A scaled-up catalytic chemical vapor deposition synthesis approach has been used to grow carbon nanotubes (CNTs) onto quartz and alumina fibers to prepare novel hierarchically structured composites with high volume fractions of CNTs. The assynthesized CNT-coated fibers were functionalized using atmospheric oxygen plasma, prior to infusing an epoxy polymer matrix, in order to improve wettability and bonding.The polymer matrix was further modified by adding a triblock copolymer to provide nanoscale toughening. Direct adhesion tests show that CNT and plasma treatments increase the shear strength of quartz or alumina/epoxy films by 30%. CNT-quartz/epoxy composites show an 80% increase in the in-plane shear strength while the CNTalumina/epoxy composites show more than a three-fold increase in shear strength. The failure mechanisms of the films and the fiber composites are dominated by fracture through the catalytic iron nanoparticles and the improvements are limited by the CNT to fiber adhesion. The composites also have very high in-plane electrical conductivity, 2 over 3 S/cm. The carbon nanotubes form a piezoresistive sensing network surrounding the fibers. Under flexural loading the change in electrical resistance can detect damage initiation and impending fracture, particularly for the alumina composites.
Many modern military aircraft are constructed from composite and bonded structure, such as thin carbon-epoxy laminate bonded to Kevlar® and Nomex® honeycomb. Operation of these platforms in Australian and global conditions will subject the structure to potentially high levels of humidity, extremes in temperature, and for maritime operations, exposure to salt spray conditions. The thin composite laminate is likely to rapidly absorb moisture in a humid environment and enable permeation of moisture into the adhesive and core. In addition to the chemical influence of moisture on the composite structure, the moisture trapped in the honeycomb structure may freeze and expand with changes in altitude during operations or simply due to daily temperature fluctuations at the resident airbase. The combination of moisture ingress in the honeycomb structure and thermal cycling may lead to deteriorated strength of the honeycomb panels over time that would not be observed for long term humid exposure alone. Long term salt water absorption may also have an adverse effect on composites structures. This study investigates the effects of humid environments and thermal cycling on the mechanical properties of composite and honeycomb structures.
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