Page 37 - Mechanics Analysis Composite Materials

22 Mechanics and analysis of composite materials

Being a heterogeneous media, a composite material has two levels of heterogeneity.
The first level represents a microheterogeneityinduced by at least two phases (fibers
and matrix) that form the material microstructure. At the second level the material
is characterized with a macroheterogeneity caused by the laminated or more
complicated macrostructure of the material which consists usually of a set of layers
with different orientations.
The first basic process yielding material microstructure involves the application of
a matrix material to fibers. The simplest way to do it used in the technology of
composites with thermosetting polymeric matrices is a direct impregnation of tows,
yarns, fabrics or more complicated fibrous structures with liquid resins. Thermo-
setting resin has relatively low viscosity (I 0-100 Pa s) which can be controlled with
solvents or heating and good wetting ability for the majority of fibers. There exist
two versions of this process. According to the so-called “wet” process, impregnated
fibrous material (tows, fabrics, etc.) is used to fabricate composite parts directly,
without any additional treatment or interruption of the process. In contrast to that,
in “dry” or “prepreg” processes impregnated fibrous material is dried (not cured)
and thus obtained preimpregnated tapes (prepregs) are stored for further utilization
(usually under low temperature to prevent uncontrolled polymerization of the
resin). Machine making prepregs is shown in Fig. 1.16. Both processes having
mutual advantages and shortcomings are widely used for composites with
thermosetting matrices.
For thermoplastic matrices, application of the direct impregnation (“wet”
processing) is limited by relatively high viscosity (about 10l2Pa s) of thermoplastic
polymer solutions or melts. For this reason, “prepreg” processes with preliminary
fabricated tapes in which fibers are already combined with thermoplastic matrix are
used to manufacture composite parts. There also exist other processes that involve
application of heating and pressure to hybrid materials including reinforcing fibers
and a thermoplastic polymer in the form of powder, films or fibers. A promising
process (called fibrous technology) utilizes tows, tapes or fabrics with two types of
fibers - reinforcing and thermoplastic. Under heating and pressure thermoplastic
fibers melt and form the matrix of the composite material.
Metal and ceramic matrices are applied to fibers by means of casting, diffusion
welding, chemical deposition, plasma spraying, processing by compression molding
and with the aid of powder metallurgy methods.
The second basic process provides the proper macrostructure of a composite
material corresponding to loading and operational conditions of the composite part
that is fabricated. There exist three main types of material macrostructure - linear
structure which is specific for bars, profiles and beams, plane laminated structure
typical for thin-walled plates and shells, and spatial structure which is necessary for
thick-walled and solid composite parts.
Linear structure is formed by pultrusion, table rolling or braiding and provides
high strength and stiffness in one direction coinciding with the axis of a bar, profile
or a beam. Pultrusion results in a unidirectionallyreinforced composite profile made
by pulling a bundle of fibersimpregnated with resin through a heated die to cure the
resin and, to provide the proper shape of the profile cross-section.Profiles made by