Page 203 - 3D Fibre Reinforced Polymer Composites

192 30 Fibre Reinforced Polymer Composites

increasing plastic shear deformation and axial rotation the further they are behind the
crack tip. As the stitches are deformed they are ploughed laterally into the crack faces
of the composite. At a high amount of axial rotation the stitches experience splitting
cracks and spalling, and this generally occurs when the sliding displacement rises above
0.6 mm. This deformation and damage to a sheared stitch is shown in Figure 8.27, and
it is obvious a large degree of axial rotation has occurred on the fracture plane. In this
thread the fibres have been rotated by an angle (S, of up to about 45'. The plastic
deformation and ploughing of the stitches absorbs a large amount of the applied shear
stress. Furthermore, the large amount of axial rotation to the stitches causes them to
bend near the fracture plane so a significant load of the applied shear stress is carried by
the stitches in tension. The combination of these effects lowers the shear strain acting
on the crack tip and thereby improves the delamination resistance. Eventually the
stitches at the rear of the stitch bridging zone break when the sliding displacement
exceeds about 1 mm (Figure 8.27b). The stitch bridging zone can grow for long
distances (up to -50 mm) before the stitches fail, and this is the principle toughening
mechanism against mode I1 delamination cracks.

Figure 8.27 Scanning electron micrograph showing (a) plastic shear deformation and
(b) shear failure to a stitch subject to mode I1 interlaminar loading.