The Structure of Wood

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1 WOOD Wood Wood is the oldest and still most widely used of structural materials. In the 17th and 18th centuries the demand for wood is so great that much of Europe was deforested. Today, about 10 9 tons of wood was consumed each year. 1

2 The Structure of Wood At macroscopic level, it can be seen that the main cells (fibers or tracheids) run axially up and down the tree: this is the direction in which the strength is the greatest. The wood is divided radially by the growth rings: differences in density and cell size caused by rapid growth in the spring and summer, and sluggish growth in the fall and winter. Most of the growth of the tree takes place in the cambium, a thin layer just below the bark. The Structure of Wood At microscopic (light microscope or SEM) level, the wood is made up of long hollow cells, squeezed together like straws. The axial section shows roughly hexagonal cross-sections of the cells; the radial and the tangential sections show their long, thin shape. There are fat tubular sap channels running up the axis of the tree carrying fluids from the roots to the branches. The strings of smaller cells are called rays, which run radically outwards from the center of the tree to the bark. The mechanical strength is mostly provided by the fibers. 2

3 The Structure of Wood At molecular level, the walls of the tracheid cells have a composite structure like fiberglass. The strong fibers are made of crystalline cellulose, a high polymer (C 6 H 10 O 5 ) n (DP ~10 4 ) made from glucose (C 6 H 12 O 6 ) by the tree through a condensation reaction. Cellulose has no side group so that it can crystallize easily to form microfibrils. The microfibrils (~45% of the wall) are embedded in the lignin, an amorphous polymer, and hemicellulose, a partly crystalline cellulose with smaller DP (~40% of the wall). The Structure of Wood There are also water and extractives (oils and salts which give wood its color, smell, and its resistance to beetles, bugs, and bacteria). They make up ~10% of the cell wall. 3

4 The Structure of Wood At the molecular level, the basic structures of the cell walls in different woods are quite similar. The cell wall is a little less stiff, but nearly as strong as an aluminum. The cell walls are helically wound (like the handle of CFRP golf club), with the fiber direction nearer the cell axis rather than across it, which gives the cell wall a modulus and strength that are large parallel to the axis and smaller (by a factor ~3) across it. The Structure of Wood Both of the foam cells and the cellulose fibers in the cell wall are aligned predominantly along the grain of the wood. Wood is mechanically very anisotropic. The properties along the grain are quite different from those across it. Since all woods are made of the same stuff, the difference between them is mostly caused by the difference of the relative density. 4

5 The mechanical properties depend on the water content (~50% in green wood). Seasoning (several years) or kiln drying (several days) can bring the water content down to ~14%. The wood shrinks, and its modulus and strength increase, since the cellulose cells pack more closely. To avoid movement, the water content should be maintained at the level of the air humidity. Thermal expansion of wood is small compared the volume change caused by the moisture variation. Mechanical Properties Woods are visco-elastic solids. On loading they show an immediate elastic deformation followed by a further slow creep or delayed elasticity. For design purpose, we can take wood as elastic, with a rather lower modulus for long-term loading than for short-term loading times (a factor of 3 is typical) to allow for the creep. 5

6 The modulus primarily depends on density (also the water content and the loading direction). The axial modulus changes with density nearly linearly while the transverse modulus 2. Thus, anisotropy increases as the density (and thus modulus) decreases (Balsas > Oak). Mechanical Properties When loaded along the grain, the cell walls are extended or compressed. The overall modulus is E wood = E s ( / s ) where E s and s are the modulus and density of the solid cell wall, respectively. The transverse modulus is lower because: (1) the cell wall is less stiff along this direction; (2) the cell wall is bent. It behaves like a foam. E woodv = E s ( / s ) 2 Thus, the elastic anisotropy E wood / E woodv = ( s / ) 6

7 The axial strength of wood is ~100MPa - about the same as strong polymers such as epoxy. The ductility is ~1%. Compression along the axial direction leads to kinking of the cell walls. The kin usually initiates at points where the cells bend to room for a ray, and the kink band forms at an angle of 45 o -60 o. Because of the kinking, the compression strength ~50MPa. The strength depends mainly on density. Similar to modulus = s ( / s ) and v = s ( / s ) 2 The cell walls bend like beams, and collapse occurs when these beams reach their plastic collapse load. Moisture and temperature are also important to the strength. 7

8 Mechanical Properties In woods, the initial defect may be a knot, or a saw cut, or cell damage. Testing for toughness usually involves three-point bending of square beams. The toughness is measured by the fracture work (the area under the load-deflection curve). K IC depends of density ( / s ) 3/2. Woods split easily along the grain but with difficulty across the grain, since the fracture toughness is more than a factor of 10 smaller along the grain than across it. The toughness of wood is higher than any simple polymer, close to reinforced composites: (1) the complicated microstructure leads to rough fracture surface; (2) fiber pull-out reinforces the cracking area. 8

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