Eddy currents develop

The first system called LiSSA has been developed for interpretation of data from eddy-current inspection of heat exchangers. The data that has to be interpreted consists of a complex impedance signal which can be absolute and/or differential and may be acquired in several frequencies. The interpretation of data is done on the basis of the plot of the signal in the impedance plane the type of defect and/or construction is inferred from the signal shape, the depth from the phase, and the volume is roughly proportional to the signal amplitude. 

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. 

The simulation of the actual distortion of the eddy current flow caused by a crack turns out to be too time consuming with present means. We therefore have developed a simple model for calculating the optimum excitation frequencies for cracks in different depths of arbitrary test sarriples Using Equ. (2.5), we are able to calculate the decrease in eddy current density with increasing depth in the conductor for a given excitation method, taking into account the dependence of the penetration depth c on coil geometry and excitation frequency. 

Exciting developments based on electromagnetic induction raced along from that time, giving us the sophisticated products our everyday lives depend on. During most of the period productive uses for eddy current technology were few and few people believed in it as a usefiil tool eddy currents caused power loss in electrical circuits and, due to the skin effect, currents flowed only in the outer surfaces of conductors when the user had paid for all the copper in the cable. The speedometer and the familiar household power meter are examples of everyday uses that we may tend to forget about. The brakes on some models of exercise bicycle are based on the same principle. 

The war itself also drove the development of improved and miniaturised electronic components for creating oscillators and amplifiers and, ultimately, semiconductors, which made practical the electronic systems needed in portable eddy current test instruments. The refinement of those systems continues to the present day and advances continue to be triggered by performance improvements of components and systems. In the same way that today s pocket calculator outperforms the large, hot room full of intercormected thermionic valves that I first saw in the 50 s, so it is with eddy current instrumentation. Today s handheld eddy current inspection instrument is a powerful tool which has the capability needed in a crack detector, corrosion detector, metal sorter, conductivity meter, coating thickness meter and so on. 

Technology has converged and seems likely to continue to do so, particularly with the parallel developments in the personal computer. The P.C. today has the power needed to analyse and interpret the often confusing responses obtained in eddy current tests, in real time as the tests are being made. The responses we get from a testpiece come, as we all know, equally from features we are not interested in as they do from significant features. 

Very many types of eddy current instruments available tend to look remarkably similar in much the same way as automobiles tend to look alike. This may be an inevitable trend since developments are converging on users needs, plus PC platforms are being used more and these too are tending to look more and more alike. In the way that the DSP has changed the inside of the instrument, the PC. is changing the outside. 

The new generation of eddy current instruments mainly uses digital technology. When developing a new instrument, it is therefore necessary to get the most out of irmovation. 

The developed method is used in eddy current defectoscopes like Zond VD used for detecting corrosion spots in the body of the plane through aluminium cover, cracks detecting in helicopter blades under dielectric covers up to 8-10 mm thick, in pipelines under protective covers up to 10 mm thick, etc... 

The application of eddy currents in non destructive testing was very developed during the recent years. Adding to the defects characterization, actual studies deal with the metallurgical evaluation of materials. Surface processing allow to increase the material endurance and consequently its life duration. 

Abstract An Eddy current method applying a High Temperature Superconductor ( HTS ) DC SQUID sensor operating at Uquid nitrogen temperature (77K) is presented. The method is developed for the detection of surface or surface near defects. We compare the performance of the SQUID system with the performance gained from a commercial Eddy current system, while using identical probes. The experimental data are obtained on defects in gas turbine blades. The advantage of planar conformable probes for the use with the SQUID is discussed. 

The Continued Development of Eddy-Current Imaging Scanner Systems Illustrated by Tests on Aircraft Wheels - From Manual Testing to Image-Processing. 

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. 

Remote Field Eddy Current (RFEC) technology is a variation of the conventional eddy-current method, developed for detecting flaws at any point in the walls of (particularly) ferromagnetic (Fe) tubes and pipes from the inside diameter. 

The Zond VD - 96 portable eddy-current flaw detector-tester is an original Russian development possessing heightened sensitivity for the surface defects and high inspection capacity. (Russia patent Xs 2063025. All-Union state standard certificate of Russian Federation JVa 2846 of 14. 07. 97)... 

A physical theory about the effect of the eddy current on the impedance of the solenoid can be established by a simple way when we develop the fundamentals principles... 

The results of measurements and comparisons of the different magnetic field are presented and discussed. Further development will be shown, that this theory can be applied to solve inverse eddy-current problems as well. 

The equation system of eq.(6) can be used to find the input signal (for example a crack) corresponding to a measured output and a known impulse response of a system as well. This way gives a possibility to solve different inverse problems of the non-destructive eddy-current testing. Further developments will be shown the solving of eq.(6) by special numerical operations, like Gauss-Seidel-Method [4]. 

This work presents the theoretieal results and their experimental verifications concerning two possible methods for predicting the material discontinuities shape and severity. The methods are developed for the case of the eddy current transducer with orthogonal coils, for two situations for long crack-tjfpe discontinuities, a metod based on the geometrical diffraction has been used, while in the ease of short discontinuities the holographic method is prefered. 

The methods have been developed for an eddy current transducer with orthogonal coils... 

The first method, based on the geometric theory of diffiaction, can be used in the case of cracks, while the second - the holographic one, can be used for small discontinuities. The both methods have been developed for the case of an eddy current transducer with orthogonal coils. 

The 16-bit and 32-bit software for eddy current testing using PC measurement cards manufactured by SOFRATEST, France, is presented in this article. Microsoft Visual C/C++ Development System and Tools have been used. 

One of the techniques capable of being used "on stream" is the novel INCOTEST technique, see figure 2, which measures wall thickness of piping and vessels through an insulation layer with a maximum thickness of 10 centimetres. This system is based on pulsed eddy currents, was first developed in the United States, and is now being commercialised and extensively validated for a number of applications. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Eddy currents The examination of non-ferrous tubing using external coils is a well-tried and successful inspection technique, owing mainly to the pioneering work of Forster in Germany. The adoption of this method for in-situ inspection of condenser tubes, by mounting eddy-current coils in probes (or bobbins) that can be inserted in condenser tubes, was a logical development of the technique. Suitable apparatus was developed in the immediate post-war period more or less independently by several oil and chemical companies. The principle of operation has been described in the literature... 

Nondestructive testing (NDT) is used to assess a component or structure during its operational lifetime. Radiography, ultrasonics, eddy currents, acoustic emissions, and other methods are used to detect and monitor flaws that develop during operation (Chapter 7). 

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