Page 225 - Plastics Engineering

208 Mechanical Behaviour of Composites

3.1 1 General Deformation Behaviour of Laminates

The previous section has illustrated a simple convenient means of analysing
in-plane loading of symmetric laminates. Many laminates are of this type and
so this approach is justified. However, there are also many situations where
other types of loading (including bending) are applied to laminates which may
be symmetric or non-symmetric. In order to deal with these situations it is
necessary to adopt a more general type of analysis.

Convention for defining thicknesses and positions of plies
In this more general analysis it is essential to be able to define the position and
thickness of each ply within a laminate. The convention is that the geometrical
mid-plane is taken as the datum. The top and bottom of each ply are then defined
relative to this. Those above the mid-plane will have negative co-ordinates and
those below will be positive. The bottom surface of the fth ply has address
hf and the top surface of this ply has address hf-1. Hence the thickness of the
fth ply is given by
h(f) = hf - hf-1
For the 6 ply laminate shown in Fig. 3.21, the thickness of ply 5 is given by

h(5) = hs -h4 = 3 - 1 = 2 mm

Fig. 3.21 Six ply laminate

The thickness of ply 1 is given by

h(1) = hl - h~ = (-3) - (-6) = 3 mm