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.