Figure 5. Effect of volumetric carbon loading on the electrical
conductivity of the catalytically-grown multi-walled carbon
nanotube-reinforced epoxy composite.
The typical tensile stress-strain curves are presented in Figure 6 for
the catalytically-grown multi-walled carbon nanotube-reinforced epoxy
composite. Dynamic material properties for elastomers are often
specified for structural response in the frequency rather than the time
domain. Vibration experiments are used to find complex modulus in the
frequency domain and this data is often used to design shock absorbers
and base isolation bearings. However, the concept of a complex modulus
is based on linear viscoelastic material behavior, namely material for
which stress is directly proportional to strain and strain rate.
Frequency domain material properties are therefore limited to
applications where strains are small and stress is approximately linear
with strain and the strain rate. Frequency domain material properties
become irrelevant if the material exhibits nonlinear elastic behavior or
is subjected to large strains. Under tension, elastomers are not only
nonlinear elastic but also hyper-elastic. Clearly, new dynamic material
properties are needed to characterize elastomeric structures undergoing
high strain rates and nonlinear, hyper-elastic behavior. Thus, there
exist a need to develop a tensile impact apparatus that is designed to
give dynamic stress-strain curves of a rubber specimen undergoing
tensile impact loading. Particularly, there is a need for an apparatus
that is capable of achieving strains sufficient to fracture virtually
any elastomer sample. The data provided by the tensile impact apparatus
would enable one to predict tensile fracture of rubber components under
shock or impact loads. Generally, for all type of matrixes, and
especially for the thermosets, the lower the viscosity of the precursor
of the polymeric matrix, the lower the resulting chemical and physical
final properties of the solidified matrix. The solidified matrixes with
the highest physical and chemical properties usually also have the
highest viscosity in the liquid precursor state, with the resulting
restrictions in terms of their processability. In some cases, it is
possible to reduce the global viscosity of the precursor polymer
composition used to impregnate the substrate by using solvents. The
drawback is due to the fact that the solvents need to be eliminated from
the final composite, before the cross-linking of the precursor starts.
For the processes which do not use solvents at all, the necessary
viscosity for the processability of the precursor is achieved by a
temperature increase. Nevertheless, the temperature cannot be increased
for the hand lay-up process or can only slightly be increased for all
other processes. Depending on the type of polymers in the matrix, above
a certain temperature limit, degradation starts or cross-linking starts,
thereby reducing the impregnation time window.