PREDICTION OF ELASTIC CONSTANTS AND THERMAL EXPANSION
from the fiber volume fraction and the fiber and matrix properties are available in the
literature. The simplest are rules of mixtures (e.g., [5.1,5.2]). Better models offer more
realistic partitioning of stress between fibers and resin [5.3-5.7]. The main features of
some important models are compared in Table 5.1 and some tow properties computed for
AS4/1895 are shown as functions of fiber volume fraction in Fig. 5-2. Each model
predicts five elastic constants corresponding to the five degrees of freedom allowed by
transverse isotropy. Only Hashin's model admits anisotropy in the fibers themselves; all
the other models assume the fibers and resin are separately isotropic. Assuming glass
fibers are isotropic is very plausible. But the anisotropy in common carbon fibers is
substantial and Hashin's model should then be preferred [5.8].
Figure 5-2. Comparison of the elastic constants predicted for tows in an AS4/1895
composite using various models from the literature (Table 5.1). Hashin I: assuming
the same isotropic fiber elastic constants as in the other models. Hashin II: using
anisotropic fiber elastic constants for AS4 fibers.
