Page 193 - 3D Fibre Reinforced Polymer Composites

182 30 Fibre Reinforced Polymer Composites

8.4 INTERLAMINAR PROPERTIES OF STITCHED COMPOSITES

8.4.1 Mode I Interlaminar Fracture Toughness Properties
A key benefit of stitching is a vast improvement to the delamination resistance of
composites. Stitching is a remarkably effective technique for increasing the
interlaminar fracture toughness of laminates under mode I loading conditions (ie. crack
opening). The toughening effect of stitching is shown in Figure 8.15, which shows
mode I crack growth resistance (or R-) curves for a stitched and unstitched composite.
This figure shows the R-curve for a glasslvinyl ester composite that has been stitched
with Kevlar yam. The R-curve behaviour of other types of stitched composites is
similar to that shown in Figure 8.15. The mode I strain energy needed to start the
growth of a delamination in a stitched composite is normally the same as the unstitched
laminate. However, the R-curve for the stitched composite rises rapidly with crack
length up to about 20 mm due to the increased toughening provided by the stitches. At
longer crack lengths, the curve becomes relatively constant, and this is taken to be the
steady-state interlaminar fracture toughness of the stitched composite (GIR). At this
stage the delamination resistance of the stitched composite is much higher than the
unstitched laminate.

,3000
<
7
v
g 2500
a,
C
c
92000
z Stitched Composite
5 1500
.-
5
5 1000
C -
c
-
2 500
r"
0' I t t I 1
0 20 40 60 80 100
Delamination Length (mm)

Figure 8.15 Mode I R-curves for a stitched and unstitched glasdvinyl ester composite.

The improved delamination resistance is due to the bridging action of the stitches,
which exert a closure traction force that lowers the tensile strain acting on the crack tip.
A schematic illustration is presented in Figure 8.16 of the interlaminar toughening
provided by stitching under mode I loading. Ahead of an advancing delamination the