Page 157 - 3D Fibre Reinforced Polymer Composites

146 30 Fibre Reinforced Polymer Composites

composite are then obtained by averaging the properties of the RVE for the different
yarn orientations present in the global coordinate system. More recently Chen et al.
(1999) described the use of a Finite Multiphase Element method to predict the elastic
properties of 3D braids. This process uses a two step numerical approach of generating
a fine local mesh at the unit cell level to analyse the stresdstrain response of the unit
cell, then a coarse global mesh to obtain the overall response of the composite at the
macroscopic level.
To date these models have only been used for the prediction of elastic constants and
there does not appear to be any attempt made to predict the strength of 3D braided,
polymer matrix composites. The comparison of predicted and experimental elastic
constants is reasonable good, mostly within 10% (Chen et al., 1999) but in general the
predicted properties are less than that measured via experiment.

6.6 SUMMARY

3D braided preforms are very versatile forms of textile reinforcement for composite
structures. As discussed in Chapter 2, 3D braids can be produced in a wide range of
cross-sectional shapes and these shapes can be varied along their length to form
structural details such as tapers, bifurcations, holes, etc. However, there has only been
relatively limited data published on the mechanical properties of 3D braided polymer
matrix composites, much of the development in the area of 3D braids appears to be
focussed on ceramic and metal matrix composites. In particular, there has been little
comparison made between the performance of 3D braided composites made by different
braiding processes and between 3D braids and 2D laminates.
From the data that has been published it is evident that the presence (or absence) of
axial fibres and the angle of the braiding yams both play an important role in controlling
the mechanical properties. Improved longitudinal performance results from increased
axial fibre content and decreased braiding angle, but at the expense of transverse
properties. The damage resistance and tolerance of 3D braided composites are also
significantly better than 2D tape laminates due to the highly interlinked nature of the 3D
architecture, however the fatigue performance has been shown to be worse.
A result of particular interest is the high sensitivity that 3D braided composites have
to cut edges. The act of machining the specimen edge and thus cutting the braiding
yams into discontinuous sections was found to significantly decrease the tensile and
flexural properties of the composite. This indicates the need to produce 3D braided
composite components to net-shape, thus removing any need for machining that will
reduce its performance.
Before 3D braided preforms can be generally accepted as reinforcements for
composite structures, a great deal more information must be gathered on their
mechanical properties. In particular, the effect on the mechanical performance of the
braiding technique and the various processing parameters within each technique must be
understood in order for these reinforcement styles to be used with confidence.