Series editors' preface - Wood Tools

84 Conservation of Furniture
Figure 2.20 Diagrammatic representation of a beam
supported at each end with a load concentrated at its
mid span. The large arrows indicate the direction of the
applied load on the beam, whilst the small arrows
indicate the resulting stresses within the beam as it
bends. Compression and tension are concentrated on
the surface of the beam at mid span, whilst shear stress
is concentrated along the neutral axis at the ends of the
beam
(the neutral axis). Given the span, cross-sectional
dimensions and the magnitude and position
of loads on a beam (regardless of the
material used), the magnitude of the bending
stresses can be computed using an engineering
concept known as the Flexure Formula
(Hoadley, 1980). In wood, the level of bending
stress which results in failure of a wooden
beam, i.e. the breaking strength, is called the
modulus of rupture.
It is tempting to consider strength to mean simply
the maximum resistance of wood. However,
another important aspect of strength is the reality
that mechanical loading of a material is always
accompanied by deformation. Deformation in an
object under load is expressed as strain. Strain is
defined as the change in dimension per unit of
original dimension:
strain =
change in dimension (in)
original dimension (in)
Though strain is expressed in units per unit
(e.g. inches per inch), it is nevertheless simply
a fraction or ratio.
Consider the simultaneous stress and strain
behaviour during the progressive loading of a
(a) (b) (c) (d)
Stress σ
X
σ 1
x
Proportional
limit
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Strain E
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Strain E
Strain E
Figure 2.21 Stress strain diagram. A block of wood under a load in compression perpendicular to the grain is
shown in (a). The relationship between stress and strain (defined in section 2.5.1) is shown in (b), where moderate
loading is applied and stress (e.g. x) does not exceed the proportional limit ('). Behaviour in this range is elastic,
i.e. when the load (stress) is removed, the wood returns to its original dimension (strain is recovered). In (c), as
additional load (stress) is applied (X) and the proportional limit (') is exceeded, dimensional change (strain) is no
longer proportional to the applied load (stress). In the graph, this is the point at which the line that shows the
effect of loading begins to curve. With most wood properties, the proportional limit is also the elastic limit, and
dimensional change (strain) beyond the elastic limit is not reversible when the load (stress) is removed. Once the
proportional limit has been exceeded in (d), even though the applied load (stress) is removed, the wood does not
return to its original dimension (residual strain). Residual strain in wood is called set
Set
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