Capacitive self tension sensing properties of steel beam: Electrode configuration and stress regime

Abstract

A structural material is called self-sensing when it is able to reveal its own state without the need for any external sensors. Self-sensing for steel is valuable for the control and safety of steel structures such as buildings and bridges. The changes in the capacitance of steel in response to cyclic tensile stresses applied in low, medium and high stress regimes are measured by using a Inductnce-Capacitance-Resistance (LCR) meter. Coplanar and parallel plate electrode configuration is used for capacitance measurements. Aluminum foil is used as electrode. A steel beam of 100 mm in length, 30 mm in width and 2.5 mm in thickness is tensioned by holding it at both ends to produce direct tensile stresses in the material. The maximum stresses applied for low, medium and high stress regimes are 6.7 MPa, 33.3 MPa and 66.7 MPa. The capacitance value of the sample with coplanar and parallel plate electrode configurations measured without applying load are 203.42 pF and 196.00 pf, respectively. The fractional changes in capacitance are 0.059%, 0.192% and 0.275% when 6.7 MPa, 33.3 MPa and 66.7 MPa direct tensile stress is created in the steel beam. These values are 0.12%, 0.20% and 0.29% for parallel plate electrode configuration. Test results demonstrates that there is a relation between stress and fractional change in capacitance. In other words, measuring fractional change in capacitance gives information about the stress variations in the material. From the experimental results, parallel plate electrode configuration is found to be more effective in tensile stress self-sensing. In addition, the relationship between stress and fractional change in capacitance is more linear for both electrode configurations in the low stress regime. This paper aims to reveal the tensile stresses occurring in steel by means of capacitance-based sensing. Sensing capability in larger scale structures and factors effecting sensing sensitivity are to be addressed in future work.