The eddy current probe, which is flexible, array typed, highly sensitive and capable of quantitative inspection is one practical requirement in nondestructive testing and also a research hotspot. A novel flexible planar eddy current sensor array for the inspection of microcrack presentation in critical parts of airplanes is developed in this paper. Both exciting and sensing coils are etched on polyimide films using a flexible printed circuit board technique, thus conforming the sensor to complex geometric structures. In order to serve the needs of condition-based maintenance (CBM), the proposed sensor array is comprised of 64 elements. Its spatial resolution is only 0.8 mm, and it is not only sensitive to shallow microcracks, but also capable of sizing the length of fatigue cracks. The details and advantages of our sensor design are introduced. The working principal and the crack responses are analyzed by finite element simulation, with which a crack length sizing algorithm is proposed. Experiments based on standard specimens are implemented to verify the validity of our simulation and the efficiency of the crack length sizing algorithm. Experimental results show that the sensor array is sensitive to microcracks, and is capable of crack length sizing with an accuracy within ±0.2 mm.

Aiming at the problem of low resolution of traditional eddy current sensor for buried defects detection in metal materials, a null-offset planar eddy current sensor array is designed, realized and tested. The arrayed sensor is composed of a spatially periodic excitation coil and multiple differential detection coils, which are skillfully designed and can effectively suppress the common mode component in the induced voltage and improve the signal-noise-ratio (SNR) of sensor. The null-offset eddy current sensor is fabricated and utilized to detect the buried defects 0.5 mm depth beneath the surface of aluminum alloy test pieces. Simulation and experimental results show that the null-offset eddy current detection unit improves the detection resolution of buried defects by more than 1000 times.

Thermal barrier coatings (TBCs) can significantly reduce the operating temperature of the aeroengine turbine blade substrate, and their testing technology is very urgently demanded. Due to their complex multi-layer structure, it is hard to evaluate TBCs with a single function sensor. In this paper, an electromagnetic/capacitive composite sensor is proposed for the testing of thermal barrier coatings. The dielectric material is tested with planar capacitor, and the metallic material is tested with electromagnetic coils. Then, the comprehensive test and evaluation of thermal barrier coating system can be realized. The sensor is optimized by means of theoretical and simulation analysis, and the interaction between the planar capacitor and the electromagnetic coil is studied. The experimental system is built based on an impedance analyser and multiplex unit to evaluate the performance of the composite sensor. The transimpedances and capacitances are measured under different coating parameters, such as thickness and permittivity of top coating as well as bond layer conductivity. The experimental results agree with the simulation analysis, and the feasibility of the sensor is proved.

1/f noise is one of the main noise sources of magnetoresistance (MR) field sensors, which can cause intrinsic detection limit at low frequency. To suppress this noise, a novel magnetic flux vertical motion modulation (MFVMM) design using micro-electromechanical system flux concentrator (FC) and actuator is proposed in this paper, which can mitigate the 1/f noise by shifting the measurement frequency to kilohertz range. The proposed MFVMM has high efficiency and simple structure. The magnetic FC and the modulation film of MFVMM were analyzed with a numerical simulation. The simulation results show that the modulation efficiency of MFVMM exceeds 21%. A preliminary prototype sensor based on the multilayered giant MR AA002 and MFVMM was fabricated, and its modulation efficiency achieves 18.8% at a frequency of 7.01 kHz, which exceeds the efficiency of most existing sensors with the other modulation schemes. In addition, the magnetostatic detection ability is improved to 0.44 nT/√Hz.

1/f noise is one of the main noise sources of magnetoresistive (MR) sensors, which can cause intrinsic detection limit at low frequency. To suppress this noise, the solution of flux concentration and vertical motion modulation (VMM) has been proposed. Magnetic hysteresis in MR sensors is another problem, which degrades their response linearity and detection ability. To reduce this impact, the method of pulse magnetization and magnetic compensation field with integrated planar coils has been introduced. A flux concentration and VMM based magnetoresistive prototype sensor with integrated planar coils was fabricated using microelectromechanical-system technology. The response linearity of the prototype sensors is improved from 0.8% to 0.12%. The noise level is reduced near to the thermal noise level, and the low-frequency detection ability of the prototype sensor is enhanced with a factor of more than 80.