Water occurs in living wood in three conditions, namely: (1) in the cell walls, (2) in the protoplasmic contents of the cells, and (3) as free water in the cell cavities and spaces. In heartwood it occurs only in the first and last forms. Wood that is thoroughly air-dried retains from 8 to 16 per cent of water in the cell walls, and none, or practically none, in the other forms. Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry.
The general effect of the water content upon the wood substance is to render it softer and more pliable. A similar effect of common observation is in the softening action of water on rawhide, paper, or cloth. Within certain limits the greater the water content the greater its softening effect.
Drying produces a decided increase in the strength of wood, particularly in small specimens. An extreme example is the case of a completely dry spruce block two inches in section, which will sustain a permanent load four times as great as that which a green block of the same size will support.
The greatest increase due to drying is in the ultimate crushing strength, and strength at elastic limit in endwise compression; these are followed by the modulus of rupture, and stress at elastic limit in cross-bending, while the modulus of elasticity is least affected. These ratios are shown in Table XV, but it is to be noted that they apply only to wood in a much drier condition than is used in practice. For air-dry wood the ratios are considerably lower, particularly in the case of the ultimate strength and the elastic limit. Stiffness (within the elastic limit), while following a similar law, is less affected. In the case of shear parallel to the grain, the general effect of drying is to increase the strength, but this is often offset by small splits and checks caused by shrinkage.
| TABLE XV | ||||||
|---|---|---|---|---|---|---|
| EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD,SHOWN IN RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM THE GREENCONDITION TO KILN-DRY (3.5 PER CENT) | ||||||
| (Forest Service Bul. 70, p. 89) | ||||||
| KIND OF STRENGTH | Longleaf pine | Spruce | Chestnut | |||
| (1) | (2) | (1) | (2) | (1) | (2) | |
| Crushing strength parallel to grain | 2.89 | 2.60 | 3.71 | 3.41 | 2.83 | 2.55 |
| Elastic limit in compression parallel to grain | 2.60 | 2.34 | 3.80 | 3.49 | 2.40 | 2.26 |
| Modulus of rupture in bending | 2.50 | 2.20 | 2.81 | 2.50 | 2.09 | 1.82 |
| Stress at elastic limit in bending | 2.90 | 2.55 | 2.90 | 2.58 | 2.30 | 2.00 |
| Crushing strength at right angles to grain | 2.58 | 2.48 | ||||
| Shearing strength parallel to grain | 2.01 | 1.91 | 2.03 | 1.95 | 1.55 | 1.47 |
| Modulus of elasticity in compression parallel to grain | 1.63 | 1.47 | 2.26 | 2.08 | 1.43 | 1.29 |
| Modulus of elasticity in bending | 1.59 | 1.35 | 1.43 | 1.23 | 1.44 | 1.21 |
| NOTE.—The figures in the firstcolumn show the relative increase in strength between a green specimenand a kiln-dry specimen of equal size. The figures in the second columnshow the relative increase of strength of the same block after beingdried from a green condition to 3.5 per cent moisture, correctionhaving been made for shrinkage. That is, in the first column thestrength values per actual unit of area are used; in the secondthe values per unit of area of green wood which shrinks to smaller sizewhen dried. See also Cir. 108, Fig. 1, p. 8. | ||||||
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.
Figure 24
Relation of the moisture content to the various strength values of spruce. FSP = fibre-saturation point.