Page 198 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 187
to the toughness of the equivalent unstitched laminate (G,,). The figure shows a general
increase to the interlaminar fracture toughness with increasing stitch density. A few
outlying data points show that the delamination resistance can be improved by over 30
times by stitching with exceptionally thick, strong threads. For most composites,
however, stitching increases the delamination resistance by a factor of up to 10-15. This
compares favourably with other types of 3D composites that have interlaminar fracture
toughness properties that are up to 20 times higher than the equivalent 2D laminate.
A number of micromechanical models have been proposed to determine the
improvement to the mode I interlaminar fracture toughness properties of composites due
to stitching. Of the models, there are two models proposed by Jain and Mai that have
proven the most accurate (Jain and Mai, 1994a, 1994b, 1994~). Both models are based
on Euler-Bernoulli linear-elastic beam theory applied to a stitched composite with the
double cantilever beam (DCB) geometry, as illustrated in Figure 8.22. The models can
be used to caIculate the effect of various stitching parameters (eg. stitch density, thread
strength, thread diameter) on the R-curve behaviour and GIR value of any laminated
composite.
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- Stitch Rupture I
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I I I I I

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I I I I I
Stitch Pull-Out
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Figure 8.22 The DCB specimen geometry used as the basis for the Jain and Mai model
for mode I interlaminar fracture toughness of stitched composites. Models have been
developed for the cases where the stitches (a) rupture along the delamination crack path
(continuous stitching model) and (b) failure at the surface and then pull-out from the
composite (discontinuous stitching model) (From Jain and Mai, 1997).