Page 228 - 3D Fibre Reinforced Polymer Composites

Z-Pinned Composites 217

A) Control
5000 6) Fastened
-
C) 2% 20 mil primed ATTI
z - - 0) 5% 20 mil prkned AT Ti

0 5 10 15 20 25 30 35

Deflection, mrn

Figure 9.14 Tensile pull-off results of stiffened joints (Freitas et al., 1996)

9.6 Z-PINNED SANDWICH COMPOSITES

Conventional sandwich composite materials used in aircraft, marine craft and civil
structures are prone to delamination cracking and failure at the edge of the face skins
when subject to high peel stresses. Various techniques have been developed to improve
the peel resistance of sandwich composites, including tapered ends to the face skins,
bolting or riveting of the face skins, and using high toughness adhesives between the
face skins and core. More recently, Z-fiberm technology has been used to increase the
peel strength and provide through-thickness reinforcement to sandwich composites.
Aztex Inc., the manufactures of 2-fibersm, produce two products known as X-Corm
and K-Corm which are structural sandwich materials reinforced with z-pins. The z-
pins are inserted through the entire thickness of sandwich materials during processing in
an autoclave. The process is similar to that shown in Figure 9.2 for the insertion of z-
pins into single-skin composites. During processing the z-pins penetrate both skins to
create a 3D fibre structure, with the pins orientated in a tetragonal truss network as
illustrated in Figure 9.15 to provide maximum resistance to shear and compression
loads.
Frietas et al. (1996) report that z-pinned sandwich composites have shear and
compressive strengths that are about 4 and 10 times higher than unreinforced foam,
respectively. It is also claimed that z-pinned sandwich composites have higher skin-to-
core bond strength, better impact damage tolerance, and are more resistant to moisture
ingress than conventional honeycomb sandwich materials (Freitas et al., 1996; Palazotto