Shoukai Wang

Sensing damage in carbon fiber and its polymer-matrix and carbon-matrix composites by electrical resistance measurement

Fatigue damage was sensed in real time in continuous carbon fiber and its polymer-matrix and carbon-matrix composites by electrical resistance measurement in the fiber direction. In a polymer-matrix composite, fiber breakage overshadows fiber damage in causing the resistivity of the composite to increase irreversibly. In a carbon-matrix composite, fiber breakage and matrix cracking caused the resistivity to increase irreversibly, such that these two mechanisms cannot be distinguished. Fatigue damage was detected from 50% of the fatigue life onward for the polymer-matrix composite, and from 0% of the fatigue life onward for the carbon-matrix composite.

Temperature/light sensing using carbon fiber polymer-matrix composite

Abstract A polymer (epoxy)-matrix composite with the top two laminae of continuous carbon fibers in a crossply configuration was found to be a temperature sensor. Each junction between crossply fiber tow groups of adjacent laminae is a sensor, while the fiber groups serve as electrical leads. A junction array provided by two crossply laminae allows sensing of the temperature/light distribution. The contact electrical resistivity of the junction decreases reversibly upon heating (whether using light or hot plate to heat), due to the activation energy involved in the jump of electrons across the junction. The contact resistivity decreases with increasing pressure during composite fabrication, due to the increase in pressure exerted by fibers of one lamina on those of the other lamina. The absolute value of the fractional change in contact resistivity per degree C increases with increasing pressure during composite fabrication, due to decrease in composite thickness, increase in fiber volume fraction and consequent increases in interlaminar stress and activation energy. A junction between unidirectional fiber tow groups of adjacent laminae is much less effective for temperature/light sensing, due to the absence of interlaminar stress.

Electrical behavior of carbon fiber polymer-matrix composites in the through-thickness direction

The electrical behavior of continuous carbon fiber epoxy-matrix composites in the through-thickness direction was studied by measuring the contact electrical resistivity (DC) of the interlaminar interface in the through-thickness direction. The contact resistivity was found to decrease with increasing curing pressure and to be higher for unidirectional than crossply composites. The lower the contact resistivity, the greater was the extent of direct contact between fibers of adjacent laminae. The activation energy for electrical conduction in the through-thickness direction was found to increase with increasing curing pressure and to be lower for unidirectional than crossply composites. The higher the activation energy, the greater was the residual interlaminar stress. Apparent negative electrical resistance was observed, quantified, and controlled through composite engineering. Its mechanism involves electrons traveling in the unexpected direction relative to the applied voltage gradient, due to backflow across a composite interface. The observation was made in the through-thickness direction of a continuous carbon fiber epoxy-matrix two-lamina composite, such that the fibers in the adjacent laminae were not in the same direction and that the curing pressure during composite fabrication was unusually high (1.4 MPa).

Early fatigue damage in carbon-fibre composites observed by electrical resistance measurement

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fibre composites by d.c. electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continuous carbon fibre carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fibre fracture as well. For the short carbon fibre polymer-matrix and cement-matrix composities, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibres and the resulting increase in the chance that adjacent fibres touched one another.

Piezoresistivity in continuous carbon fiber polymer-matrix and cement-matrix composites

Piezoresistivity was observed in continuous unidirectional carbonfiber cement-matrix and polymer-matrix composites. The fiber volumefraction was 2.6–7.4% and 58% for cement-matrix andpolymer-matrix composites respectively. The DC electrical resistancein the fiber direction increased upon tension in the fiber directionfor the cement-matrix composite, due to fiber-matrix interfacedegradation, but decreased upon tension for the polymer-matrixcomposite due to increase in the degree of fiber alignment.

Self-monitoring of strain and damage by a carbon-carbon composite

Abstract A carbon-carbon composite was found to be able to sense its own damage and dynamic strain, as its electrical resistance increased irreversibly upon damage and increased reversibly upon tensile strain. Even damage after the first cycle of tensile loading within the elastic regime was detected. The reversible resistance increase upon cyclic tension was mainly due to dimensional changes, but it was partly due to a phenomenon that intensified as damage increased. The gage factor, which is the reversible part of the fractional resistance change per unit reversible strain, was 1.2–2.4.

Carbon fiber polymer-matrix composite interfaces as thermocouple junctions

Thermocouples made from dissimilar continuous carbon fibers in the form of epoxy-matrix composite, using the interlaminar interface as the thermocouple junction, were found to exhibit thermocouple ...

Carbon fiber structural composites as thermistors

Abstract Carbon fiber structural composites, in the form of short-fiber silica fume cement–matrix composite and crossply continuous-fiber polymer–matrix composite, were found to be thermistors due to the decrease in electrical resistivity with increasing temperature. The resistivity is the volume resistivity for the cement–matrix composite and the contact resistivity between crossply laminae for the polymer–matrix composite. The activation energy of electrical conduction is up to 0.41 and 0.12 eV for cement–matrix and polymer–matrix composites, respectively. For the polymer–matrix composite, each junction between crossply fiber groups of adjacent laminae is a thermistor, while the fiber groups serve as electrical leads.

Piezoresistivity in Silicon Carbide Fibers

The polycrystalline β-SiC fiber of diameter 14 μm (without a carbon core) is piezoresistive under tension, with gage factor 5. The resistivity increases linearly and reversibly with strain in the elastic regime. The fiber of diameter 140 μm (with a carbon core) is not piezoresistive, due to the carbon core controlling the electrical resistance.

Carbon fiber polymer-matrix structural composite as a semiconductor

An epoxy-matrix composite with continuous crossply carbon fibers was found to be a semiconductor in the through- thickness direction, with a tunable energy gap of 10-2-10-1 eV. The higher the pressure during composite fabrication by lamination, the higher the interlaminar stress and the greater the energy gap, which is the activation energy for electron jumping from one lamina to the adjacent one in the composite. The semiconducting behavior involves the contact electrical resistivity between adjacent laminae in the composite decreasing reversibly with increasing temperature. The concept of optoelectronic and electronic devices made from carbon fiber polymer-matrix composites is provided. Devices include solar cells, light emitting diodes, lasers, infrared detectors and transistors.Thus, a new dimension is added to smart structures and a new field of electronics is born.