Carbon fiber polymer-matrix structural composites for electrical-resistance-based sensing
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This dissertation has advanced the science and technology of electrical-resistance-based sensing of strain/stress and damage using continuous carbon fiber epoxy-matrix composites, which are widely used for aircraft structures. In particular, it has extended the technology of self-sensing of carbon fiber polymer-matrix composites from uniaxial longitudinal loading and flexural loading to uniaxial through-thickness loading and has extended the technology from structural composite self-sensing to the use of the composite (specifically a one-lamina composite) as an attached sensor. Through-thickness compression is encountered in the joining of composite components by fastening. Uniaxial through-thickness compression results in strain-induced reversible decreases in the through-thickness and longitudinal volume resistivities, due to increase in the fiber-fiber contact in the through-thickness direction, and minor-damage-induced irreversible changes in these resistivities. The Poisson effect plays a minor role. The effects in the longitudinal resistivity are small compared to those in the through-thickness direction, but longitudinal resistance measurement is more amenable to practical implementation in structures than through-thickness resistance measurement. The irreversible effects are associated with an increase in the through-thickness resistivity and a decrease in the longitudinal resistivity. The through-thickness gage factor is up to 5.1 and decreases with increasing compressive strain above 0.2%. The reversible fractional change in through-thickness resistivity per through-thickness strain is up to 4.0 and decreases with increasing compressive strain. The irreversible fractional change in through-thickness resistivity per unit through-thickness strain is around -1.1 and is independent of the strain. The sensing is feasible by measuring the resistance away from the stressed region, though the effectiveness is less than that at the stressed region. A one-lamina carbon fiber epoxy-matrix composite is an effective attached flexural sensor, with effectiveness comparable to a commercially manufactured self-sensing 24-lamina quasi-isotropic carbon fiber epoxy-matrix composite. In the one-lamina sensor, the arrangement of the fibers is such that adjacent fibers make contact with one another at points along their length, as shown by the substantial conductivity in the transverse direction. The surface resistance of the sensor attached to the tension surface of the beam increases upon flexure, due to decrease in the degree of current penetration within thickness of the sensor. The surface resistance of the sensor attached to the compression surface of the beam decreases upon flexure, due to increase in the degree of current penetration. The sensing effectiveness is superior for the tension surface than the compression surface. Minor/major/catastrophic damage and damage evolution during flexure are indicated by characteristic increases in the surface resistance of the one-lamina sensor; the characteristics are simpler and easier to interpret than those of previously reported 24-lamina quasi-isotropic carbon fiber composites without glass fiber.