Page 33 - Mechanics Analysis Composite Materials

18 Mechanics and analysis of composite materials

matrix cannot be reset, dissolved or melted. Heating of a thermoset material results
first in degradation of its strength and stiffness and then in thermal destruction.
In contrast to thermoset resins, thermoplastic matrices (PSU, PEEK, PPS and
others - see Table 1.1) do not require any curing reaction. They melt under heating
and convert to a solid state under cooling. Possibility to re-melt and dissolve
thermoplastic matrices allows us to reshape composite parts forming them under
heating and simplifies their recycling which is a problem for thermoset materials.
PoIymeric matrices being combined with glass, carbon, organic, and boron fibers
yield a wide class of polymeric composites with high strength and stiffness, low
density, high fatigue resistance, and excellent chemical resistance. The main
disadvantage of these materials is their relatively low (in comparison with metals)
temperature resistance limited by the matrix. The so-called thermo-mechanical
curves are plotted to determine this important (for applications) characteristic of the
matrix. These curves, presented for typical epoxy resins in Fig. 1.12, show the
dependence of some stiffness parameter on the temperature and allow us to find the
so-called glass transition temperature, Tg,which indicates dramatic reduction of
material stiffness.There exist several methods to obtain material thermo-mechanical
diagram. The one used to plot the curves presented in Fig. 1.12 involves
compression tests of heated polymeric discs. Naturally, to retain the complete set
of properties of polymericcomposites, the operating temperature, in general, should
not exceed Tg.However, actual material behavior depends on the type of loading.
As follows from Fig. 1.13, heating above the glass transition temperature only
slightly influences material properties under tension in the fiber direction and
dramatically reduces strength in longitudinal compression and transverse bending.
Glass transition temperature depends on the processing temperature, Tp, under
which material is fabricated, and higher Tp results, as a rule, in higher Tg.
Thermoset epoxy matrices cured under 120-160°C have Tg=60-140°C. There also

0’ I I I I I T,”C
0 40 80 120 160 200
Fig. 1.12. Typical thermo-mechanical diagrams for cured epoxy resins with glass transition temperatures
80°C (-) and 130°C (- -- -).