Page 136 - 3D Fibre Reinforced Polymer Composites

30 Woven Composites 125
binder, as shown in Figure 5.16. Once the surface tow has failed it loses stiffness, but is
constrained from buckling outwards by the surface loop. Upon further loading kink
bands form in other distorted tows. Cox et al. (1992, 1994) found that kink bands
within 3D woven composites develop as discrete geometric flaws rather than as
coplanar bands that occur in unidirectional laminates. As a result, 3D woven
composites fail gradually at discrete locations throughout the material, leading to very
high strains to ultimate failure.

-kink bands

/-stut(er

Figure 5.16 Schematic showing the locations of two kink bands within a surface axial
tow (From Cox et al., 1992).

The high failure strains of 3D woven composites under compression loading is shown in
Figure 5.17 that compares the compressive stress-strain behaviour of a unidirectional
carbodepoxy prepreg tape against a 3D carbodepoxy composite measured by Cox et al.
(1992). It is seen that the curve for the tape laminate increases steadily until
catastrophic failure occurs at a strain of -1.4%. In contrast, the curve for the 3D
composite shows a sudden load drop at a strain of -OS%, although complete failure
does not occur. Instead, the load decreases very gradually over a large strain. Cox et al.
( 1992) found that 3D woven composites still have significant strength after compressive
strains of more than 15%, indicating extraordinary high ductility. This extreme ductility
is a unique property of 3D woven composites, and is due to kink bands forming as
discrete geometric flaws that inhibit catastrophic failure which facilitates the gradual
failure of the material under increasing strain.