Eddy-current tests

B. A. Auld, F. G. Muennemann and M. Riaziat, Quantitative modelling of flaw responses in eddy current testing. In R. S. Sharpe (ed.), Research techniques in nondestructive testing, Vol. 7. Academic, London, 1984. 

In this paper we present simulations and measurements of several types of excitation coils, which match the special requirements for a SQUID based eddy current NDE system. We note however that all calculations presented here on penetration depths, current distributions and crack-detecting algorithms are also useful for conventional eddy current testing systems. 

The next phase which resulted in the worldwide acceptance of eddy current technology for testing metals was the work of Dr Friedrich Foerster. Dr Foerster, a modem Giant, has rightly been called the father of modern eddy current testing (Ref 5). His early work was driven by the priorities of the Second World War, after which he embarked upon major research and... 

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. 

A method has been worked out for eddy current testing of surfaces and surface cracks or corrosion under dielectric or non-magnetic metal layer of up to 10 mm. The method is based on excitation of eddy currents by a coil with U - type core and information reading by a sensitive gradientometric element located on a axis of symmetry of the core (fig. 1). 

So in order to improve selective characteristics of eddy current testing one should minimize phase change under interference factors influence. Analysis of the above characteristics has indicated that in case of interacting under-surface defects, there is an optimal frequency providing the best sensitivity to defect in amplitude. 

The eddy currents testing probe realization was satisfactory at the sight of the control and the testing results of coating samples. The fundamental parameters to keep in mind for a probe construction are ... 

DOD (C.V.), CHENG (C.C.), SIMPSON (W.A.), DEEDA (D.A.), SMITH (J.H.), The analysis of reflection type coils for eddy current testing, DAK Ridge Normal Laboratory, US atomic energy commission. 

Based on the above mentioned requirements, you can therefore deduet the necessity of achieving a higher degree of test reliability by employing eddy-current test procedures that can fulfil these requirements. 

A Retrospective Look at the Development of Eddy-Current Testing on Aircraft Wheels ... 

In addition to the distortions caused by the probes, there were also distortions caused by filtering the signals within the eddy-current test instruments. To achieve the highest possible dynamics with the test instruments, high-pass filters with a high rate of rise, but also a long reverberation time were used. Thus, the recorded C-scan pictures sometimes shows strong echo effects. 

Eddy Current Testing of Turbo Generators Rotors. 

A the time of eddy-current testing, we exploit information given at distance by the variation of the impedance [L,R] of a solenoid near a conducting piece to defect and characterize the defects. 

Magnetic Field Sensors on 3D-Eddy-Current Testing. 

Special probe geometries and combinations of different types of magnetic field sensors make an important contribution to the further improvement of the eddy-current testing method and results in new applications. 

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]. 

Figure 2 Principle of eddy-current testing with relaxation circuit...

So, a comparison of different types of magnetic field sensors is possible by using the impulse response function. High amplitude and small width of this bell-formed function represent a high local resolution and a high signal-to-noise-characteristic of a sensor system. On the other hand the impulse response can be used for calculation of an unknown output. In a next step it will be shown a solution of an inverse eddy-current testing problem. 

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. 

Detailed view of the eddy current testing device... 

The measuren.cnts are performed on a sample made from Inconell IN 718 placed at our disposal by Me oren- und Turbinen-Union Miinchen GmbH (MTU). The sample is of disk shape and has a curved side and a flat side, see also fig. 6. On the curved side MTU had detected a segregation reaching to the surface but with unknown depth. It had been detected by segregation etching and its existence was confirmed by eddy current testing. Both methods are successful only in such a special case where the segregation extends to the surface. 

D. J. Hagemaier, Fundamentals of Eddy-Current Testing, American Society for Nondestmctive Testing, Columbus, Ohio, 1990. 

NOTE Nonferromagnetic (metals that cannot be magnetized, such as copper and brass) tubes that are not visually accessible can be examined nondestructively using eddy current testing. 

The eddy current tests performed earlier detected no pits because pit formation depended on the contamination of the system by seed hairs that occurred subsequently to the eddy current testing. 

Among the available nondestructive testing techniques are ultrasonic tests, eddy-current tests, air-underwater and hydrostatic tests, and visual examinations. All of these techniques must be executed by technically qualified personnel. 

Identification. Incomplete fusion generally results in discontinuities along the side walls of a joint. When these discontinuities emerge at surfaces, they can be observed visually if accessible (Figs. 15.10 and 15.11). Defects of this type may also be detected by ultrasonics, radiography, magnetic particle inspection, and eddy-current testing. 

Identification. Slag inclusions will not be visually identifiable unless slag particles emerge at the weldment surfaces. Radiography, eddy-current testing, and ultrasonics are nondestructive testing techniques that can disclose slag inclusions. 

R6, for eddy current testing of the steam generator tubes, is performed in a refueling ouiagL by quickly draining the RCS loops, hence, the decay heat rate is high. 

The nondestructive electrical eddy current test is a method in which eddy current flow is induced in the test object. Changes in the flow by variations in the test specimen... 

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