Annular flexible eddy current array (A-FECA) sensor for quantitative monitoring of cracks in ferromagnetic steels under varying loads and temperatures

Abstract

In this study, we investigated an annular flexible eddy current array sensor for quantitative monitoring of cracks in ferromagnetic steels under varying loads and temperatures. First, the sensor was designed to monitor the length of cracks in ferromagnetic materials. Using numerical simulations, the effects of permeability, lift-off distance, and conductivity on the response of the sensor were studied. Simulation results showed that when the rate of increase in conductivity and the decrease in permeability were the same, it had the same effect on the trans-impedance response of the sensor. Based on this relation, a new sensor with a reference channel was proposed. At the same time, a new signal, ΔP, for ferromagnetic materials was defined to improve sensitivity to the cracks and to remove the influence of temperature variations. Then, simulated crack-monitoring experiments were carried out to verify the sensitivity of the new sensor. The results showed that the sensitivity of the new sensor improved more than three times that of the original. Moreover, to verify the temperature-compensation capability of the new sensor, a temperature experiment was carried out. When the ambient temperature increased from −20 °C to 70 °C, the ΔP signal remained almost unchanged, which did not affect the monitoring of cracks. Subsequently, quantitative monitoring of cracks in a ferromagnetic steel structure under varying loads was performed. Varying the load was found to change the permeability of ferromagnetic materials, which can result in fluctuations of the output signals. Because of the higher sensitivity of the optimized sensor, the sensor can achieve quantitative monitoring of cracks under various loads. Finally, a crack-monitoring experiment under environmental temperature interference was carried out. The sensor could provide quantitative monitoring of the cracks in ferromagnetic steels under varying environmental temperatures and loads.