The moisture content has a decided bearing also upon the manner in which wood fails.  In compression tests on very dry specimens the entire piece splits suddenly into pieces before any buckling takes place (see Fig. 9.), while with wet material the block gives way gradually, due to the buckling or bending of the walls of the fibres along one or more shearing planes. (See Fig. 14.) In bending tests on wet beams, first failure occurs by compression on top of the beam, gradually extending downward toward the neutral axis.  Finally the beam ruptures at the bottom.  In the case of very dry beams the failure is usually by splitting or tension on the under side (see Fig. 17.), without compression on the upper, and is often sudden and without warning, and even while the load is still increasing.  The effect varies somewhat with different species, chestnut, for example, becoming more brittle upon drying than do ash, hemlock, and longleaf pine.  The tensile strength of wood is least affected by drying, as a rule.

In drying wood no increase in strength results until the free water is evaporated and the cell walls begin to dry[49].  This critical point has been called the fibre-saturation point.  (See Fig. 24.) Conversely, after the cell walls are saturated with water, any increase in the amount of water absorbed merely fills the cavities and intercellular spaces, and has no effect on the mechanical properties.  Hence, soaking green wood does not lessen its strength unless the water is heated, whereupon a decided weakening results.

[Footnote 49:  The wood of Eucalyptus globulus (blue gum) appears to be an exception to this rule.  Tiemann says:  “The wood of blue gum begins to shrink immediately from the green condition, even at 70 to 90 per cent moisture content, instead of from 30 or 25 per cent as in other species of hardwoods.”  Proc.  Soc.  Am.  For., Washington, Vol.  VIII, No. 3, Oct., 1913, p. 313.]

[Illustration:  FIG. 24.—­Relation of the moisture content to the various strength values of spruce.  FSP = fibre-saturation point.]

The strengthening effects of drying, while very marked in the case of small pieces, may be fully offset in structural timbers by inherent weakening effects due to the splitting apart of the wood elements as a result of irregular shrinkage, and in some cases also to the slitting of the cell walls (see Fig. 25).  Consequently with large timbers in commercial use it is unsafe to count upon any greater strength, even after seasoning, than that of the green or fresh condition.

[Illustration:  FIG. 25.—­Cross section of the wood of western larch showing fissures in the thick-walled cells of the late wood.  Highly magnified. Photo by U. S. Forest Service.]

In green wood the cells are all intimately joined together and are at their natural or normal size when saturated with water.  The cell walls may be considered as made up of little particles with water between them.  When wood is dried the films of water between the particles become thinner and thinner until almost entirely gone.  As a result the cell walls grow thinner with loss of moisture,—­in other words, the cell shrinks.