WoodMacrostructure & Microstructure Praveen P Department of Materials Science Mangalore University Karnataka, India 1
Wood…. Most widely used engineering construction material Naturally occurring composite material Consists complex array of cellulose cells reinforced by apolymeric substance, lignin and other organic compounds. Wood is a heterogeneous, hygroscopic, cellular andanisotropic material. It is composed of cells, and the cellwalls are composed of micro-fibrils of cellulose (40% – 50%)and hemicellulose (15% – 25%) impregnated with lignin(15% – 30%) 2
Wood….Natural product with complex structureHighly anisotropic, not homogeneousPosses high tensile strength in thedirection parallel to tree stem 3
Cross section of a typical treea. Outer bark layer: Dry, dead tissue andprovides external protection for the tree.b. Inner bark layer: carries food from theleaves to all the growing parts.c. Cambium layer: Tissue layer between thebark and wood that forms the wood & barkcells.d. Sapwood: Light coloured wood whichforms outer part of the tree stem. Containsome living cells which function for foodstorage and carry water from root to leaves.e. Heartwood: Old inner region of the treestem which is no longer living. It is darkerthan sapwood and provides strength for thetree.f. Pith: Soft tissue at the centre of the treearound which the first growth of the tree takesplace.g. Wood rays: Connect the tree layers frompith to the bark and function is food storageand transfer of food. 4
Classification Trees are classified in to two major groups Softwoods (Gymnosperms) and Hardwoods (Angiosperms)Softwood HardwoodTree seed is exposed Seed is coveredRetains its leaves throughout the year Sheds its leaves annuallyEvergreen trees Deciduous treesPhysically soft Physically hardLight colour Dark colouredExamples: pine, spruce etc Examples: Mahogany, Oak, Teak etc 5
Annual growth ringsDuring each growth seasona new layer of wood isformed annually around thetree stem. This layers arecalled annual growth rings.Each ring has two subrings: earlywood (spring)and latewood (summer). Insoftwoods the earlywoodhas a lighter colour and thecell size is larger. 6
Microstructure of woodExplain the microstructure of wood? Explain the effect of water absorption on the properties ofwood? 8
Microstructure of a softwoodThree complete growth rings can be seenEarlywood: larger size cellsSoftwood consists mainly of long, thin walled tubularcells called tracheids.The length of a longitudinal tracheid is about 3 to 5mm and and its diameter is about 20 to 80micrometer.The longitudinal tracheids constitute about 90%volume of the softwood.The large open space in the centre of the cell iscalled the lumen and is used for water conduction.The earlywood cells have a relatively large diameter,thin walls and a large size lumen.The latewood cells have a smaller diameter andthick walls with a smaller lumen.Wood rays which run in the transverse directionsfrom the bark to the centre of the tree consist of anaggregate of small parenchyma cells that arebricklike in shape and which are used for food 9storage.
Microstructure of Hardwood Have large diameter vessels for the conduction of fluids. The vessels are thin walled structures consisting ofindividual elements called vessel elements and are formed inthe longitudinal direction of the tree stem. The wood of hardwood trees is classified as either ring-porous or diffuse-porous depending on how the vessels arearranged in the growth rings. In a ring-porous hardwood thevessels formed in the earlywood are larger than thoseformed in the latewood. In a diffuse-porous hardwood, the vessel diameters areessentially the same throughout all the growth rings. The longitudinal cells in the hardwood tree stem are fibers. These are elongated cells with close pointed ends and areusually thick walled. Length of this fibers are about 0.7 to 3 mm and averagediameter is 20 micrometer. The food storage cells of hardwood are the ray(transverse) and longitudinal parenchyma which are brick orbox shaped. The rays for hardwoods are usually much larger than for 10softwood, having many cells across their width.
Wood-Moisture RelationsMoisture content: Wood is a hygroscopicmaterial, i.e., it will absorb water vaporfrom the atmosphere. Water may be held in wood in two ways:Moisture content in wood is expressed as apercentage of ovendry weight. A moisture (1) bound water, & (2) free water.content of 50% means that there are 50 • Bound water is held within the cell wallsparts of water to 100 parts of dry wood by adsorption force.substance by weight. – It is generally believed that bound water is not in the crystalline regions of the cell wall, but is adsorbed in the amorphous regions. – This has important implications for the volume changes associated with moisture changes. • Free water is not held by any forces and is situated in the cell cavities know as lumen. 11
Fiber saturation pointThe moisture content at which the cell wall is saturated with boundwater & at which no free water is present is called the fibersaturation point, (FSP).• The FSP varies from species to species, but it averages about28% moisture content.• Addition or removal of water below the FSP has a pronouncedeffect on practically all wood properties.• Addition or removal of water above the FSP has a almost no effecton any wood properties. 12
Shrinkage and swelling• The variation of shrinkage betweendifferent directions can be attributedto the microstructure of wood.• The latewood cells dominateshrinkage since they absorb muchmore water and in the tangentialdirection there is an unbrokenalignment of latewood. The greater shrinkage in the tangential direction causes distortion in lumber with different orientations. 13
Effects of Moisture on the Strength of Wood• The strength of wood is constant above the fiber saturation point.• Below the fiber saturation the strength of wood increases with decreasingmoisture content. This can be related to where the water is absorbed in themicrostructure. 14
Mechanical Properties of Wood Effects of Moisture on the Strength of Wood The strength of wood is constant above the fibre saturation point. Below the fibreThe strength of wood is highly saturation the strength of wooddependent upon direction: increases with decreasing Tensile strength values in moisture content. This can belongitudinal:radial:tangential directions related to where the water ison average are in the ratio of 20:1.5:1 absorbed in the microstructure. The variation of strengthbetween different directions can beattributed to the fine structure of thewood cells. 15