Page 137 - 3D Fibre Reinforced Polymer Composites

126 313 Fibre Reinforced Polymer Composites

-
1600
-
1400
-
1200 /I
- Unidirectional Tape laminate
1000

3D Woven Composite

I I I I I
0 1 2 3 4 5
Strain (%)

Figure 5.17 Compression stress-strain curves for a unidirectional tape laminate and a
3D woven composite (The curves for the laminate and 3D composite are from Daniel
and Ishai (1994) and Cox et al. (1992), respectively).

The compressive properties of 3D woven composites under fatigue loading have also
been investigated. Dadkhah et al. (1995) examined the compression-compression
fatigue performance of various 3D woven carbodepoxy composites, and found that the
fatigue failure mechanism is similar to the failure process described above for
monotonic compression loading. Under cyclic compression loading, a kink band
initiates at a site of high fibre distortion, with the most common location being where
the surface tow is crimped by the z-binder (see Figure 5.4). Upon further load cycles
the fibres within the crimped tow progressively rotate to greater angles until failure
occurs by kinking. It is believed that the fatigue life does not extend greatly beyond the
formation of the first few kink bands. The fatigue endurance of 3D woven composites
have not yet been compared against 2D laminates with the same fibre content, although
it is expected to be lower due to the heavily distorted fibres lowering the cyclic stress
needed to induce kinking failure.

5.3.3 Flexural Properties
The flexural properties of 3D woven composites has been investigated by Chou et al.
(1992), Cox et al. (1994), Ding et al. (1993) and Guess and Reedy (1985). In most