Eddy current probes

At sufficiently high frequency, the electromagnetic skin depth is several times smaller than a typical defect and induced currents flow in a thin skin at the conductor surface and the crack faces. It is profitable to develop a theoretical model dedicated to this regime. Making certain assumptions, a boundary value problem can be defined and solved relatively simply leading to rapid numerical calculation of eddy-current probe impedance changes due to a variety of surface cracks. 

Fig. 1 Shematic of eddy current probe for corrosion detection.

SQID Eddy Current Technique Applying Conformable Eddy Current Probes. 

Lew frequency Eddy current probing For frequencies below some 100 Hz the SQUID is coupled with a completely superconducting flux antenna. This antenna has to be within the cryogenic vessel. The Eddy current excitation is done in a conventional way. But care must be taken, that interference between the excitation field and the flux anteima and SQUID is... 

We realized an Eddy current SQUID system of the high frequency type a room temperature Eddy current probe is connected to a SQUID sensor at hquid nitrogen temperature. Fig.3 gives an overview over the components of the system, fig, 5 shows a schematic diagram of the electronics. 

Several types of Eddy current probes were used with the SQUID system and the commercial system as well. High inductance wire wound probes with a ferritie eore and low induetance planar thick frhn coils were applied. The wire wound probe is the commonly used probe for high resolution conventional testing. The low inductance planar cod is more suited to be apphed in combination with the SQUID system. It is well adapted for surfaee defects and shallow defects. 

As stated above the SQUID amplifier demands a low inductance Eddy current probe in order to be able to amplify signals up to 1 MHz. Low inductance Eddy current probes can be obtained by reducing the number of turns and by loosing the magnetic coupling between the tums. So magnetic cores should be avoided as well as tight wounded tums. For this purpose planar coils are the best... 

This type of coil was prepared from copper cladded printed circuit board material by applying photolithographic techniques. The p.c. board material is available with difierent copper thicknesses and with either a stiff or a flexible carrier. The flexible material offers the opportunity to adapt the planar coil to a curved three dimensional test object. In our turbine blade application this is a major advantage. The thickness of the copper layer was chosen to be 17 pm The period of the coil was 100 pm The coils were patterned by wet etching, A major advantage of this approach is the parallel processing with narrow tolerances, resulting in many identical Eddy current probes. An example of such a probe is shown in fig. 10. 

New Aspects for Remote Field Eddy Current Probe Development. 

The development of Remote Field Eddy Current probes requires experience and expensive experiments. The numerical simulation of electromagnetic fields can be used not only for a better understanding of the Remote Field effect but also for the probe lay out. Geometrical parameters of the prohe can be derived from calculation results as well as inspection parameters. An important requirement for a realistic prediction of the probe performance is the consideration of material properties of the tube for which the probe is designed. The experimental determination of magnetization curves is necessary and can be satisfactory done with a simple experimental setup. 

P.Y. lotibert, D. Miller, D. Placko, and E. Savin. New eddy current probe and associated preprocessings for 3d image reconstruction in non ferromagnetic tubes testing. In To be published in the proceedings of the Seventh ECNDT, 1998. 

New Eddy Current Probe and Associated Preprocessing for 3D Image Reconstruction in Non Ferromagnetic Tube Testing. 

Progress in mean of modelisation and inverse problem solving [1] let us hope to dispose soon of these tools for flaws 3D imaging in Non Destructive Control with eddy current sensors. This will achieve a real improvement of the actual methods, mainly based upon signature analysis. But the actual eddy current probes used for steam generators tubes inspection in nuclear industry do not produce the adequate measurements and/or are not modelisable. 

Therefore we present here a new eddy current probe designed with imaging objectives. The elements of this probe have been choosen after a relevant analysis of electromagnetic interactions. 

The second method applies eddy-current probes in a new measurement method. The principle circuit of this method is presented in fig. 2. 

Fig. 1 The view of the polar display with monitors arcs and control panel for SFT6000N board parameters. Recorded signal is from the eddy current probe moved along in a brass tube of inner diameter 20 mm with 2 mm holes as artificial flaws. SFT6000N card operates with 40 kHz injection voltage firequency.

Displacement, or eddy-current, probes are designed to measure the actual movement, or displacement, of a machine s shaft relative to the probe. Data are normally recorded as peak-to-peak in mils, or thousandths of an inch. This value represents the maximum deflection or displacement from the true centerline of a machine s shaft. Such a device must be rigidly mounted to a stationary stmcture to obtain accurate, repeatable data. See Figure 43.21 for an illustration of a displacement probe and signal conditioning system. 

Displacement or eddy-current probes are designed to measure the actual movement, i.e. displacement, of a machine s shaft relative to the probe. Therefore, the displacement probe must be rigidly mounted to a stationary stmcture to gain accurate, repeatable data. 

Electrical output transducers exist in many forms and consist simply of elastic elements coupled with suitable displacement transducers such as capacitance, inductance and reluctance pick-ups, certain types of strain gauge, piezoelectric elements, potentiometers and eddy current probes. A selection of these is described in the following paragraphs. 

Eddy current (EC) Inspection of carbon fibre-reinforced structures bases on their anisotropic electrical properties. Great differences in conductivity between carbon fibres, polymer matrix and integrated functional components contribute to this circumstance. The paper describes suitable eddy current probes, the fundamental idea of modelling and measurement of EC distribution and provides a short survey of application potential to characterise CFRP non-destructively. 

Non-centric eddy current probe for CFRP inspection... 

Eddy currents have also been u,scd to measure the volume fraction of titanium alloy reinforced with silicon carbide fibers [124], Beissner studied the effect of abnormal micro-structure on eddy current probe response, a change in response occurring due to the movement of fibers within rows leading to changes in volume fraction. 

Equation 6 was used to calculate the sensitivity of a coil with the diameter of 11 mm and an wire diameter of 0.1 mm. The 10 turns were arranged a 2 x 5 wiring scheme in figure 1. The coil diameter is a medium diameter for eddy current distance sensors. The number of turns is chosen to provide a high inductance and resonance frequency above 10 MHz, similar to other eddy current probe experiments [7]. The wire diameter and the wiring scheme provide an compact coil, which can be manufactured using self bonding wire. 

C. Dodd and W. Deeds, Analytical solutions to eddy-current probe-coil problems, Journal of applied physics, vol. 39, no. 6, pp. 2829-2838, 1968. 

When an electromagnetic field is introduced into an electrical conductor, eddy currents flow in the material and any variations in material conductivity due to cracks, voids, or thickness changes can alter the path of the eddy current. Probes are used to detect the current movement and thus describe the flaws. 

In 1979 a number of surface marks were noted in the plates adjacent to seam welds on Calder Reactor 4. Two of the features had the appearance of crack-like defects (Figure 4 but more detailed investigations using a hydraulic manipulator with television cameras, eddy current probes, ultrasonic probes and a crack depth gauge, revealed them to be surface imperfections and fabrication flaws. 

Auld, B., F. Muennemann, and D. Winslow (1981). Eddy current probe response to open and closed surface flaws. Journal of Nondestructive Evaluation 2(1), 1-21. 

The eddy-current probe is placed in contact with the test object. 

Eddy-current methods are used to measure a variety of material characteristics and conditions. They are applied in the flaw detection mode for the detection of surface-connected or near-surface anomalies. The test objects must be electrically conductive and be capable of uniform contact by an eddy-current probe. Special equipment and specialized probes are required to perform the inspection. Procedure development, calibration artifacts, and process control are required to assure reproducibility of response in the selected test object. 

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