Fatigue crack growth in bolted metallic joints is one of the most serious and concerning types of damage in aircraft structures. The problem of how to quantitatively track the size of a crack in order to determine structural integrity has attracted much attention. In this paper, a novel eddy current array sensing film, with one exciting coil covering the entire thickness and several sensing coils distributed along the axial length of the hole, bonded onto the bolt, is proposed to quantitatively monitor a bolt-hole crack in the radial and the axial directions. Finite element simulation is utilized to investigate the eddy current disturbance caused by the crack growth and to optimize the coil configuration of both exciting and sensing coils in order to improve the crack monitoring capability of the sensing film. The simulation results show that the proposed sensing film has very good capability for tracking cracks when the traces of the exciting coil are designed to be similar to the sensing coils' and have opposite current direction at the boundary of two adjacent sensing coils. An experiment is conducted to verify the feasibility and effectiveness of the proposed eddy current array sensing film to quantitatively track hole-edge crack growth in bolted joints.

https://iopscience.iop.org/article/10.1088/1361-665X/aae54e/pdf

A novel eddy current array sensing film for quantitatively monitoring hole-edge crack growth in bolted joints

Array eddy current testing (AECT) sensor is capable of inspecting a large area in a single probe pass resulting in a C-scan image, which is beneficial for customizing the profile of the part being inspected. This paper presents a novel AECT sensor with three-phase excitation. The sensor has two rows of excitation coils and a row of pickup coils. The excitation coils are driven by three-phase currents that are 120° apart in phase. The induced eddy current in the conductive sample shifts electrically along the phase variation direction of the excitation currents. As a result, the sensor does not need multiplexer resulting in advantages of lower cost, less noise and faster inspection speed. The background signal measured by the sensor is small if there is no defect in the sample. Therefore, the sensor has high relative sensitivity to the presence of defect. A 3D finite element method (FEM) model is utilized to study the operating principle of the sensor. A prototype sensor has been developed and tested, with which aluminum sample and carbon fiber reinforced polymer (CFRP) sample with machined defects are inspected. The experiment results demonstrate the feasibility of the sensor for fast imaging of conductive materials.

Novel Array Eddy Current Sensor With Three-Phase Excitation

Bolted joints are the key components for enduring primary load and play a vital role in ensuring structural safety for aircrafts. But it is very difficult to analyze the strength and failure modes ...

A two-dimensional eddy current array–based sensing film for estimating failure modes and tracking damage growth of bolted joints:

An excitation coil layout method for improving the sensitivity of a rosette flexible eddy current array sensor

Quantitative characterization of subsurface defects is a challenging problem in eddy current testing (ECT), not only due to the limitation of the penetration depth of eddy currents, but also due to the ill-posed nature of the inverse problem associated with quantifying a defect's 3D dimensions and its location in the test sample from measurements made on the sample surface. This paper presents a new ECT method for subsurface defect detection and quantification. Eddy currents with continuously varying frequency in a selected frequency band is induced in the conductive sample by using a chirp modulated excitation scheme. The frequency band of the chirp waveform is chosen so that the penetration depth of eddy current in the selected frequency band covers the thickness of the sample under test. The presence of a defect will disturb the distribution of the induced eddy currents. This disturbance is measured in terms of a varying magnetic field using an array of tunneling magneto-resistance (TMR) sensors. A C-scan image is obtained by linearly scanning the specimen. The footprint of the defect is calculated using spatial features of the C-scan image while the depth of the defect is estimated by analyzing the frequency characteristics of the measured chirp signal. A three-dimensional estimate of the dimensions of any subsurface defect can be obtained using the empirical procedure. The operating principle of this method is studied with a finite element method (FEM) model and then validated experimentally with a prototype array probe consisting of 180 TMR sensors. Thanks to its high sensitivity, the TMR sensors are suitable for measuring the weak low frequency magnetic field signal caused by a deep defect. The spacing between each two adjacent sensors is as small as 0.5 mm providing a magnetic field image with fine spatial resolution, which is beneficial for defect quantification. An aluminum sample with embedded defects was tested using the prototype probe. It is seen that the proposed scheme utilizing spatial and frequency domain features is capable of quantitatively characterizing subsurface defects.

Frequency domain analysis of magnetic field images obtained using TMR array sensors for subsurface defect detection and quantification

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.

Yuting He

Papers

A High-Sensitivity Flexible Eddy Current Array Sensor for Crack Monitoring of Welded Structures under Varying Environment.