Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
runs at right-angles in successive layers. For indoor use, urea formaldehyde
(UF) is employed, whereas marine ply uses phenol formaldehyde (PF) as the
adhesive.
Chipboard is formed by the compression moulding of wood chips bonded
with about 10% UF. The material is significantly cheaper than plywood, but
its properties are somewhat inferior.
Hardboard is made from wood chips which have been separated under
pressurized steam into fibres. A mat of these fibres is then hot pressed with
UF into a board with one smooth glossy surface and one textured surface. It
is a cheaper option than plywood where strength is not required, but because
it absorbs water, hardboard cannot be used outside.
Medium Density Fibreboard (MDF) is a type of hardboard which is made
from wood fibres glued with UF under heat and pressure: it is dense, flat,
stiff and has no knots and is easily machined. Traditional woodwork joints
may even be cut. Because it is made up of fine particles it does not have an
identifiable grain and it can be painted to produce a smooth surface.
6.4 Modelling composite behaviour
A number of composite models have been developed with the aim of predicting
the mechanical properties of composites from a knowledge of those of the
constituent phases (matrix and reinforcement). We will consider some of the
simpler models below.
6.4.1 Stiffness
The elastic behaviour of composite materials can be modelled most readily
if simplifying assumptions are made. One such simplification is to assume
that the two components have identical Poisson ratios. It is also commonly
assumed either that the elastic strain is uniform throughout the composite,
which implies differences in stress distribution, or that the elastic stress field
is uniform, implying variations in local strain.
Assuming uniform strain within the composite: Consider a composite
material consisting of two phases, 1 and 2, which possess a similar Poisson
ratio but differing Young’s moduli of values E 1 and E 2 . Let us assume that,
when the material is elastically distorted, the macroscopic stress and strain
are reproduced in a typical unit volume which consists of a single particle of
material 2 in a cube of matrix material 1. Assuming that the cube is loaded
across two opposite faces by a force F (Fig. 6.4) and considering a crosssection
of the composite of thickness dx at a distance x from an end face,
which intersects an area A 1 of matrix and an area A 2 of the dispersed material,
if the strain (ε) is uniform within this element, we can write the normal stress
on area A 1 as E 1 ε and that on area A 2 as E 2 ε.