Page 32 - Mechanics Analysis Composite Materials

Chapter 1. Introduction 17

term traditionally used for this phenomenon in analysis of stiffened structures
(Goodey, 1946)) in the material under given operational temperature. That is why
this temperature is limited, as a rule, by the matrix rather than by the fibers. But
on the other hand, to provide material integrity up to the failure of the fibers, the
matrix material should possess high compliance. Obviously, for a linear elastic
material (see Fig. 1.3), combination of high stiffness and high ultimate strain 5
results in high strength which is not the case for modern matrix materials. Thus,
close to optimal (with respect to the foregoing requirements) and realistic matrix
material should have nonlinear stress-strain diagram (of the type shown in Fig. 1.5)
and possess high initial modulus of elasticity and high ultimate strain.
However, matrix properties, even being optimal for the corresponding fibers, do
not demonstrate themselves in the composite material if the adhesion (the strength
of fiber-matrix interface bonding) is not high enough. High adhesion between fibers
and matrices providing material integrity up to the failure of the fibers is a necessary
condition for high-performance composites. Proper adhesion can be reached for
properly selected combinations of fiber and matrix materials under some additional
conditions. First, a liquid matrix should have viscosity low enough to allow the
matrix to penetrate between the fibers of such dense systems of fibers as tows, yarns,
and fabrics. Second, the fiber surface should have good wetability with the matrix.
Third, the matrix viscosity should be high enough to retain the liquid matrix in the
impregnated tow, yarn or fabric in the process of fabrication of a composite part.
And finally, the manufacturing process providing the proper quality of the resulting
material should not require high temperature and pressure to make a composite
part.
By now, typical matrices are made from polymeric, metal, carbon, and ceramic
materials.
Polymeric matrices are divided into two main types, thermoset and thermoplastic.
Thermoset polymers which are the most widely used matrix materials for advanced
composites include polyester, epoxy, polyimide and other resins (see Table 1.1)
cured under elevated or room temperature. A typical stress-strain diagram for a
cured epoxy resin is shown in Fig. 1.11. Being cured (polymerized) a thermoset

0
0 2 4 8 8

Fig. 1.1 1. Stress-strain diagram for a typical cured epoxy matrix.