Page 213 - 3D Fibre Reinforced Polymer Composites

202 30 Fibre Reinforced Polymer Composites

The strength of typical composite lap joints can be limited by the interlaminar strength,
which is the weak link for composite adherends as it relies on the brittle matrix tensile
properties and the bonding strength of the fibedmatrix interface. To improve composite
lap joint strength, one can choose a toughened resin system for the composite substrate
to increase the interlaminar fracture toughness and/or taper the composite substrate in a
form of ply drop-off to reduce the positive normal stress.

Figure 8.36 Peel stress induced interlaminar delamination in composite lap joints

Placement of fibres in the through-thickness direction using the stitching and z-pinning
technique provides a bridging mechanism holding the two delaminated substrates
together. Sawyer (1985) utilized prepreg to laminate the composite substrates in single-
lap joints, which were then transversely stitched using a shoe-making sewing machine.
Comparison of the failure loads of the joints with and without transverse stitching
revealed that transverse stitching can significantly improve the static strength of the
joints.
Instead of stitching the prepreg, which causes appreciable fibre damage, Tong et a1
(1998) stitched dry fabric preform, which was then placed in a mould and resin was
injected using the resin transfer moulding technique, to demonstrate the promising
effect of transverse stitching. Figures 8.37 and 8.38 illustrate the configurations of the
single-lap joint specimen and the stitching pattern.

Average thickness 1.64 mm
Lay-up: [0/*45/90],
r *luminumtab

Specimen width 25.4 mm
Figure 8.37 Configuration of the single-lap joint specimen manufactured using the
RTM process