Page 196 - Mechanics Analysis Composite Materials

Chapter 4. Mechanics of a composite layer 181

matrix is relatively low, and the second circumstance arises - matrix material with
low stiffness cannot provide the proper stress diffusion in the vicinity of damaged or
broken fibers (see Section 3.2.3). As a result, the main material characteristic - its
longitudinal tensile strength - decreases. Experimental results corresponding to
composites with resins 1, 2, 3, and 4 are presented in Fig. 4.50. Thus, significant rise
in transverse elongation is accompanied with unacceptable drop in longitudinal
strength (see also Chiao, 1979).
One of the possible ways for synthesizing composite materials with high
transverse elongation and high longitudinal strength is to combine two matrix
materials - one with high stiffness to bind the fibers and the other with high
elongation to provide the proper transverse deformability (Vasiliev and Salov,
1984). The manufacturing process involves two-stage impregnation. At the first
stage a fine tow is impregnated with the high-stiffness epoxy resin (of the type 2
in Fig. 4.48) and cured. The properties of thus fabricated composite fiber are as
follows:

0' " " " EZ,%
0 0.2 0.4 0.6 0.8 1 1.2

Fig. 4.49. Stress-strain curves for transverse tension of unidirectional fiberglass compositeswith different
epoxy matrices (numbers on the curves correspond to Fig. 4.48).

F,,MPa
1500

1400

1300

1200

I100
I000
0 2 0 4 0 6 0
Fig. 4.50. Dependence of the longitudinal strength on the matrix ultimate elongation (numbers on the
curve correspond to Figs. 4.48 and 4.49).