Factors Affecting The Strength Of Wood

Besides the normal tendency to change of strength among and within species, many other factors may affect the strength of wood. These factors may be broadly grouped into natural defects and irregularities, factors related to the ecosystem and the effects of biological agents.

When the grain direction is not similar to the long axis of a wooden part, it is said to be cross-grained (sometimes referred to as short-grained). Cross grain may occur from spiral grain in the tree or by the manner in which the timber is sawn. In linear furniture parts such as legs and spindles, whose performance depends on longitudinal similarities such as bending resistance, cross grain may consequence in serious strength loss. A slope-of-grain of one in five, for example, may consequence in 50-60% reduction in the modulus of burst. Knots in wood are another major weakening defect. Loss in strength results not only from the abnormal tissue and grain direction of the knot itself, but from the cross grain of wood distorted around the knot. Compression wood, the reaction wood formed in conifers as a consequence of crooked or leaning stems, is usually higher in density and compression strength than normal wood, but the wood is weaker in tensile strength and in both modulus of burst and modulus of elasticity in bending. In hardwoods, tension wood is exceptionally ineffective in compression similar to the grain though it may be stronger in tension and tougher than normal wood of the same density. It displays abnormally high longitudinal shrinkage and slightly increased tangential, but normal radial, shrinkage. The lignin content of the cell wall is deficient compared with normal wood and gelatinous fibres may be present.

As wood dries below the fibre saturation point, strength increases with the loss of bound water. The greatest increases are in compression along the grain: strength is approximately doubled when wood is dried to 12% moisture content, tripled when oven-dried. Modulus of burst is increased much less, and modulus of elasticity is increased least upon drying.

Strength of wood is also affected by temperature, increased as temperature is lowered, decreased as temperature is increased. Over the range and duration of naturally occurring temperature changes, strength changes are permanent. However, if exposed to higher than natural temperatures, or for prolonged periods, long-lasting loss of strength may consequence. Effects of heat in reducing strength are least in dry air, greatest in moist air or steam. The use of steaming to temporarily plasticize wood for long-lasting bending of furniture parts is well known.

Strength of wood is also related to duration of loading. Time-related crawl in wood reduces strength over longterm load periods. For example, a beam might carry a short-term (5 minutes) load three times as great as it could carry for a long term (a hundred years or longer).

The destructive effects of wood-inhabiting insects such as termites, carpenter ants and beetles need little elaboration, as the physical loss of wood will consequence in proportional loss of strength. Fungi are a major cause of decline in wood. In order for the threadlike hyphae of fungi to develop in wood, four major requirements are necessary: favourable temperature (70-85 °F is ideal), oxygen (20% or more air quantity in the wood), moisture (fibre saturation point or above is ideal), and food. Wood-staining fungi utilize the residues of stored materials in parenchyma cells of sapwood but they do not attack cell walls. consequently, although the staining fungi discolour the wood, they do not reduce its strength. However, the wood-destroying fungi utilize enzymes to break down and assimilate the cell wall substance, producing various forms of decay or decay. Initial stages of fungal invasion, termed incipient decay, may at first have insignificant effect on strength. Impact strength is the first strength character to be affected. If allowed to continue, total loss of strength may consequence. It should be noted that Chlorociboria both deposits a green stain in the wood and causes losses of strength. Controlling moisture is the principal approach to preventing decay. If wood is maintained below 20% moisture content, decay fungi cannot develop.

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