The paper deals with the theory of the coupling of strongly anisotropic fibers along their axis with anisotropic matrices along either the fiber direction or the transverse plane to the direction of the fibers. It is shown that the first arrangement deteriorates, whereas the second improves significantly the mechanical behavior of the composites. Indeed, it is shown that anisotropy of the matrix, increasing its mechanical properties on the transverse isotropic plane of the composite, increases also the transverse-transverse Poisson's ratio, v23, whereas decreases the longitudinal shear modulus, G,2. This results in values of the eigenangle wx approaching the corresponding value
Wic
for the equivalent isotropic material and caused a deterioration of the strength and toughness of the composite, since the material now has the tendency to develop higher stress concentrations for equivalent loadings [1,2]. On the contrary, a strong anisotropic matrix along the direction of the fibers yields the inverse results for the respective moduli of the anisotropic composite.
Finally, the most important result is a perceptible decrease of the ratio E,1/2G,2 which yields values of the eigenangle wc tending to approach the critical value wi for the isotropic material. The decrease of wc indicates the improvement of the quality of the composite, since it develops relatively lower stress concentration factors, which approach their respective isotropic values [2]. Thus, the anisotropic composite material is approaching an equivalent state of quasi-isotropy and therefore improves its strength by reducing considerably the eventual anisotropic stress concentration factors of the respective structural elements [1]. Examples with T300/N5208 graphite-epoxy composites and Borsic-1100 aluminum metal-metal composites indicate clearly the beneficial effect of the anisotropy of their matrices.
