Materials for engineering, 3rd Edition - (Malestrom)

Composite materials 193
diagrammatically in Fig. 6.3. The outer layer is mainly of lignin and
hemicellulose and acts as a bond layer to adjacent cells. The three secondary
inner layers, S-1, S-2 and S-3 differ mainly in the helical orientation of the
microfibrils. The thick S-2 layer largely determines the strength properties
of the wood: in it the cellulose-based microfibrils are oriented in a helix
nearly parallel to the axis of the cell. Helices in the S-1 and S-3 layers are
tighter than in S-2.
Hardwoods (Fig. 6.2(b)) have many cell types, chiefly longitudinal fibres
analogous to tracheids in the softwood but smaller, longitudinal vessels or
pores much larger than tracheids, and radial ray cells, which can constitute
up to 30% in hardwoods.
6.3.1 Wood composites
Plywood consists of an uneven number of wood laminae (known as veneer)
bonded together with a thermoset polymer adhesive so that the grain direction
S 1
Secondary
wall
S 2
S 3
Primary wall
6.3 Idealized drawing of cell wall.