In the present study, the fracture energy of hybrid carbon fiber reinforced polymers was investigated. The composites were modified by the addition of multi-walled carbon nanotubes into the matrix material. The interlaminar fracture properties under Mode I and Mode II remote loading were studied as a function of the carbon nanotube content in the matrix. With the addition of carbon nanotubes in the epoxy matrix, a significant increase in the load bearing ability as well as in the fracture energy was observed, for both Mode I and Mode II tests. It is speculated that carbon nanotubes due to their large aspect ratio have a significant toughening effect since extra energy is needed in order to pull them out from the matrix and start the crack propagation following a kinking out pattern at nanoscale.
In this study, carbon nanotubes (CNTs) were used as additives in the epoxy matrix of unidirectional (UD) carbon fiber reinforced laminates. CNTs were employed not only for improving the mechanical performance of composite, but also for providing an innovative way for monitoring the damage accumulation during the loading of the laminate via the monitoring of the changes in the conductivity of the material. The CNT inclusion improved the transverse conductivity rendering the UD composite more electrically isotropic. Both plain and modified laminates were subjected to monotonic and cyclic tensile loading with simultaneous monitoring of the longitudinal resistance. The resistance change due to mechanical loading was more pronounced for the laminates with the modified matrices. As it was expected, the presence of the electrically conductive CNT network acted as a direct sensor of matrix related damage phenomena, which was complementary to changes related to failure of the reinforcing phase. This was not the case for the reference composite laminates where the monitored changes of the electrical conductivity mirrored the damage exclusively related to the reinforcing phase i.e., the carbon fibers.
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