If the wood shrinkage takes place below the fiber saturation point, why wood disks cut from freshly felled trees, still saturated, develop cracks?
Some colleagues think it is due to growth stress.
As the split occurs in transverse direction and growth stress takes place in longitudinal direction, I do not see how growth stress acts on this phenomenon.
The enclosed picture shows the kind of crack I am talking about. The disc shown in the Figure is dried, but I observed some green discs from young Eucalyptus trees presenting this type of crack after cutting.
I am afraid I can´t give you a satisfying answer to your question but maybe my observations help you on your way to solve this riddle.
I am familiar with this (sometimes annoying) phenomenon since I have worked on quiet a lot of stem discs myself and I think the radial cracks are mostly the result of the shrinking process within the rings. Since the circumference of the outer rings is comparatively longer with increasing distance from the pith the crack there is accordingly wider.
I also experienced that thicker discs contain a lower risk to crack, I suspect due to a slower drying process. Another observation of mine is, that discs we took from very slow growing coniferous trees of the understory in the Siberian boreal forest with a high proportion of compression wood did not crack often, maybe due to the relative increase to cell wall thickness versus lumen diameter. But when such a slow growing tree suddenly experiences a fast increase of radial growth due to changed conditions, these discs are not only showing radial cracks but also tangential cracks along the ring boundaries between the last narrow and the following wide ring. Since different tree species express different crack patterns I think woodanatomical features (soft vs. hard wood, ring vs. diffused porouse, etc) play also a role here.
Further I think the initial water content plays an important role…that’s why the professional wood harvests in our region traditionally takes place in winter, when the water content of the wood is lowest. It is also common practice to cover the cutting edges of valuable hardwood logs with latex paint to slow down the drying and therefore prevent or reduce the crack formation.
Concerning the cracks right after cutting the discs I can imagine that the loss of the water pressure within the cells due to the cutting already initiate the shrinking and therefore the crack formation...but this is just a suspition.
These crack lines are the first area that the moisture can be turn to vapor and leave the wood piece.
Its a very good question. Let me see if i can help you.
The shrinkage is not homogeneous. While the disk surface, due the contact with the external environmental, reach faster to below of the saturation point, the inner part still is near of the maximum moisture content. Due this delta, the surface shrinks while the middle not, starting the split in the weaker point of the tissue: rays parenchyma cells (if you have boundaries parenchyma type like jatobá, mahogany, etc. the split could be in that way).
The wood tissue have an great resistance to compression, flexion, and another others... but we can not say the same to split. In this way, the open newly surface by the split start to shrink in a different compass of the middle, generating an deeper split, occurring that again and again until we have an big crack.
I thinks this wood has growth stress in own. and when the be cut. this stress be free and make crack and split in wood and the end thanks for your good ask.
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