Page 220 - 3D Fibre Reinforced Polymer Composites

Z-Pinned Composites 209

small angle (less than -7'). The damage caused to composites by.the insertion of Z-
fibersTM is still under investigation, and more research is needed to understand the types
and amounts of damage caused in the z-pinning process. The limited amount of
information that has been published on damage reveals that the most common types are
misalignment and fracture of in-plane fibres (Steeves and Fleck, 1999a). The in-plane
fibres of unidirectional tape laminates are misaligned and distorted when pushed aside
during insertion of Z-fibersTM. An example of the distortion of fibres around a z-pin is
shown in Figure 9.5. The misalignment angle of in-plane fibres around the pins is
dependent on a number of factors, including the type of composite (Le. prepreg,
thermoplastic, fabric preform), fibre lay-up orientations, and the fibre volume content of
the laminate. Misalignment angles of between 5" and 15' have been measured in
unidirectional tape composites reinforced with Z-fibersTM, compared to unreinforced
tape laminates with an average misalignment angle of about 3'. In some cases the
fibres can be so severely misaligned that they break. The incidence of fibre breakage is
not known, although it is expected to be small. Due to the misalignment of in-plane
tows, resin rich regions are formed at each side of the pin, as shown in Figure 9.5.
These regions can be up to -1 mm long, and in composites reinforced with a high
density of Z-fibersTM these regions may join up to form continuous channels of resin.

Figure 9.5 In-plane fibre distortion around a z-pin in a composite. Note the presence of
the large resin-rich zones

9.3 MECHANICAL PROPERTIES OF Z-PINNED COMPOSITES

The in-plane mechanical properties and failure mechanisms of composites reinforced
with Z-fibersTM have not been studied in detail. The effect of Z-fiberTM reinforcement
on in-plane properties such as flexural modulus and strength, shear strength, fatigue-
life, and open-hole tensile and compressive strengths have not been reported, and it is
an important topic of future research to determine these mechanical properties and