correct, then [near as dammit is to swearing], thinking in term of a sequence of circles, the percentage of crack-space in an inner circle should be the same as the percentage of space in the outer edge, and in fact throughout the material: More material, more space, same percentage.

If still confused, think of expansion and contraction of a steel rod due to heating/cooling. A longer rod will expand and contract further than a small one. The measure of wood around the outside of a log is greater than around the center, and the tree grows in definite rings.

Why does it crack in the first place? Bonding. Even steel cracks under pressure since it is uneven throughout, even though it looks uniform outside of a microscope. I'll quit now. My brain is exploding.

[Andy Dingley]

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.

[Lawrence Wasserman]

I imagine this would work, say you very carefully controlled the drying of a tree trunk cross section so that it took 100 years, then it would probably not split until after the first fifty years or so had passed.