Page 153 - 3D Fibre Reinforced Polymer Composites

142 30 Fibre Reinforced Polymer Composites

yams. Note that the strength and stiffness results for the Celion 6K specimens have also
been normalised to a fibre volume fraction of 68% to allow for better comparison.

Table 6.4 Comparison of E-glasdepoxy 2-step and 4-step braided composite (from
Byun et al., 1991a)
2-step (V, = 40%) 4-step (V, = 40%)
Number of axial yarns 38 0
Number of braider yarns 11 34
Surface braider yarn angle (“) 55 25
Tensile modulus (GPa) 35.2 25.5
Tensile Strength (MPa) 502 420
Poisson’s ratio 0.3 1 0.58
Tensile failure strain(%) 1.33 1.83
Compressive modulus (GPa) 23.1 15.3
Compressive strength (ma) 418 194
Compressive failure strain (%) 1.87 1.4
Short beam shear strength (MPa) 71 76.8

TabIe 6.5 Effect of yarn size upon the mechanical properties of carbodepoxy 1x1 4-
step braided composites (from Macander et al., 1986)
AS4 AS4 AS4 Celion6K Celion
3K 6K 12K 12K
Fibre volume fraction (%) 68 68 68 56 68
(68 normalised)
Tensile strength (MPa) 736.8 841.4 1067.2 857.7 1219.8
(1041.5)
Tensile modulus (GPa) 83.5 119.3 114.7 87.8 113.1
(106.6)
Short beam shear strength 114.8 126 121.4 71.4 71.4
(MPa) (86.7)
Poisson’s ratio 0.945 1.051 0.98 0.968 0.874
Flexural strength (MPa) 885.3 739.8 1063.3 -
Flexural modulus (GPa) 84.5 95.2 136.5 -
Apparent fibre angle (deg.) k19 k15 *I5 k15 k17.5

The results in Table 6.5 suggest that the tensile strength and modulus of the 4-step
braided composites increase with increasing yam size. This could be related more to the
dependence of the braid angle on the yarn size, as larger yarn sizes were generally
observed to produce lower braiding angles in the specimens. However, Macander et al.
(1986) did not propose this as the only influence that yarn size had upon the tensile
properties and concluded that other variables not clearly identified play a significant
role in this effect. They suggested that the effect of “crowding” of the braider yarns at
the specimen edges can orient them more along the axial direction and thus improve the