Materials for engineering, 3rd Edition - (Malestrom)

192
Materials for engineering
Table 6.2 Mechanical properties of some woods, parallel (||) and perpendicular (⊥) to
the grain
Species ρ E(||) E(⊥) σ t (||) σ c (⊥) K c (||) K c (⊥)
(kg m –3 ) (GPa) (GPa) (MPa) (MPa)
1
(MPa m2) (MPa m
Balsa 200 6.3 0.2 23 12 0.05 1.2
Mahogany 440 10.2 0.8 90 46 0.25 6.3
Ash 670 15.8 1.2 116 53 0.61 9.0
Oak 690 13.6 1.0 97 52 0.51 4.0
Beech 750 13.7 1.7 100 45 0.95 8.9
Douglas fir 590 16.4 1.1 120 50 0.34 6.2
Scots pine 550 16.3 0.8 90 47 0.35 6.1
3-plywood 520 (isotropic)
12.1
Chipboard 720 1.9
Hardboard 1030 4.6
Medium 750 2.5
Density
Fibreboard
1
2 )
from deciduous broad-leaved trees. The terms are misleading, since many
softwoods are harder than many hardwoods, but the two types differ in their
macrostructures, Fig. 6.2.
Softwoods (Fig. 6.2(a)) are composed of about 90% long, slender rectangular
cells called tracheids, about 3 to 5 mm in length. These can be modelled as
a bundle of tubes or drinking straws and the stiffness is much greater parallel
to the axis of the tubes than in the perpendicular direction. This partially
accounts for why wood is 10 to 20 times stiffer parallel to the grain than
perpendicular to it. About 10% of wood cells grow as rays running in the
radial direction. These ray cells act as reinforcement in this direction, increasing
the radial strength and stiffness, and they also cause the tracheids to align
themselves fairly regularly in radial rows (Fig. 6.2(a)).
The principal reason for the high longitudinal strength and stiffness is
found in the structure of the walls of the longitudinal cells, shown
a
6.2 SEM images of wood with tangential surface on RHS and radial
surface on LHS, (a) softwood, showing part of four growth rings and
(b) hardwood showing longitudinal pores.
b