Mendomers are a group of polymers that are mendable upon heating. Specifically, cracks in these polymers have been shown to heal themselves when heated close to the glass transition temperature. The main mechanism behind the healing is the thermally reversible Diels—Alder reaction, where a dicyclopentadiene unit in the polymer backbone breaks apart into two cyclopentadiene terminal groups, which then reunite upon heating. The present study investigates the feasibility of using a mendomer as a matrix for re-mending composites reinforced with graphite fibers. The graphite fibers are used as electrical conductors to provide the necessary heat to the polymer. Specimens were prepared by spreading a monomer, called mendomer, powder on a graphite/epoxy laminate substrate and curing in a vacuum oven. Microcracks were introduced by bending the substrate coupon, and the latter was heated by applying electric currents. The healing behavior was confirmed by disappearance of microcracks that were observed with an optical microscope and a scanning electron microscope (SEM).

This article investigates the dependency of electrical resistance on temperature for graphite/polymer composites. The electrical resistance change of a unidirectional laminate is measured with elevated temperature in the fiber, transverse and through-thickness directions. Because of the properties of graphite fibers, the electrical resistance decreases as temperature increases in any direction. The activation energy is calculated from an Arrhenius plot to evaluate the difference among the different directions. The activation energy is the lowest in the fiber direction due to the high electrical conductivity of graphite fibers. Since electric current flows thorough the contacts between neighboring graphite fibers, similar activation energies are required in the transverse and through-thickness directions, indicating transversely isotropic properties. These characteristics will be used to establish the relationship between electrical resistance outputs and structural conditions, and will be eventually applied to an in-service structural management system using addressable conducting network, which is a grid arrangement of conducting lines on a composite panel.

The initial stage of fatigue failure has not been thoroughly clarified for carbon fiber reinforced plastics (CFRPs). Although the initiation of fatigue cracks has been regarded to be interfacial debonding between the carbon fiber and polymer matrix, their detection among numerous carbon fibers, whose diameter is only 7 µm, is extremely difficult. In this study, a single carbon fiber was transversely embedded in a dumbbell-shaped epoxy sample to focus on the interfacial debonding and was observed using synchrotron radiation (SR) X-ray computed tomography (CT). A tabletop fatigue testing machine driven by a piezoelectric actuator was developed to apply static and cyclic loads along the beamline. SR X-ray multiscale CT imaging was conducted by switching between an absorption-contrast projection method (micro-CT) and a phase-contrast imaging-type X-ray microscopic CT (nano-CT). The carbon fiber was entirely captured by micro-CT and then magnified at both ends on the free surfaces. Nano-CT clearly visualized the interfacial debonding under 30 MPa static tensile load and the implication of the coalescence of nano-voids along the interface under 50 MPa. Under cyclic loads, the interfacial debonding gradually progressed under a 8–40 MPa sinusoidal stress after 10,000 cycles, whereas it did not propagate under a stress below 30 MPa.