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• A novel FECA sensor with polyimide (PI) reinforcing plate was proposed for the monitoring of fatigue cracks at the invisible part of the bolt hole edge. • Both conventional FECA sensors and FECA sensors with PI reinforcing plates can maintain normal operation under the static extrusion action of 5 N.m to 63 N.m bolt tightening torque. • The FECA sensor with PI reinforcing plate has the same monitoring accuracy as the conventional FECA sensor, both with a resolution of 1 mm and a monitoring error within 0.1 mm. • The conventional FECA sensor can maintain normal operation under the action of the bolt tightening torque of 5 N.m and quantitatively monitor the propagation of fatigue cracks. However, when the bolt tightening torque exceeds 5 N.m, the sensor will fail due to the damage of the cover layer or the excitation coil during the fatigue experiment. • The FECA sensor with PI reinforcing plate can effectively resist the effects of bolt compression and friction, enabling quantitative monitoring of fatigue cracks under the bolt tightening torque of 45 N.m and 63 N.m. This paper proposes a flexible eddy current array (FECA) sensor with polyimide (PI) reinforcing plate based on conventional co-direction excitation layout sensors, for quantitative monitoring of cracks in the invisible part under the bolt gasket. Firstly, the stress distribution of conventional FECA sensors and FECA sensors with PI reinforcing plate under different tightening torques was analyzed by finite element analysis. Then, the monitoring accuracy of two different types of sensors was verified by fatigue crack accuracy monitoring verification experiments. Finally, fatigue experiments were conducted to compare and analyze the crack monitoring effect of the two different types of sensors under different bolt tightening torques. The results show that FECA sensors with PI reinforcing plate can quantitatively monitor the crack under the bolt tightening torque of 45 N.m and 63 N.m. The research results are helpful for the engineering application of FECA sensors, and also has certain reference value for other types of sensors. [ABSTRACT FROM AUTHOR]

Sensors for structural health monitoring (SHM) need to be permanently integrated on structures and withstand the harsh service environments, which has been a big challenge for the application of SHM in aircrafts. This paper focuses on the durability of flexible eddy current array (FECA) sensors in harsh service environments of aircrafts, including vibration environment and several typical exposed environments. First, a kind of FECA sensor is illustrated and its integration method is proposed. Moreover, in order to study the durability of the sensor in vibration environment, the modal analysis is performed by the finite element method. According to the simulation results, the durability experiment in vibration environment is carried out under the fourth order vibration mode, which makes the sensor suffer the harshest vibration loads. During the vibration experiment, output signals of the sensor keep stable and the sensor is well bonded to the structure, which shows the integrated sensor has high durability in vibration environment. Finally, the durability of integrated sensors is separately tested in three exposed environments, including salt fog corrosion environment, fluid immersion environment, as well as hygrothermal and ultraviolet-radiation environment. After these environmental exposure experiments, all sensors are well bonded to structures and can effectively monitor fatigue cracks, which shows great durability. Therefore, FECA sensors can survive in harsh service environments of aircrafts, which provides important support for the engineering applications of FECA sensors. [ABSTRACT FROM AUTHOR]