Page 234 - Plastics Engineering

Mechanical Behaviour of Composites 217

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0
0 1 0 2 0 ~ 4 0 5 0 6 0 7 0 8 0 9 0
Angle (e)

Fig. 3.25 Variation of elastic properties for a (+/ - 45) symmetric laminate of carbodepoxy

It is interesting to compare the behaviour of the single ply and the laminate
as shown in Figs 3.24 and 3.25. Firstly it is immediately evident that the lami-
nate offers a better balance of properties as well as improvements in absolute
terms. The shear modulus in particular is much better in the laminate. Its peak
value at 45" arises because shear is equivalent to a state of stress where equal
tensile and compressive stresses are applied at 45" to the shear direction. Thus
shear loading on a [& 451, laminate is equivalent to tensile and compressive
loading on a [0/90], laminate. Thus the fibres are effectively aligned in the
direction of loading and this provides the large stiffness (or modulus) which is
observed.
It is also worthy of note that large values of Poisson's Ratio can occur
in a laminate. In this case a peak value of over 1.5 is observed - something
which would be impossible in an isotropic material. Large values of Poisson's
Ratio are a characteristic of unidirectional fibre composites and arise due to the
coupling effects between extension and shear which were referred to earlier.
It is also important to note that although the laminae [It 451, indicates that
Ex = E, = 18.1 GN/m2, this laminate is not isotropic or even quasi-isotropic.
As shown in Chapter 2, in an isotropic material, the shear modulus is linked
to the other elastic properties by the following equation