Page 127 - 3D Fibre Reinforced Polymer Composites

116 30 Fibre Reinforced Polymer Composites

Tan et a1 (2001) also measured and predicted the in-plane elastic constants for 3D
orthogonal woven E-glass/epoxy composites. Table 5.2 compares the in-plane Young’s
moduli, the shear modulus and the Poison’s ratio that were measured experimentally
and predicted using both the analytical and finite element analysis based laminate block
models. A good agreement between the experimental and predicted results is noted.

Table 5.2 Comparison of predicted and measured in-plane elastic constants for 3D
orthogonal woven E-glass/epoxy composites
Model El (GPa) E2(GPa) vi2 G2 (GPa)
Analytical Laminate block model 29.59 27.05 0.1342 4.4790
EA Laminate Block Model 29.46 28.03 0.1329 5.3987
Average experimental results 31.37 29.68 0.1 158 4.5289

A unique feature of many 3D woven composites is that they begin to permanently
deform or ‘soften’ at relatively low tensile stress levels (Callus et al., 1999; Ding et al.,
1993; Guess and Reedy, 1985; Lee et al., 2002). This softening is shown by the kink in
the stress-strain curve for the 3D composite in Figure 5.7, which does not usually occur
in 2D laminates. The softening can reduce the stiffness by 20 to 50%, depending on the
type of composite, and is attributed to the onset of plastic deformation of the most
heavily distorted load-bearing tows, as depicted in Figure 5.4 (Cox et al., 1994; Callus
et al., 1999). As reported earlier, the load-bearing tows in a 3D woven composite can
be severely misaligned from the in-plane direction by the z-binders. These heavily
distorted tows begin to plastically straighten when the applied tensile strain reaches a
critical value sufficient to induce permanent shear flow of the resin within the fibre
bundle. The critical tensile stress (od for plastic tow straightening can be estimated by
(Cox et al., 1994):

where f, is the volume fraction of load-bearing tows, 1TI31 is the axial shear strength of
the tow, and 151 is a fibre waviness parameter which is defined as the average
misalignment angle for 90% of all load-bearing tows. Using this equation, the effect of
fibre waviness on the plastic tow straightening stress is plotted in Figure 5.10. Shown
in this figure are typical fibre waviness values for prepreg tape, 2D woven and 3D
woven composites. From this figure it is obvious that tensile softening of 3D woven
composites occurs at much lower stress values than 2D composites. Therefore, to
overcome this softening it is necessary to minimise in-plane fibre waviness or use a
resin having a high yield shear strength.
At tensile stresses above the onset of plastic tow straightening, 3D woven
composites experience matrix cracking (both tensile and delamination), z-binder
debonding, tow rupture and, in some materials, tow pull-out (Callus et al., 1999; Cox et
al. 1994; Lee et al., 2000). Tensile failure generally occurs by rupture of the load-