Self-sensing of flexural damage and strain in carbon fiber reinforced cement and effect of embedded steel reinforcing bars

Self-sensing of flexural damage and strain in carbon fiber reinforced cement is attained by measuring the volume or surface resistance with the four-probe method and electrical contacts on the compression and/or tension surfaces. The oblique resistance (volume resistance in a direction between the longitudinal and through-thickness directions) increases upon loading and is a good indicator of damage and strain in combination. The surface resistance on the compression side decreases upon loading and is a good indicator of strain. The surface resistance on the tension side increases upon loading and is a good indicator of damage. The effectiveness for the self-sensing of flexural strain in carbon fiber reinforced cement is enhanced by the presence of embedded steel rebars on the tension side. For the same midspan deflection, the fractional change in surface electrical resistance is increased in magnitude, whether the surface resistance is that of the tension side or the compression side. The fractional change in resistance of the tension surface is increased by 40%, while the magnitude of the fractional change in resistance of the compression surface is increased by 70%, due to the steel.     

The role of electronic and ionic conduction in the electrical conductivity of carbon fiber reinforced cement

Electrical conduction in carbon fiber reinforced cement with a fiber volume fraction below the percolation threshold involves electrons and ions. The fiber affects both the electronic conduction and the ionic conduction. The ozone treatment of the fiber surface helps the ionic conduction. Latex as an admixture helps provide a relatively high ionic conductivity; silica fume as an admixture helps provide a relatively high electronic conductivity. In the dry state (the state of practical importance attained by room temperature drying), electronic conduction is more significant than ionic conduction. In the wet state (water saturated state), ionic conduction dominates. When silica fume is present with the fiber, the fractional electronic contribution in the dry state is 0.99. When latex is present with the fiber, the corresponding value is 0.72-0.78. The ratio of the wet ionic conductivity to the dry ionic conductivity is much increased by fiber surface treatment and is higher when latex rather than silica fume is used. The wet ionic conductivity is much higher than the dry overall conductivity when latex is present, but is lower than or comparable to the dry overall conductivity when silica fume is present; the wet ionic conductivity is lower than the dry overall conductivity when the fiber is not treated and silica fume is present.     

Interlaminar damage in carbon fiber polymer-matrix composites, studied by electrical resistance measurement

Interlaminar thermal damage in continuous carbon fiber polymer-matrix composites was monitored in real time during thermal cycling by measurement of the contact electrical resistivity of the interlaminar interface. Damage was accompanied by an abrupt increase of the resistivity for a thermoset-matrix composite, and by an abrupt decrease of the resistivity for a thermoplastic-matrix composite. Both phenomena are due to the effect of matrix damage on the chance of fibers of one lamina touching those of an adjacent lamina. The damage involved matrix molecular movement in the thermoplastic case, but not in the thermoset case.     

The pull-out and failure of a fiber bundle in a carbon fiber reinforced carbon matrix composite

The interfacial failure is examined for a unidirectionally reinforced carbon fiber/carbon matrix composite. A novel tensile test is conducted which realizes the processes of interfacial debonding and subsequent pull-out of a fiber bundle from the surrounding composite medium. The critical stress at the onset of delamination cracking is related to the fracture energy (the critical energy release rate for mode II cracking). A force-balance equation of a fiber bundle, which is quasi-statically pulled-out of the composite socket, is formulated in terms of the inter- and intra-laminar shear strengths of the composite. This equation is successfully used to estimate the delamination crack length along the debonded fiber bundle, as a function of the stress applied to the bundle.     

Sensing delamination in a carbon fiber polymer-matrix composite during fatigue by electrical resistance measurement

Delamination in a crossply [0/90] continuous carbon fiber polymer-matrix composite was sensed in real time during fatigue by measuring the electrical resistance of the composite in the through-thickness direction. Upon 0° tension-tension fatigue at a maximum stress of 57% of the fracture stress, the resistance irreversibly increased both in spurts and continuously, because of delamination, which started at 33% of the fatigue life. The resistance increased upon loading and decreased upon subsequent unloading in every cycle, thereby allowing strain sensing. The minimum resistance at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life. The resistance became noisy starting at 62% of the fatigue life, at which delamination occurred rapidly and the fraction of laminate area delaminated reached 4.3%.     

Biaxial compression in carbon-fiber-reinforced mortar, sensed by electrical resistance measurement

The stress-sensing behavior of carbon-fiber-reinforced mortar under biaxial compression was found to be different with that under uniaxial compression. For both the case of uniaxial compression and the case of biaxial compression, with an increase in the compressive stress, the fractional change in resistance in any direction decreases gradually at first, and then increases with increasing stress. But the stress level at which the fractional change in resistance starts to increase, referred to as the critical stress in this paper, varies with the loading style and the direction in which the resistance is measured. The critical stress related to the biaxial compression and the stress direction is larger than that related to the uniaxial compression and the stress direction. But on the other hand, the critical stress related to the biaxial compression and the direction other than the two stress directions is less than that related to the uniaxial compression and the direction other than the stress direction. The piezoresistivity of carbon-fiber-reinforced mortar under biaxial compression is more sensitive than that under uniaxial compression.     

Lignin-based carbon fibers for composite fiber applications

Carbon fibers have been produced for the first time from a commercially available kraft lignin, without any chemical modification, by thermal spinning followed by carbonization. A fusible lignin with excellent spinnability to form a fine filament was produced with a thermal pretreatment under vacuum. Blending the lignin with poly(ethylene oxide) (PEO) further facilitated fiber spinning, but at PEO levels greater than 5%, the blends could not be stabilized without the individual fibers fusing together. Carbon fibers produced had an over-all yield of 45%. The tensile strength and modulus increased with decreasing fiber diameter, and are comparable to those of much smaller diameter carbon fibers produced from phenolated exploded lignins. In view of the mechanical properties, tensile 400–550 MPa and modulus 30–60 GPa, kraft lignin should be further investigated as a precursor for general grade carbon fibers.     

Study of carbon films from PAN/VGCF composites by gelation/crystallization from solution

A carbon film with a cross-sectional area much larger than that of a commercial carbon fiber (>6000 times) and a thickness of about 0.3 mm was obtained using a new method. In this method, composite materials of polyacrylonitrile (PAN) and vapor-grown carbon fiber (VGCF) prepared by gelation/crystallization from dilute solutions were used as starting materials The gelation/crystallization method was adopted to ensure high orientation of PAN chains. The composite materials were heat-treated at 200-300°C in an oxidizing atmosphere for thermal stabilization and then heat-treated to 1500°C in argon gas to promote carbonization. The tensile modulus and electric conductivity for the carbon materials with cross-sectional areas of about 0.6 mm² (thickness 0.3 mm and width 2 mm) reached 18 GPa and 10 Ω⁻¹ cm⁻¹, respectively. The mechanical and electrical properties of the final carbonized materials were sensitive to the PAN/VGCF composition and the draw ratio. These phenomena were analyzed using Fourier transform IR and X-ray diffraction.     

Anomalous electrical transport properties of amorphous carbon films on Si substrates

Amorphous carbon (a-C) films are deposited on n-Si substrates at different temperatures using pulsed laser deposition. Some anomalous current-voltage (I-V) characteristics of the a-C/n-Si are reported. The films deposited at 27 °C have an apparent voltage-induced switch effect, and the value of the switch voltage decreases with increasing temperature. However, the I-V characteristics of the a-C/n-Si deposited at 300 °C and 500 °C are completely different from those deposited at 27 °C. The anomalous I-V characteristics should be of interest for various applications such as field effect devices. In addition, the magnetoresistance (MR) and the resistance of the a-C/n-Si have been studied. Finally, we interpret the anomalous I-V characteristics and MR observed by use of energy band theory.     

Comparison of 2D and 3D carbon/carbon composites with respect to damage and fracture resistance

The static mechanical responses of two- and three-dimensionally reinforced carbon/carbon composites (2D- and 3D-C/Cs) were compared. The mechanical properties examined included tensile and shear stress-strain (S-S) relations, and fracture behavior using compact tension and double edge notch configurations. Compared with 2D-C/Cs, 3D-C/Cs were shown to possess a similar tensile S-S relation, lower shear strength, higher ultimate deformation in shear, and much higher fracture resistance. The differences in shear and fracture resistance were shown to be derived from a weaker fiber/matrix interface and weaker bonding between fiber bundles in the 3D-C/Cs. These weak interface characteristics of 3D-C/Cs are due to the high value of residual stresses caused by the three-dimensional fiber constraint of 3D-C/Cs.     

Effect of H2O adsorption on the electrical transport properties of double-walled carbon nanotubes

Water molecules adsorbed on a double-walled carbon nanotube (DWCNT) serve as charge trapping centers when present in low density and as electron donors when present in high density. There is a discontinuous change between the low- and high-density regions. H₂O molecules are apt to be adsorbed on the outer surface of DWCNTs, and in this case the electrical transport properties are extremely sensitive to environment, which suggests that DWCNTs are hole doped and act as an electric dipole with the inner tube.     

Hybrid electro-conductive composites with improved toughness, filled by carbon black

The influence of both carbon black (CB), and an ethylene-propylene copolymer grafted by maleic anhydride (EP-g-MA), on the static mechanical properties, impact strength, peel and shear strengths as well as on the electrical conductivity of composites based on high-density polyethylene (HDPE) matrix, was investigated in this paper. It was found that CB improves the stress at yield, the stress at break, and Young’s modulus, as well as the shear strength and peel strength, of the HDPE/CB composites. The percolation threshold was found at 4.5 vol.% of CB. The addition of EP-g-MA to the HDPE/CB composites improves their impact strength, the peel and shear strengths, and the electrical conductivity.     

Simulation of carbon nanotube based p-n junction diodes

Taking advantage of the unique characteristics of an ambipolar carbon nanotube field effect transistor (CNTFET), a ‘p-n junction’ is simulated along the single-walled carbon nanotube channel using two separate gates close to the source and drain of the CNTFET, respectively. The current-voltage characteristics of the double-gated CNTFET are calculated using a semiclassical method based on the Schottky barrier field effect transistor mechanism. The calculation results show a good rectification performance of the p-n junction.     

Structure and electrical properties of carbon black

The incorporation of conductive particles into a polymer matrix modifies fundamentally the electrical conductivity of the composite. The importance of carbon black structure on other mechanical and rheological properties is also well documented. Besides common techniques, like DBP-absorption and CDBP-absorption, void-volume, determined by the measurement of the volume of a given quantity of carbon black under a given pressure, has been considered as a more absolute technique. The present work considers the void-volume technique under a broader angle. The evolution of the volume of a given carbon black weight under increasing pressure as well as the evolution of the electrical resistivity is recorded and analysed.     

Compressive strength of three-dimensionally reinforced carbon/carbon composite

Compressive behavior of three-dimensionally reinforced carbon/carbon composite (3D-C/C) was examined from room temperature to elevated temperatures up to about 3000 K. Three-dimensionally reinforced C/C was found to have an inclination to induce kinks at the ends of specimens due to extremely low shear strength. In order to avoid this type of premature fracture and to conduct high-temperature tests, discussion was made on specimen geometry and testing procedure, and the combination of a dumbbell-shape specimen and test configuration without a supporting jig were found to be suitable for the present study. Using this set-up, the compressive strength of a 3D-C/C was evaluated as a function of temperature up to about 3000 K. The compressive strength of the 3D-C/C monotonically increased with the increase in temperature up to 2300 K, but decreased above this temperature. The strength enhancement was suggested to be caused by improvement in the fiber/matrix interfacial bonding, and the degradation over 2300 K was by softening of the matrix at high temperatures.     

Diblock copolymer stabilization of multi-wall carbon nanotubes in organic solvents and their use in composites

A versatile method for the preparation of dispersed nanotubes using polystyrene-b-polyisoprene diblock copolymers in different selective organic solvents is presented. Stable dispersions have been obtained in polar (DMF) and apolar (heptane) media depending on the selectivity of the diblock copolymers. They have been characterized by means of optical microscopy, TEM imaging and dynamic light scattering, showing the first demonstration of multiwall carbon nanotubes (MWCNTs) solutions with in situ characterization of diblock copolymer stabilization. The most effectively stabilized dispersions have been used to make nanotube/polystyrene composites. We find that the coating of the nanotubes by the diblock polymer does not prevent electrical transport, so that the system can exhibit a relatively high surface conductivity above the percolation threshold. The low percolation threshold experimentally determined is presumably due to weak attractive interactions between the nanotubes as the composites are dried.     

Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites

The novel properties of carbon nanotubes have generated scientific and technical interest in the development of nanotube-reinforced polymer composites. In order to utilize nanotubes in multi-functional material systems it is crucial to develop processing techniques that are amenable to scale-up for high volume, high rate production. In this research we investigate a scalable calendering approach for achieving dispersion of CVD-grown multi-walled carbon nanotubes through intense shear mixing. Electron microscopy was utilized to study the micro and nanoscale structure evolution during the manufacturing process and optimize the processing conditions for producing highly-dispersed nanocomposites. After processing protocols were established, nanotube/epoxy composites were processed with varying reinforcement fractions and the fracture toughness and electrical/thermal transport properties were evaluated. The as-processed nanocomposites exhibited significantly enhanced fracture toughness at low nanotube concentrations. The high aspect ratios of the carbon nanotubes in the as-processed composites enabled the formation of a conductive percolating network at concentrations below 0.1% by weight. The thermal conductivity increased linearly with nanotube concentration to a maximum increase of 60% at 5 wt.% carbon nanotubes.