Page 197 - Mechanics Analysis Composite Materials

182 Mechanics and analysis of composite materials

number of elementary glass fibers in the cross-section - 500;
mean cross-sectional area - 0.15 mm2;
fiber volume fraction - 0.75;
density - 2.2 g/cm3;
longitudinal modulus - 53.5 GPa;
longitudinal strength - 2100 MPa;
longitudinal elongation - 4.5%;
transverse modulus - 13.5 GPa,
transverse strength - 400 MPa;
transverse elongation - 0.32%.
At the second stage, a tape formed of composite fibers is impregnated with a highly
deformable epoxy matrix whose stress-strain diagram is presented in Fig. 4.51. The
microstructure of the resulting two-matrix unidirectional composite is shown in
Fig. 4.52 (dark areas are cross-sections of composite fibers, magnification is not
enough to see elementary glass fibers). Stress-strain diagrams corresponding to
transverse tension, compression, and in-plane shear of this material are presented in
Fig. 4.16.
The main mechanical characteristics of the two-matrix fiberglass composite are
listed in Table 4.3 (material No. 1). As can be seen, two-stage impregnation results
in relatively low fiber volume content (about 50%). Material No. 2 that is composed
from composite fibers and a traditional epoxy matrix has also low fiber fraction, but
its transverse elongation is 10 times less than that of material No. 1. Material No. 3
is a traditional glass-epoxy composite that has the highest longitudinal strength and
the lowest transverse strain. Comparing materials No. 1 and No. 3 we can see that
though the fiber volume fraction of the two-matrix composite is lower by 24%, its
longitudinal strength is less than that of a traditional composite by 3.4% only
(because the composite fibers are not damaged in the processing of composite
materials), while its specific strength is a bit higher (due to lower density). Material
No. 4 demonstrates that direct application of a highly deformable matrix allows us

0 20 40 60 80 100 120
Fig. 4.51. Stress-strain diagram of a deformable epoxy matrix.