Series editors' preface - Wood Tools

wood member, for example, a block of wood
under a load in compression perpendicular to
grain, as shown in Figure 2.21a.
Under moderate loading, the relationship
between stress and strain is indicated by Figure
2.21b. Note that strain is proportional to stress,
and behaviour in this range is elastic, that is,
when stress is removed, strain is recovered.
However, as shown in Figure 2.21c, as additional
stress is applied, a proportional limit, σ,
is reached, beyond which strain is no longer
proportional. With most wood properties, the
proportional limit is also the elastic limit, and
strain beyond the elastic limit is not reversible;
when stress is removed, strain is not reversible.
As shown in Figure 2.21d, removal of stress
beyond the proportional limit results in residual
strain, called set. Dents in the surfaces of
furniture or the loosening of mortise and tenon
joints are examples commonly associated with
compressing wood perpendicular to grain
beyond its proportional limit. In most cases,
there is no meaningful maximum load, since
continued compression would result in increasing
resistance, even after the wood had been
crushed beyond usefulness.
With many strength properties, such as compression
parallel to grain (i.e. along the grain)
or bending, a maximum load is reached at levels
of 1.5–2 times the proportional limit load.
Maximum load may be accompanied by fracture
or other consequential failure.
Within the proportional limit, the ratio of
stress to strain is called the modulus of elasticity,
or Young’s Modulus (E):
E = stress/strain
Since the units of strain cancel out to give a
ratio or number without dimensions, the result
of this calculation is still a stress. It is that stress
which would in theory double the length of a
specimen if it did not break first. It can also be
regarded as the stress to produce 100% strain
(Gordon, 1976). Whereas strength is a measure
of the force or stress needed to break an
object, Young’s modulus or E is concerned
with how stiff, flexible, springy or floppy a
material is. To quote from J.E. Gordon’s excellent
The New Science of Strong Materials: ‘A biscuit
is stiff but weak, steel is stiff and strong,
nylon is flexible (low E) and strong, raspberry
jelly is flexible (low E) and weak. The two
properties (modulus and strength) together
Wood and wooden structures 85
describe a solid about as well as you can reasonably
expect two figures to do’ (Gordon,
1976).
The modulus of elasticity is especially
important in bending as it serves as a convenient
rating of relative stiffness among different
woods. In many applications, the rigidity of the
wood may be as critical as its breaking
strength.
Certain strength characteristics of wood cannot
be described in terms of pure stress values,
as they involve complex loading or resistance
which cannot be readily analysed. An example
is the hardness value of wood, which measures
the indentation resistance of wood. This property
is determined by an empirical test which
simply measures the amount of force required
to embed a standard tool (a hemisphere of 0.44
in diameter) into a wood surface.
2.5.2 Relative strength properties
For a given species of wood, relative density is
perhaps the best single predictor of relative
strength. It seems logical that the higher the
relative density, the harder and stronger the
wood. This is illustrated by the following comparison
of figures for a typical white oak with
those for a typical species of poplar.
Timber Relative Modulus of Modulus of
density elasticity rupture
Quercus spp. 0.68 11 700 MN/m2 105 MN/m2 1.7 106 psi 15 200 psi
Populus spp. 0.34 7600 MN/m2 47 MN/m2 1.1 106 psi 6800 psi
Within a given species of wood, there is striking
superiority of strength parallel to grain
compared with that perpendicular to the grain.
For example, among softwoods with average
relative densities in the range of approximately
0.3–0.55, the tensile strength parallel to grain
of air-dry woods is in the range of approximately
70–140 MN/m 2 (10 000–20 000 psi),
perpendicular-to-grain tensile strength averages
only about 3–8% as great. Compression
strength parallel to grain of dry softwoods is in
the range of 30–60 MN/m 2 (4000–9000 psi),
perpendicular to grain only 8–25% of this
value.
Strength properties for common species of
cabinet wood are presented in Table 2.1.