Chapter 7 FACTORS AFFECT THE MECHANICAL PROPERTIES OF

Chapter 7 FACTORS AFFECT THE MECHANICAL PROPERTIES OF WOOD

Learning Outcome At the end of this Chapter, student should be able to, Define the factors that affect the mechanical properties of wood Elaborate the relationship between density and moisture content of the wood and mechanical properties of wood

Introduction Because of natural growth characteristics of trees, wood products vary in specific gravity, may contain cross grain, or may have knots and localized slope of grain. Natural defects such as pitch pockets may occur as a result of biological or climatic elements influencing the living tree. These wood characteristics must be taken into account in assessing actual properties or estimating the actual performance of wood products.

Factors Affect Mechanical Properties of Wood Besides the normal variability 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 environment and the effects of biological agents. Natural Defects Factors related to the environment Biological agents

1. Natural Defects Wood is a natural composite that exhibits a remarkable combination of strength, stiffness and toughness. At the same time being a natural/organic product, it is a structure of infinite variation of detail and design. It is due to this reason that no two specimens of wood log are alike, in spite of being harvested from the same tree.

Cross grain - When the grain direction is not parallel to the long axis of a wooden component, 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 properties such as bending resistance, cross grain may result in serious strength loss. A slope-of-grain of one in five, for example, may result in 50 -60% reduction in the modulus of rupture.

Knots -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 result 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 rupture and modulus of elasticity in bending. In hardwoods, tension wood is exceptionally weak in compression parallel to the grain though it may be stronger in tension and tougher than normal wood of the same density. It exhibits 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.

2. Environment Factors Wood is a porous material made up of cells of various kinds. Depending on the nature of these cells, some woods have more or less solid wood substance for a given sized piece. with wood, the fewer holes (cells), the more wood substance. These wood substances direct related with those environment factors, therefore the mechanical properties of wood are affected by various factors, mainly moisture, density, temperature, duration of loading, and defects.

Moisture Content- 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 rupture is increased much less, and modulus of elasticity is increased least upon drying. The magnitude of moisture influence is different in different properties. According to studies of this relationship, a 1 % change of moisture changes the strength in axial compression by 6 %, bending strength 5 %, hardness 2. 5 – 4 %.

Density - density is the best and simplest index of the strength of wood. With increasing density, strength also increases. Greater density derives from a greater proportion of cells with thick walls and small cavities, and this result in higher strength of wood. The amount of wood substance for a given volume determines density. Woods with more weight for a given volume have a higher density than woods with less weight.

Temperature - 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 temporary. However, if exposed to higher than natural temperatures, or for prolonged periods, permanent loss of strength may result. Effects of heat in reducing strength are least in dry air, greatest in moist air or steam.

3. Biological Agents Two types of biological agents cause the wood loss in strength. The destructive effects of wood-inhabiting insects such as termites, carpenter ants and beetles need little elaboration, as the physical loss of wood will result in proportional loss of strength. Fungi are a major cause of deterioration 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 volume 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. Therefore, although the staining fungi discolour the wood, they do not reduce its strength.

Wood-destroying (decay) fungi - seriously reduce strength by metabolizing the cellulose fraction of wood that gives wood its strength. Early stages of decay are virtually impossible to detect. For example, brown-rot fungi may reduce mechanical properties in excess of 10% before a measurable weight loss is observed and before decay is visible. When weight loss reaches 5% to 10%, mechanical properties are reduced from 20% to 80%. White rot wood decay under Malaysian (humid tropical) terrestrial conditions pose more serious threats to the in-ground service life of hardwoods than other common fungal decay types. Hevea brasiliensis (rubberwood) suffered the most severe wood decay with average percentage mass loss of 43. 9% challenged with a representative virulent Malaysian white rot Basidiomycete Pycnoporus sanguineus.

Insect – Insect damage may occur in standing trees, logs, and undried (unseasoned) or dried (seasoned) lumber. Although damage is difficult to control in the standing tree, insect damage can be eliminated to a great extent by proper control methods. Insect holes are generally classified as pinholes, grub holes, and powderpost holes. Because of their irregular burrows, powderpost larvae may destroy most of a piece’s interior while only small holes appear on the surface, and the strength of the piece may be reduced virtually to zero.

Summary The mechanical properties of wood are affected by various factors, mainly moisture, density, temperature, decay and defects. These factors will reduced the strength and stiffness of the wood. Some of the factors will reduced the usage of wood for further application. Care should be taken to protect the timber/wood for the factors that might affect the mechanical properties of wood.