A robust feature in multi-frequency eddy current (MEC) testing has been found that can be directly linked to the thickness of the plate under test. It is shown mathematically that the peak frequency of the imaginary part of the inductance change when an air-cored coil is placed next to a non-magnetic metallic plate is inversely proportional to the thickness of the plate for a given material. Experimental results indicate that this relationship also holds for a ferrite-cored U-shaped coil. In addition, this peak frequency has been shown to be relatively independent of lift-off variations. Use of this new feature provides a fast and accurate method to gauge the thickness of plates. Measurements made for a sample air-cored and ferrite U-cored coil next to copper and aluminium plates of various thicknesses verified the proposed method.

Thickness measurement of non-magnetic plates using multi-frequency eddy current sensors

The characterization of carbon-fiber-reinforced plastics (CFRPs) using multifrequency eddy current sensors is presented in this paper. Three sensors are designed for bulk conductivity measurements, directionality characterization, and fault detection and imaging of unidirection, cross-ply, and impact-damaged CFRP samples. Analytical and finite-element (FE) models describing the interaction of the sensors with the CFRP plate samples are developed to provide an explanation of, and physical insights into, the measured results and observed phenomena. A signal processing method is developed to compensate for the variation in lift-off during the measurements.

Noncontact Characterization of Carbon-Fiber-Reinforced Plastics Using Multifrequency Eddy Current Sensors

This paper presents an analytical model that describes the inductance change when a double air-cored coil sensor is placed next to a conducting plate. Analysis of the analytical model reveals that the phase signature of such a sensor is virtually liftoff independent. This finding is verified by numerical evaluations. This paper also finds that the phase signature of a ferrite U-cored sensor can be approximated by that of a double air-cored sensor of similar size and, therefore, possesses a similar liftoff-independent property. Measurements made with a sample U-cored sensor next to plates of nonmagnetic and magnetic materials verified the theoretical results.

Analysis of the Liftoff Effect of Phase Spectra for Eddy Current Sensors

This paper presents the measurement of ferrite/austenite phase fraction using a multi-frequency electromagnetic sensor. A simple analytical model was established that can describe the response of the sensor for samples containing varying fractions of ferromagnetic phase over a wide range of frequencies (100 Hz–1 MHz). In particular, a new feature, the peak frequency of the imaginary part of the inductance, is found to be able to distinguish between samples across the whole range of the ferrite percentages. FEM models were used to simulate representative real microstructures from the samples and to relate the relative permeability to the ferrite fraction. Experimental results suggest that the accuracy of ferrite/austenite percentage measurement is within 8%.

Measurement of permeability and ferrite/austenite phase fraction using a multi-frequency electromagnetic sensor

Ensuring the correct fiber orientation in draped textiles and 3D preforms is one of the current challenges in the production of carbon-fiber reinforced plastics (CFRP), especially in resin transfer molding (RTM). Small deviations in fiber angle during preforming have a considerable effect on the mechanical properties of the final composite. Therefore, this paper presents an automated method for determining local yarn orientation in three-dimensionally draped, multi-layered fabrics. The draped fabric is scanned with a robot-guided high-frequency eddy current sensor to obtain an image of the sample's local conductivity and permittivity. From this image, the fiber orientation not only of the upper, but also of the lower, optically non-visible layers can be analyzed. A 2D Fast Fourier Transform is applied to local segments of the eddy current image to determine the local yarn orientation. Guidelines for processing the eddy current data, including phase rotation, filtering and evaluation segment size, are derived. For an intuitive visualization and analysis of the determined yarn orientation, reference yarn paths are reconstructed from the determined yarn angles. The developed process can be applied to quality inspection, process development and the validation of forming simulation results.

Automated detection of yarn orientation in 3D-draped carbon fiber fabrics and preforms from eddy current data

http://pure-oai.bham.ac.uk/ws/files/16481023/Liu_Measurement_magnetic_properties_JMMM_2014.pdf

Measurement of the magnetic properties of P9 and T22 steel taken from service in power station

An electromagnetic (EM) sensor, capable of detecting the formation of ferromagnetic ferrite from paramagnetic austenite below the Curie temperature, has been developed and assessed. The long term aim of this work is to develop a method for monitoring microstructure online during strip steel processing. In the present paper, the initial results of variation in trans-impedance with microstructure obtained for three different types of steel with varying carbon contents are discussed. It was found that the EM sensor can successfully detect the formation of ferrite below the Curie temperature, but trans-impedance values are affected by the presence of a decarburised ferrite ring that forms around the specimens tested in a furnace. It was also found that the trans-impedance value is monotonically (non-linearly) related to ferrite volume fraction, and depends on the morphology and distribution of the ferromagnetic phase and, hence, is influenced by the prior austenite grain size.

Measurement of phase transformation in steels using electromagnetic sensors

Measurement of water level and conductivity is important in many applications, including geophysics, ocean engineering, and the oil industry, one example being gas/oil/water separation. Non-contact measurement is ideal for those applications. Capacitance, radar, ultrasonic and laser techniques have been used in measuring level, but simultaneous measurement of level and conductivity has not been reported. In this paper, we developed a novel inductive sensor, which uses two coils of different size to infer conductivity and level simultaneously based on a simplified model deduced from an analytical solution. The simplified model is verified with both simulation and experimental data. The experimental results have shown that measurement accuracy for both level and conductivity are within 3%.

Simultaneous Non-contact Measurement of Water Level and Conductivity

This paper describes the development of an instrument to analyze the phase transformations of hot strip steel using an electromagnetic sensor. The sensor exploits variations in the electrical conductivity and magnetic permeability of the steel to monitor microstructure evolution during processing. The sensor is an inductive device based on a H-shaped ferrite core, which is interrogated with a multi-frequency impedance analyzer (MFIA) containing a digital signal processor (DSP). An overview of the instrument and measurements from a range of carbon steels sample are presented. The results verify the ability of the instrument both to monitor the microstructural changes and to reject variations in liftoff distance between the sensor and the hot strip

/pdf/the-development-of-a-multi-frequency-electromagnetic-qazy9tebgs.pdf

The Development of a Multi-frequency Electromagnetic Instrument for Monitoring the Phase Transformation of Hot Strip Steel

https://research.birmingham.ac.uk/portal/files/16480992/Liu_Differential_permeability_behaviour_JMMM_2014.pdf

Differential permeability behaviour of P9 and T22 power station Steels

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