Self-sensing refers to structural material sensing by auxiliary devices without intelligent features. The analysis of the electrical parameters of a single carbon fiber is the foundation of CFRP self-sensing. Focusing on electrical-resistance-based strain, this study conducts a theoretical analysis of the electrical parameters of a single carbon fiber. The relationship between stress-induced strain and resistance is established, yielding the gage factor (GF) under the load effect. Drawing upon the impurity scattering mechanism, the relationship between thermal-induced strain and resistance is formulated, leading to the GF under thermal effects. According to the quasi-static equivalent superposition principle, strain vs. resistance in the effect of thermal–mechanical coupling was established, and a GF model is proposed. The analysis of a single carbon fiber demonstrates that under load effect the contribution of the piezoresistive effect reaches 13.4%, which is non-negligible. Thermal-resistance tests were conducted on a single carbon fiber with different initial states. The thermal-resistance analysis indicated that the resistance of a single carbon fiber decreased with an increase in temperature. The initial state had a significant impact on the GF. The thermal resistance of a free single carbon fiber can be expressed by two types of models, each with an error of less than 0.2% from 223 K to 473 K. Based on four-point bending specimens, the force-resistance test of a single carbon fiber was conducted indirectly. The improvement in the production process has led to an increase in the graphitization degree of carbon fibers. The KSF values of A3 and B3 are 1.411 and 1.405, respectively, both of which are higher than those of carbon fibers in the earlier literature. The strain-resistance analysis showed that the stress-induced GF of a single carbon fiber is lower than the thermal-induced GF. When the deformation was constrained, the stress-induced GF of the single carbon fiber was reduced. Together, the thermal and mechanical properties of a single carbon fiber make it more suitable as a temperature sensor than as a damage sensor.
Li et al. (Thu,) studied this question.