In-situ health monitoring of CFRP composites using electrical characteristics

The increase in electrical resistance of Carbon Fiber Reinforced Plastic (CFRP) composites caused by mechanical damage are studied experimentally and analytically. Considering the contacts of misaligned conducting carbon fibers of CFRP, we proposed a discrete network model as an equivalent electrical circuit for CFRP. This model incorporates both an electrical ineffective length, over which broken fibers carry no current, and the Weibull-Poisson statistics of fiber breakage. The model is also used to explain experimental data showing that the resistance change does not depend on the gage length of specimen and that the resistance and electrical ineffective length are controlled by the fiber volume fraction. To establish a correlation between the derived electrical ineffective length and actual physical fiber contacts, we apply percolation theory of a two dimensional composite with continuous slightly misaligned overlapping fibers to demonstrate that the electrical ineffective length can depend on the fiber volume fraction in a manner consistent with that derived from analysis of the experimental data. Through these experimental and analytical study, the physical meaning of the proposed model was solidified. It was also shown that the resistance change of CFRP under tensile and cyclic loading, especially the useful function of memorizing the applied maximum strain, can be predicted with reasonable accuracy by a simple analytic equation.