Page 81 - Mechanics Analysis Composite Materials

66 Mechanics and analysis of composite materials

Table 3.2
Strength of bundles consisting of fibers with different strength.

Fiber no. Bundle no.
1 2 3 4 5

0.6 0.7 0.85 0.9 0.95
0.8 0.9 0.9 0.85 0.95
1.o 1.2 1.1 1.o 0.95
1.6 1.4 1.15 1.05 0.95
3.0 I .6 1.4 1.1 0.95
1.4 1.16 1 .os I .o 0.95
*"I
r, (%I 95.0 66.0 22.0 7.8 0
F 3.2 3.6 4.25 4.5 4.75

relatively low stiffness changes the mechanism of fibers interaction and considerably
increases their effective strength. To show this, the strength of dry fiber bundles can be
compared with the strength of the same bundles after they were impregnated with
epoxy resin and cured. The results are listed in Table 3.3. As can be seen, composite
bundles in which fibers are joined together with matrix demonstrate significantly
higher strength, and the higher the fiber sensitivity to damage, the higher is the
difference in strength of dry and composite bundles. The influence of matrix on the
variation of strength is even more significant. As follows from Table 3.4, variation
coefficientsof composite bundles are by an order lower than those of individual fibers.
To clarify the role of matrix in composite materials consider a simple model of a
unidirectional ply shown in Fig. 3.15 and apply the method of analysis developed
for stringer panels (Goodey, 1946).
Let the ply of thickness 6 consist of 2k fibers symmetrically distributed on both
sides of the central fiber n = 0. The fibers are joined with layers of the matrix
material, and the fiber volume fraction is

(3.17)

Table 3.3
Strength of dry bundles and composite bundles.

Fibers Sensitivity of fibers Ultimate tensile load P (N) Strength
to damage increase (X)
Dry bundle Composite bundle
Carbon High 14 26 85.7
Glass Moderate 21 36 71.4
Aramid Low 66 84 21.3