Dr. von Flugen, M.B.S., is famous throughout the field of non-destructive testing for his brilliant Glossary of Nondestructive Testing Terms, for his out-standing contributions to the airframe industry through his column in Aviation Week magazine, and for his monumental survey of washroom graffiti, "Pornography for the Millions." Dr. von Flugen is now associated with the Society for advanced Research in Probability Relationships between Revolving Cubes and is soon expected to release the results of his labors under the title, "Dice Loading for Fun and Profit." Dr. von Flugen's outstanding paper on Eddy Currents is presented here at the request of Centurion NDT, without whose money Dr. von Flugen would not have been able to carry out the laborious researches necessary for its presentation. It may be confidently stated that Dr. von Flugen's work will now enable Eddy Current to occupy the rightful place it has always deserved and, with wide circulation to all those interested in this subject, will almost certainly set nondestructive testing back a hundred years.
The use of induced eddy currents as a means of evaluating the metallurgical characteristics of conductive materials has become increasingly important during the past few years not only to the inspector and to the nondestructive test engineer but also to the designer. It seems particularly appropriate at this time, therefore, that the basic ground rules governing eddy current applications be clearly and concisely delineated in order to provide for standardized interpretation and evaluation throughout the industry. To this end, the following discussion is presented in as simple and direct a manner as possible. For convenience of the uninitiated, a glossary of technical terms and phrases peculiar to the field of eddy current testing is appended hereto for ready reference purposes.
Whenever relative motion occurs between a metal conductor and magnetic lines of forceirrespective of whether that motion is produced by actual movement of the metal, itself, or by a variation in the direction or intensity of the magnetic lines of forceelectric currents are induced in the surface of the metal conductor. These induced electric currents are commonly referred to as "Eddy Currents."1 Conversely, whenever electric currents flow thought a metal conductor, magnetic lines of force are induced in the surrounding atmosphere. These induced magnetic lines of force are, of course, capable of inducing secondary electric currents in a second metal conductor in the immediate vicinity and these, in turn, are capable of inducing magnetic lines of force in the surrounding atmosphere which, per se, are again capable of inducing electric currents in the first metal conductor and so on, ad infiniturn. The resulting phenomenon is known as "mutual induction," or "inductive coupling."2 The whole process of induction is therefore, a completely reversible transformation of magnetic and/or electrical energy into electrical and/ or magnetic energy and vice versa. In the simplest terms, then, the basic principle of induced eddy currents is merely the transformation of magnetic energy into electrical energy and the and the subsequent transformation of the electrical energy previously transformed from magnetic energy into electrical energy into magnetic energy which may subsequently be transformed into electrical energy.
At approximately the turn of the century, Lugrid Von Krugrid established the fact that by far the larger majority of metals are noncubic in their crystalline structure and, consequently, posses different electrical conductivities when measured along different crystalline axes. This same conclusion may be found in the independent observation of Charles Menopause and Juan Pablo Van der Schlemmer whose contention it was that they (the metals) possess different electrical conductivities along different crystalline axes even when they (the conductivities) are not measured at all!3 In spite of the fact that these rather electrifying conclusions were in direct conflict with the original hypothesis as promulgated by McNamara and Rabinowitz,4 they offered sufficient promise to inspire the Centurion NDT, Inc. to undertake the construction of a production induction system. During the initial stages of the theoretical investigation, Doktor Rausmitt Von Raddschidd was placed in complete charge of the project. All experimental work was, of course, performed in the now-famous Centurion NDT Laboratory for Theoretical Investigations, located on a rocky plateau atop McGraw Hill overlooking the River Delta. This laboratory is by the way - one of the most completely equipped facilities for theoretical investigation in the entire world. In addition to the usual theoretical laboratory equipment, Centurion NDT has provided one medium size cyclotron, a fifty-thousand-ton hydropress, a Bessemer converter, and two camel's hair test tube brushes.
After numerous unsuccessful attempts to construct a production induction apparatus, it became apparent that Dr. Von Raddschidd's investigations were too theoretical. A move was made to relieve the Herr Doktor of his responsibilities in connection with the project. However, Centurion NDT, being of the opinion that it is poor policy to change horsesor any part thereofin midstream, decided against such a movement and the Doktor was never relieved.
Following this debacle, Von Raddschidd buckled down an his ubiquitous efforts to supervise personally each and eve phase of the project led to the now-famous slogan "Raddschidd Uber Alles!" Despite the seemingly insurmountable obstacles they encountered, Dr. Von Raddschidd and his cohorts actually did develop a remarkable eddy current testing device. This device, which has since become almost universally known as "a remarkable eddy current testing device," is actually constructed in an amazing simple manner.5 It operates on the extremely elemental assumption that the electrical currents induced by magnetic lines of force induced by electrical currents in the material or part being examined are measurably affected by the effects of physical and/or mechanical variations in, on, or adjacent to the material or part being examined. Any slight variation, therefore, in the metallic configuration of the material or part being examined no matter how infinitesimally small it may beproduces a correspondingly infinitesimally small variation in the inductive reluctance of a Whitestone Bridge. This induced variation is then amplified and reamplified by a simple application of amplification and reamplification factors. By the rather obvious expedient of repeating this infinitesimal amplification and infinite number of times, the inductive reluctance is overcome and a visual signal is recorded of the visual signal recorder. There exists, of course, a remote possibility of over-exciting the inductive reluctance circuit and blowing the whole affair to Kingdom Come. This possibility has been forestalled, however, by placing a red line on the upper limit of the errif scale indicator.
The application of this instrument to production inspection operations is quite elementary. Consider, for example, the case wherein the mechanical strength of an unknown sample is desired. By merely placing a small portion of the sample in the categromic analyzer and flipping the switch marked, "Caterogmic Analyzer Switch," to the appropriate conductivity scale. The passing of a specimen through the conductivity coil (a rather difficult feat even under the most favorable circumstances) causes appreciable depreciation of the retentivity parameter which, in turn, reflects on the oscillographic recorder.6 On the direct reading type instruments, one may then obtain the inductive dislocation constant by a purely rudimentary application of the principles of advanced tensor analysis. This constant is, of course, inversely proportional to the cube root of the logarithm of an exponential function of a parameter which, when multiplied by the ration of stress to strain, will yield the modulus of elasticitywithin limits. As an alternate method, one COULD strike up an acquaintance with the unknown sample. Contrary to popular opinion, it should be noted that the investigation of Frankletter and Sanovitch have absolutely no connection with thisor any otherproblem.7
As still another example of the versatility of eddy current testing, let us consider for a moment the case where a built-up sheet metal assembly is suspected of containing one or more discontinuities of sufficient magnitude to adversely affect the structural integrity of the assembly but still not of sufficient magnitude to be detectable by routine inspection techniques such as antisynthetic refracteration, dichlrofluorobronio-hydraclanicide replacement, or intergramilar flatulation analysis. The problem in this case is, rather obviously, to determine the exact size, shape, and location of the internal discontinuity(s) without in any way detracting from the ultimate usefulness of the assembly. Unfortunately, eddy current testing is not applicable to this revolting situation.
To illustrate this extremely important aspect of eddy current testing, let us consider a hypothetical case such as a circular, thin-walled, internally stiffened cylinder of typical plastic construction. Considering the cylinder as a whole (an entirely logical consideration, and one which will be shown to be valid in this case), the configuration is conventional in all respects, being composed of a thin outer skin and stout inner members. As an extreme case, let us consider the condition of complete dimensional reversal, i.e., the cylinder has been turned inside out. (It will, of course, be assumed that the plastic is completely elastic for the purpose of this discussion.)
Under ordinary circumstances, it is a generally accepted fact that the outside diameter of a cylinder is greater than the inside diameter. These, however, are NOT ordinary circumstances. In the case under consideration, the inside diameter is outside and the outside diameter is inside. In order to explain this somewhat unusual situation, it will be necessary to examine the mechanics of dimensional reversal. What have we done? In essence, the geometrical characteristics of our specimen have been so rearranged that we now have the outside skinside inside. Or looking at it in another way, we have now placed the inside stoutside outside. Now, if the outside skinside is inside and the inside stoutside and the inside stoutside is outside, it follows that the inner skin is thinner and, conversely, that the outer part is stouter. This is rather drasticeven for a elastic plastic. However, we must not lose sight of the fact that dimensional evaluation has shown conclusively that the inside diameter, being outside, is the outside diameter and the outside diameter, being inside, is the inside diameter and, therefore, that the inside and the outside coincide. By simple logic, it follows that, if the inside and the outside coincide, the cylinder as a whole (or even as a hole) has no thickness whatsoever and, therefore, isn't! Thus, eddy current has disposed of another perplexing problem.
In summation, then, we may safely say that eddy current testing devices are particularly adaptable to automatic testing and, consequently, are recommended without qualification to the manufacturers of automatics. Eddy current evaluation depends to a great extent upon the know-how of the individual operator. It can be safely assumed, however, that an inspector who has worked with eddy currents for several years will become quite elderly.
1Less commonly - and more formally - referred to as "Edward Currents. " Eddy currents, being round in nature, are highly prized by the distributors of Son-Made raisins.
2Not to be confused with "mutual seduction " or "seductive coupling," both of which delightful pastimes are also current.
3Charles Menopause and Juan Pablo Van der Schlemmer, "Achtung, Fluxengeschtallen unter den Linden, " Transactions of the International Institute for Independent Observations, Dublin, 1902.
4Isidore McNamara and Patrick Joseph Rabinowitz, "The Original Hypothesis as Promulgated, " Science Fiction Press, Hesperus, 1982.
5The Remarkable Eddy Current Testing Device was considered to be practically useless until the discovery of remarkable eddy currents proved it to be of no value whatsoever. Notwithstanding, there are those who believe that, if it were not constructed in so amazingly simple a manner, this device might be invaluable.
6The passing of specimens through their induction coils has in the past, does, and probably always will reflect on oscillographic recorders. Such a practice would, no doubt, reflect on practically anything.
7John Edgar Hoover, "The Investigations of Frankletter and Sanovitch, " John Sneaky and Son, Washington, D.C., 1951. (Actually, these investigation proved that Frankletter and Sanovitch were really brothersjust as Hoover has suspected all along Frankletter was really a Sanovitch!)
8The analogy of the plastic cylinder was presented by the author at the National Convention of the Society for Tired Leprechauns, Flying Saucer Division. It was later published in the Los Angeles Daily Newsshortly thereafter, the Los Angeles Daily News went bankrupt.
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