This paper develops a high-sensitivity flexible eddy current array (HS-FECA) sensor for crack monitoring of welded structures under varying environment. Firstly, effects of stress, temperature and crack on output signals of the traditional flexible eddy current array (FECA) sensor were investigated by experiments that show both stress and temperature have great influences on the crack monitoring performance of the sensor. A 3-D finite element model was established using Comsol AC/DC module to analyze the perturbation effects of crack on eddy currents and output signals of the sensor, which showed perturbation effect of cracks on eddy currents is reduced by the current loop when crack propagates. Then, the HS-FECA sensor was proposed to boost the sensitivity to cracks. Simulation results show that perturbation effect of cracks on eddy currents excited by the HS-FECA sensor gradually grows stronger when the crack propagates, resulting in much higher sensitivity to cracks. Experimental result further shows that the sensitivity of the new sensor is at least 19 times that of the original one. In addition, both stress and temperature variations have little effect on signals of the new sensor.

Flexible eddy current array sensor has the characteristics of flexibility, light weight and convenient installation, and has great application prospects in metal structure crack monitoring. In allusion to the low sensitivity of traditional rosette flexible eddy current array sensor to crack identification, a co-directional excitation coil layout method is proposed, which significantly improves the sensitivity of sensor to crack identification . Firstly, by establishing the finite element model, the sensitivity of three sensing coils of the traditional sensor to crack identification is calculated to be 4.1%, 1.7% and 3.5%, respectively. The conclusion that the disturbed backflow between the excitation coils affects the sensitivity of each sensing coil to crack identification is obtained. In order to improve the sensitivity of the sensor, a new type of sensor using the co-directional excitation layout is proposed by changing the layout of the excitation coil. The simulation results show that the sensitivity of three sensing coils to crack identification is increased by 11.2, 43.1 and 36.0 times, respectively. Then, the sensitivity tests of the two sensors are carried out. The experimental results show that the sensitivity of three sensing coils of the sensor adopted co-directional excitation layout increases sequentially, which is 8.9, 23.7 and 13.3 times higher than the traditional sensor, indicating that the sensor adopted co-directional excitation layout is more sensitive to crack identification. Finally, the error analysis shows that the difference between the sensitivity of experimental results and simulation results is caused by the difference of lift-off. The research results privide an important way to improve the sensitivity of crack identification.

In order to meet the need of the quantitative monitoring of cracks in critical connection structures of aircraft, aiming at improving the durability of sensors and the reliability of signals, a structural crack monitoring (SCM) gasket, which can be used for the quantitative monitoring of cracks in bolted joints, is developed. The gasket is formed by a gasket substrate and a flexible eddy current sensor integrated to withstand the load, with the capability of temperature compensation. First of all, a new temperature-compensated rosette eddy current array (TC-RECA) sensor is designed and used to study the failure mode of the sensor. According to the failure mode of the sensor, a gasket substrate with load-bearing capacity is developed. Subsequently, the influence of permeability, conductivity and lift-off on the trans-impedance of the sensor is studied. The influence of permeability on the trans-impedance is characterized by conductivity. Based on this, a temperature compensation method for ferromagnetic materials is developed. Then, a temperature influence experiment and a structural crack quantitative monitoring experiment are carried out using a front beam connection structure of an aircraft wing. The temperature effect experimental results show that even if the ambient temperature changes from 60 °C to −40 °C, the characteristic signal ΔC does not change significantly, indicating that the gasket has a temperature compensation capability. The experimental results of the crack monitoring of the front beam connection structure show that the gasket can bear the load and has the capability to quantitatively monitor cracks, and can be used for the quantitative monitoring of cracks in the critical connection structure of aircraft structures.

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On-line quantitative monitoring of fatigue crack sensors is an important part of structural health monitoring systems. A rectangular grid-type PVD (physical vapor deposition) thin film sensor based on the potentiometric method is designed in this paper. First, the sensor finite element model is established through the AC/DC module of COMSOL Multiphysics to verify the feasibility of the sensor to monitor cracks. Second, the effect of the number of sensing channels on the sensitivity of the sensor is studied. Finally, the structural parameters of the sensor are optimized. Significantly improved sensitivity for crack detection. The results show that the rectangular grid PVD film sensor can effectively monitor structural cracks.