Wood as a Lightweight Cellular and Fiber Reinforced Material

Secondary growth produces an efficient support tissue: wood. Wood has been used by human beings for many years — dried wood has been used to construct buildings or make furniture. Such dried wood has a moisture content that depends on air temperature and humidity, and is made up of wood cells possessing empty lumina. Living trees, however, possess green wood. In green wood, cell walls are saturated, and additional water also fills up the lumina [13]. Because the mechanical properties of wood depend on the moisture content of the cell wall, the drier the wood, the stiffer and stronger it is [13]. Caution should thus be taken when using engineering literature in wood sciences because databases are not always suitable for biome-chanical analyses dealing with moist, green wood. However, mechanical properties do not vary significantly beyond a moisture content of approximately 30% (on a dry weight basis) when cell walls are saturated and lumina empty [13,14]. In living trees, water transport affects lumen water content with cell walls in the sapwood being completely or partially saturated. As a consequence, although wood moisture content varies in living trees, e.g., according to seasons, species, and ontogeny, the variations of mechanical properties of green wood, i.e., stiffness and strength, during the growing season can be neglected.

Rheological data concerning green wood are scarce (but see, for example [15,16]), and there is a need for more systematic studies in this area. Meanwhile, whenever comparisons are made, there is usually a good correlation between the properties of green and dry wood used to estimate green wood properties [14]. Because of its complex structure at different scales, wood can be considered to be a very "high tech" material. An analysis of specific properties, i.e., ratios of mechanical properties to density, reveals that at the cellular level, wood is a "honeycomb-like" lightweight material of high performance. This cellular structure is also the origin of the close relationship between dried wood specific gravity, which represents the amount of supporting material characterized by its porosity, and mechanical properties [17,18]. For instance, using the regressions established by Guitard [17] at an interspecific level on a wide sample of species with a large range of densities, and transforming mass, volume, and modulus of elasticity of air-dried wood to green wood and oven-dried properties, we can approximate the parallel to the grain modulus of elasticity of green wood for angiosperms by:

where E is the modulus of elasticity of green wood (MPa) (pooling together estimations by several methods: tension, compression, bending) and D is the basic density, i.e., the ratio of the oven-dried biomass to the volume of green wood.

These relationships show an approximately constant ratio between E and basic density. Thus, as pointed out by several authors [19-21], wood's mechanical efficiency relative to stiffness and dry biomass available is almost constant, no matter how porous the wood. However, dried biomass does not represent the true weight supported by a living tree, and the ratio of E to humid density changes as the more porous wood can absorb more water (Figure 1.1). Furthermore, an exhaustive dis-

Basic density D

FIGURE 1.1 Evolution of the specific modulus of elasticity for angiosperm green wood (ratio of the modulus of elasticity E to wood density) with basic density D. D is the amount of dried biomass per unit of green volume, i.e., the cost of support. DS (dotted line) is the density at full saturation, i.e., cell lumens are entirely filled with water, for wood density obtained in functioning sapwood, i.e., maximal self-weight of support organs. E/D is almost constant while E/DS increases significantly with wood basic density.

Basic density D

FIGURE 1.1 Evolution of the specific modulus of elasticity for angiosperm green wood (ratio of the modulus of elasticity E to wood density) with basic density D. D is the amount of dried biomass per unit of green volume, i.e., the cost of support. DS (dotted line) is the density at full saturation, i.e., cell lumens are entirely filled with water, for wood density obtained in functioning sapwood, i.e., maximal self-weight of support organs. E/D is almost constant while E/DS increases significantly with wood basic density.

cussion about design should also include additional branch and leaf weights. Thus, wood mechanical performance relative to design against, for example buckling, can change from light to dense woods. Such a distinction between mechanical efficiency, i.e., the cost of support per unit of dried mass, and performance (design safety relative to supported, humid mass) has never been considered.

At the level of the cell wall, wood is a multilayered material and can be considered as a reinforced composite made up of microfibrils composed of crystalline cellulose embedded in a matrix of lignins and hemicelluloses [22,23]. This composite structure is the major reason for the high anisotropy of wood: mechanical stiffness and strength are much greater along the grain, in the direction more or less parallel to the stem axis. This longitudinal direction is usually the most loaded direction and is held in bending in beamlike structures, such as trunks and branches. Because the cellulose microfibrils are very stiff, one important structural feature at the cell wall level is the angle between cellulose microfibrils and the cell axis in the S2 layer [22]. Significant changes in this microfibril angle (MFA) can be observed, such as in juvenile and compression wood, which have a much greater MFA [22,24]. Therefore, these types of wood are much less stiff than can be expected from their density, e.g., by using standard formulas to estimate the modulus of elasticity from wood dried density [17,18].

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