Wood Composites

22 ноября, 2015

Pokraskin

Currently a wide array of wood-to-wood or wood to other materials composites is available, varying almost indefinitely in material and geometry. The only limits are set by the imagination of the engineers and designers, and any attempt of classification will be largely in the eye of the beholder. However, for the purpose of the present discussion, the classification according to Dunky and Niemz [19] was chosen (Table 8.2).

Compared to boards and beams cut directly from the trunk, solid wood materials and veneering materials have decisive advantages. For example, layered veneer

Table 8.2 Classification of wood materials according to Dunkyand Niemz [19].

Solid wood	Veneering	Chip wood	Fiber	Composite
• Solid wood	• Laminated veneer	• Particle board	• Medium-density	• Blockboard
board	lumber		fiber board
• Gluelam		• Oriented strand	(MDF)	• Blockplywood
• Two-way joist	• Plywood	board (OSB)	• Porous fiber	• Layer
• Laminated wood	• Parallel strand	• Laminated	board (SB)	engineered
• Compound	lumber/parallam	strand lumber	• Hard fiber	parquet
board elements		• Waferboard	board (HB)	• Doors
• Prefabricated		• Extruded		• Windows
elements		particle board
		• Scrimber
		• Special boards

sheets or wood lamellae may be joined in a stacked array in order to optimize the properties of the wood:

• Drying behavior: Owing to their limited thickness, lamellae may be adjusted to a specific uniform moisture content that is in a state of balance with that of the ambient air, even in the case of high-speed industrial drying processes.

• Stability of form: In thin boards, dense and large-volume cell structures are distributed more homogeneously than in thick elements. Swelling and shrinking therefore induce less important warpage. The layers located above and below intensify this effect.

• Wood quality: Defective spots in the wood, resulting from prunings, resin ducts, or growth irregularities, are easily discovered on thin boards. They may be cut off, and the board may be joined together on the front side by means of wedge finger-joints, making it possible to produce highly elastic components (e. g. parallel bars).

• Free dimensions: Using wedge finger-joints on the front sides, lamellae of (in theory) indefinite length may be produced. By using breaking finger-joints, these in turn may be joined to produce laminated lumber.

• Forming. Wood may be plastically deformed to a certain extent by means of thermal moisture treatment. This is easier to handle with thin boards or veneer material than with solid wood boards. Bent elements may therefore be built from laminated lumber bent and glued in the forming press.

• Decorative elements: Lamellae made from woods of different shades, joined in a stacked array, may be used to create visually appealing components for furniture, interior finish of vehicles and ships, for ship decks or musical instruments.

In combination with other materials, wood composites offer an almost unlimited field of applications. Among the most varied possibilities, some examples are presented in the following section.

8.4.9