fibers since the model is not transversely isotropic but tetragonal. The square array is
conceivable applicable to boron/aluminum composites in which fibers are arranged in
patterns that resemble such arrays. However, it is not applicable to any type of boron
tapes or prepregs. The reason is that these are thin unidirectionally reinforced layers
whose thicknesss is of the order of the diameter of one boron fiber and can not be
consider composite materials.
The unit cell is modeled from the repetitive pattern of the fiber and matrix layout of
composite laminates. In the present analysis, both square and hexagonal unit cells are
considered for the micromechanics model. In both cases the dimensions are chosen such
that the fiber volume ratio is 60%, which is typical of graphite/epoxy composites. When
fibers are arranged in a square unit cell one can obtain a maximum fiber volume fraction
of 79%. The square unit cell was modeled using 1,600 quadratic solid elements with
periodic boundary conditions [7-9]. The periodic boundary conditions ensure
displacement compatibility and stress continuity on the opposite faces of the unit-cell.
** *0
Fiber
Figure 2-1. A square representative volume element and corresponding finite element
mesh.
The hexagonal pattern of unit cell can be found more commonly in fiber-matrix
composites, especially when the composite is fabricated with high fiber volume fraction.
Theoretically one can obtain a maximum fiber volume fraction of 91% with hexagonal