Series editors' preface - Wood Tools

mechanical effects of repeated shrinkage/
swelling cycles, or stress-setting of the wood.
However, experiments with wood taken from
artefacts thousands of years old have shown
the wood to have retained its hygroscopicity
and its capacity to dimensionally respond to
changes in moisture content. The assumption
should therefore prevail that wooden objects,
regardless of age, can demonstrate dimensional
movement when subjected to variable relative
humidity conditions.
2.5 Mechanical properties
In the evolution of trees, wood tissue has
developed which has highly effective axial
compression (i.e. compression along the grain)
and bending strength characteristics. For its
weight, this wood demonstrates amazing stiffness
as well as fracture resistance and it is
therefore utilized in structural products such as
furniture. However, although axial strength of
wood is impressive, the comparative weakness
of wood perpendicular to the grain, in both
compression and tension, can often be the limiting
factor of mechanical performance. This is
indicated by examples ranging from surface
indentation and splitting to the failure of joints.
Given the anisotropic nature of wood, the
many possible modes of loading wood tissue,
the many environmental conditions and defects
which influence strength, and the array of different
structural applications of wood in furniture,
a thorough discussion of mechanical
properties is hardly feasible here. Therefore
this section will merely highlight selected
aspects of the strength of clear (defect-free)
wood and the major factors that influence it in
relation to furniture.
2.5.1 Defining mechanical properties
In recognition of the anisotropic nature of
wood, it is traditional to consider the strength
of wood along its three principal axes: longitudinal,
radial and tangential. Parallel to grain
(longitudinal) strength is significantly greater
than strength across the grain; however, since
there may be only minor difference between
radial and tangential directions, it is appropriate
to simply consider average strength perpendicular
to the grain.
Wood and wooden structures 83
Strength measures the ability of a material to
resist applied force or load. The strength of
material is commonly expressed in terms of a
stress value. Stress is defined as load per unit
area, and is calculated by dividing the magnitude
of the applied load or force by the crosssectional
area over which it is distributed. A
general formula:
stress = load/area
In English-speaking countries, loads have traditionally
been measured in pounds (lb) and
areas in square inches (sq. in). In continental
Europe, kilograms and centimetres were used.
In SI units, Mega Newtons per square metre
(MN/m2 ) are used.
The basic modes of load application are
compression, tension and shear. The components
in Figure 2.19 are subjected to a single
type of stress. In practice, a combination of
stresses may occur, as in the bending of a
beam. A beam is an elongated member supported
at various points along its length with
one or more loads acting perpendicular to its
axis. For example, the beam of Figure 2.20 is
supported at each end with a concentrated
load at its mid span. As a result of the consequent
bending deformation, the upper surface
is shortened and stressed in longitudinal compression;
the lower surface is stretched and
thereby stressed in tension. These axial stresses
in tension or compression are referred to simply
as bending stresses; they are maximal at
the upper and lower surfaces of the beam and
diminish to zero at the mid plane of the beam
(a) (b) (c)
Figure 2.19 Modes of load application. Compression
(a) occurs where applied forces are aligned and tend to
crush a material. Tension (b) occurs where applied
forces are aligned and tend to pull a material apart.
Shear (c) occurs where applied forces are not aligned
and tend to slide one part of a material in one
direction and the other part of the material in the
opposite direction