Page 200 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 189

The Jain and Mai models have proven reasonably reliabIe for predicting the
delamination properties of stitched composites. For example, Figure 8.23 shows the
measured R-curve for a stitched glasdvinyl ester composite (that was shown earlier in
Figure 8.15) together with the theoretical R-curve predicted using the Jain and Mai
model, and there is good agreement between the two curves. As another example,
Figure 8.24 compares the G,R values measured for stitched carbodepoxy composites
against theoretical G,R values calculated using the continuous and modified stitching
models. Excellent agreement exists for the modified stitch model while the GIR values
are underestimated by about 50% with the continuous model. The accuracy of the
models is critically dependent on the failure mode of the stitch, that is whether failure
occurs by thread breakage, thread pull-out or a combination of these two.

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2500 Theoretical R-curve

0 20 40 60 80 100
Delamination Length (mm)

Figure 8.23 Comparison of a theoretical and experimental mode I R-curve for a stitched
glasshinyl ester composite. The theoretical curve was determined using the Jain and
Mai model.

8.4.2 Mode I1 Interlaminar Fracture Toughness Properties
Stitching is also an effective technique for improving the delamination resistance under
mode I1 loading (i.e. shear crack opening). This is particularly significant because
delamination cracks that form in composites under impact loading grow mostly under
the action of impact-induced shear strains. The effectiveness of stitching in raising the
mode I1 delamination resistance is shown in Figure 8.25, which shows a large increase
to the mode I1 interlaminar fracture toughness (GIIR) of a carbodepoxy laminate with
increasing stitch density (Dransfield et ai., 1995). It is worth noting, however, that the
improvement to the delamination resistance is usually not as high as for the mode I