Page 202 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 191
The toughening mechanisms responsible for the high mode I1 interlaminar fracture
toughness of stitched composites are complex, with a number of different mechanisms
operating along the length of a delamination crack. The shear tractions generated in
stitches with increasing sliding displacement between the opposing crack faces are
shown in Figure 8.26. This figure by Cox (1999) shows typical sliding displacement
and stress levels associated with the various mechanisms during shear loading of a
stitched composite up to the point of failure. The sliding displacement (2~1) is the
distance the two crack faces have separated under mode I1 loading. The vertical scales
show the average bridging traction across the stitches, q,. (left-hand side) and the
bridging traction for a single stitch, T (right-hand side). The values shown for q, are
representative, and will vary depending on the volume fraction of stitching and the
mechanical properties of the threads.

ploughing, debonding, and slip
/

0 -1
sliding displacement, 2u (mm)

Figure 8.26 Schematic of the shear tractions for mode I1 loading of a stitch under
increasing crack sliding distance (from Cox, 1999).

It is generally acknowledged that when an interlaminar shear stress is applied to a
stitched composite containing a delamination then the stitches ahead of the crack front
are not damaged or deformed. When the crack tip reaches the stitches, however, the
delamination causes the stitches to debond from the surrounding composite material.
The stitches are usually completely debonded from the composite when the total sliding
displacement (2~~) exceeds about 0.2 mm. As the opposing crack faces continue to slide
pass each other the stitches become permanently deformed. Plastic deformation of the
stitches can occur immediately behind the crack tip due to the low shear yield stress of
the thread material. It is estimated that permanent deformation in stitches begins when
the sliding displacement distance exceeds about 0.1 mm. The stitches experience