Correlation of Impedance and Compressive Stress of Carbon Nanofiber Aggregates for Structural Health Monitoring

A carbon nanofiber aggregate (CNFA) contains carbon nanofibers (CNFs) as an admixture of selfconsolidating mortar and produces an electrical response under stress. CNFA is a sensor that notifies the health status and provides a signal for the structural health monitoring system. It is necessary to understand the working principle and behavior of a CNFA and observe if CNFAs show consistency under various stress. In this research, three sets of test methods were tested under different loading methods to investigate the electrical impedance of CNFA under compressive stress, including the sweep-frequency test, the response from load removal, and the response from a car wheel. The behavior of CNFAs and their consistency under various loading conditions relies on the fact that they are conductive under stress. First, the sweep-frequency test compares the electrical impedance for various frequencies ranging from 1 to 500 kHz when the CNFA is scanned without the load and with a constant load. After reaching the targeted load, it is kept constant. Second, the impedance of CNFA is recorded for a constant load and a selected frequency. The electrical response of the CNFA under constant load is compared with the electrical response without any load. Third, the car wheel provides the load to the CNFA. The three methods demonstrated a positive correlation between impedance and compressive stress. The variables considered in the tests are various concentrations of CNFs and the age of samples. Irrespective of these variables, the test results from three methods show consistency in the behavior of CNFA. Therefore, the correlation between the electrical impedance and the stress is validated based on the experimental study.