1、SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirelyvoluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefro
2、m, is the sole responsibility of the user.”SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions.QUESTIONS REGARDING THIS DOCUMENT: (412) 772-8512 FAX: (412) 776-0243TO PLACE A DOCUMENT
3、 ORDER; (412) 776-4970 FAX: (412) 776-0790SAE WEB ADDRESS http:/www.sae.orgCopyright 1991 Society of Automotive Engineers, Inc.All rights reserved. Printed in U.S.A.SURFACEVEHICLE400 Commonwealth Drive, Warrendale, PA 15096-0001INFORMATIONREPORTJ425REV.MAR91Issued 1960-06Revised 1991-03Superseding J
4、425 MAR81(R) ELECTROMAGNETIC TESTING BY EDDY CURRENT METHODSForewordThis Document has not changed other than to put it into the new SAE Technical Standards BoardFormat.1. ScopeThe purpose of this SAE Information Report is to provide general information relative to the natureand use of eddy current t
5、echniques for nondestructive testing. The document is not intended to providedetailed technical information but to serve as an introduction to the principles and capabilities of eddy currenttesting, and as a guide to more extensive references listed in Section 2.2. References2.1 Related Publications
6、The following publications are provided for information purposes only and are not arequired part of this document.2.1.1 ASM PUBLICATIONATTN: MSC/Book Order, ASM International, PO Box 473, Novelty, OH 44072-9901.Metals Handbook, Eighth Edition, Vol. 11, 1976, pp. 7593.Metals Handbook, Ninth Edition,
7、Vol. 17, Nondestructive Evaluation and Quality Control, 1989. 2.1.2 ASTM PUBLICATIONAvailable from ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959ASTM Annual Standards, Part 11, Standards E 215, E 243, E 268, E 309, E 376, E 426, E 566, E 570, E571, E 690. 2.1.3 OTHER PUBLICATIONSNonde
8、structive Testing Handbook, Second Edition, Vol. 4, Electromagnetic Testing, 1986, AmericanSociety for Nondestructive Testing, Columbus, OH 43228Programmed Instruction Handbooks, PI-4-5, Eddy Current Testing, 1971. Classroom Training Handbook,CT-6-5, Eddy Current Testing, 1971. The above prepared by
9、 General Dynamics and available fromAmerican Society for Nondestructive Testing.Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, New York: John Wiley andSons, Inc., 1985Tool and Manufacturing Engineers Handbook, Vol. 4, Quality Control and Assembly, 1987, Society ofManufac
10、turing Engineers, Dearborn, MI 48121SAE J425 Revised MAR91-2-3. GeneralEddy current testing is a method of electromagnetic testing which uses induced electrical currents toindicate or measure certain characteristics of electrically conducting bodies (ferrous and nonferrous).Applications are in one o
11、f three general categories: metal sorting, surface discontinuity detection, or thicknessmeasurement. Under appropriate conditions and with proper instrumentation, eddy current testing has beenused to:a. Detect discontinuities such as seams, laps, slivers, scabs, pits, cracks, voids, inclusions, and
12、coldshuts.b. Sort for chemical composition on a qualitative basis.c. Sort for physical properties such as hardness, case depth, and heat damage.d. Measure conductivity and related properties.e. Measure dimensions such as the thickness of metallic coatings, plating, cladding, wall thickness oroutside
13、 diameter of tubing, corrosion depth, and wear.f. Measure the thickness of nonmetals, when a metallic backing sheet can be employed.4. PrincipleEddy currents are induced in a test piece by a time varying magnetic field generated by analternating current flowing in a coil. The coil configuration may
14、assume a wide variety of shapes, sizes, andarrangements. The coil may surround the test piece or may be placed on or near the surface.Eddy currents are influenced by many characteristics of the metal: conductivity, magnetic permeability,geometry, mass, and homogeneity. This fact makes it possible to
15、 evaluate many different characteristics of thetest piece with appropriate test procedures.In electromagnetic testing, energy is dissipated in the test piece by two separate processes: magnetichysteresis and eddy current flow. In magnetic materials, both effects are present. In nonmagnetic andmagnet
16、ically saturated materials, the hysteresis effect is absent or suppressed; and the prevalent losses aredue to eddy currents.Saturation is a term used generally to describe the condition of a ferromagnetic material at its maximum valueof magnetization. To provide saturation, a direct current magnetic
17、 field or a permanent magnet of sufficientstrength is applied to bring the material to a point where the ratio force approaches unity. In this condition, thematerial behaves as if it were nonmagnetic. Theoretically, magnetic saturation should not be necessary fornonferromagnetic material, but some n
18、onmagnetic materials contain small amounts of ferromagnetic materialwhich can generate electrical noise during testing. This noise can usually be eliminated by the use of asaturating field.5. ProcedureThe effect of the characteristics of the test piece on the eddy currents may be studied in a number
19、of different ways. A characteristic to be studied is related to a change in the amplitude, distribution, or phase ofthe eddy currents, or some combination of these three. These changes are reflected as changes in theexciting coil or in auxiliary coils located to be sensitive to the eddy currents. Th
20、ese changes may be measuredas voltage differences, current differences, phase differences, or changes in the impedance of the coil or coils.The coils and the instrumentation can be arranged to measure a given characteristic directly, or they may beused as a comparator. In the latter case, the measur
21、ement is the difference between the characteristics of thetest piece and a similar piece of known or acceptable characteristics. Such measurements can also be madeto determine differences between various segments of the same test piece.SAE J425 Revised MAR91-3-Even with the best instrumentation, it
22、is sometimes difficult to separate effects of the characteristics to bemeasured from effects of other characteristics. The success of an eddy current test depends on:a. Proper coil design and arrangementb. Selection of the proper test frequencyc. Selection of the proper analysis circuitd. Use of pro
23、per magnetic field strengthe. Optimization and maintenance of electromagnetic coupling between the coil and test piecef. Selection of the most suitable stage in the manufacturing process for the inspection procedureEddy current effects are most pronounced near the surface, with sensitivity for detec
24、ting irregularities ofcomposition or structure falling off as depth below the surface increases. Depth of eddy current penetration ofan object decreases as test frequency increases. Ferromagnetic metals, such as steel, are generally testedwith low frequencies in the range of 1 to 10 000 Hz (10 kHz).
25、 Nonmagnetic metals with higher conductivity,such as aluminum, are generally tested with frequencies around 100 kHz, while those with lower conductivity,such as titanium, are generally tested with frequencies in the range of 1 to 10 MHz. There are numerousexceptions to these generalities.6. Test Coi
26、l Methods6.1 Single CoilIn this method, a single coil is used. It may have one or more windings for excitation anddetection. A winding is excited from an alternating current source within the test instrument. The amplitudeand phase of the voltage across a winding is a function of the effect of the t
27、est piece on the coil.6.2 Differential CoilAn arrangement where two separate detector coils are used to compare two different testpieces, or two different portions of the same test piece. A voltage appears at the output terminal of the coilswhen the effective permeability, conductivity, mass, geomet
28、ry, or homogeneity of the metal in the two coilsdiffer.7. Method Of Analysis7.1 Lumped ImpedanceIn the lumped impedance analysis, a single coil is employed. A characteristic of thetest piece is correlated to the amplitude and phase of the coil voltage.7.2 Impedance Plane Analysis7.2.1 MAGNETIC PARAM
29、ETER AMPLITUDEThe single coil or the differential coil method may be employed in thisanalysis. The variation in amplitude and phase of the detector coil voltage is measured and plotted in animpedance plane. The coil parameters are correlated to a test piece characteristic. Some variation inchemistry
30、 and size can be tolerated in this system providing the proper test frequency is employed.7.2.2 PHASE ANGLE ANALYSISA two-coil method is more suited to this type of analysis. The phase anglebetween the voltage at the driving coil and that at the detector coil is measured and related to a test piecec
31、haracteristic.8. EquipmentEddy current test instrumentation with a wide range of test frequencies and associated coils andprobes of various sizes are commercially available to meet the needs of many applications. One of theadvantages of electromagnetic equipment is that it lends itself to automatic
32、operations for regularly shapedparts. Electromagnetic equipment can be large, elaborate, and expensive when multiple stations and materialshandling sections are included, such as are used on sheets and plates. Manual systems which are small,simple, and inexpensive are common in other instances, and
33、are used with large or irregularly shaped objects.SAE J425 Revised MAR91The electronic apparatus energizes an encircling coil or probe with alternating currents of suitable frequencyand amplitude and detects the electromagnetic response of the coil. Equipment may include a detector phasediscriminato
34、r, filter circuits, modulation circuits, magnetic saturation devices, recorders, and signaling devicesas required by the application.The encircling or probe coil assembly is capable of inducing current in the part and sensing changes in theelectric and magnetic characteristics of the part.A mechanic
35、al device capable of passing a part (such as a tube) through the encircling coil or past the probemay be used. It generally operates at uniform speed with minimum vibration of the coil, probe, or part, andmaintains the article to be inspected in proper register or concentricity with the probe or enc
36、ircling coil. Amechanism capable of uniformly rotating or moving the part or the probe may be required.An end effect suppression device, a means capable of suppressing the signals produced at the ends of tubesor bars, may be used.Reference standards are generally required to relate eddy current meas
37、urements to test part characteristics,and to adjust the sensitivity of the electronic apparatus.8.1 Typical Examples of Equipment Variations for Different Applications8.1.1 Equipment using impedance plane analysis and operable over a range of test frequencies from 1 Hz to 10kHz has been used to sort
38、 carbon steel mixtures involving different compositions and/or different heat treatconditions. A unique advantage of this instrument is that it is possible to quickly determine the optimumfrequency for performing a given test. Similar equipment has been calibrated to indicate conductivity,hardness,
39、case depth, and dimensions.8.1.2 Equipment using a single coil to scan the surface has been used to detect and indicate the depth of seams,cracks, laps, slivers, and similar surface and near-surface imperfections in bars, rounds, billets, and tubularproducts. The sensitivity of this equipment depend
40、s on the surface conditions of the product under test. Ona hot-rolled surface with thin, tightly adherent scale, seams as shallow as 0.25 mm (0.010 in) are reliablyevaluated. Product with heavy or broken scale should be cleaned by grit blasting prior to testing. Undermore favorable (smoother, less s
41、cale) surface conditions seams as shallow as 0.13 mm (0.005 in) have beenevaluated. On polished (ground) surfaces, seams and cracks as shallow as 0.025 mm (0.001 in) have beendetected.8.1.3 Equipment using differential test coils has been used to detect imperfections in carbon steel tubular and barp
42、roducts. Testing frequencies ranging from 400 Hz to over 20 kHz have been used. At the lowest testingfrequencies, and with the use of magnetic saturation, defects have been reliably detected (OD, ID, orsubsurface) in the wall of tubular products with wall thicknesses as great as 15.9 mm (0.62 in). W
43、hen testingat frequencies as low as 400 Hz, the testing speed is limited to about 30.5 m/min (100 ft/min). When highertesting frequencies are used, the testing speed can be correspondingly increased. Higher testingfrequencies can be used for testing product with thinner walls and higher resistivity.
44、8.1.4 Vector sensitive instruments operate on the impedance plane principle. The frequency range of theseinstruments is from 100 Hz to 6 MHz. This type of operation considers both the amplitude and phase of theeddy currents. This allows one to optimize the instrument response for a selected material
45、 variable, whileminimizing response to another variable, such as probe spacing.SAE J425 Revised MAR91-5-8.1.5 Multiple frequency eddy current instruments can test a product at several frequencies simultaneously or inrapid sequence. Unuseful signal variations due to overlapping responses, product geo
46、metry variations, andprobe lift-off effects can be reduced significantly. Thus the resulting signal response is enhanced.Simultaneous multiple frequency tests are generally used to overcome probe lift-off effects in surface defecttests of nonferrous products. Sequential multiple frequency tests are
47、generally used to sort ferrous productsof similar composition, or to determine two characteristics, such as hardness and case depth, in a single test.9. Notes9.1 Marginal IndiciaThe change bar (l) located in the left margin is for the convenience of the user in locatingareas where technical revision
48、s have been made to the previous issue of the report. An (R) symbol to the leftof the document title indicates a complete revision of the report.PREPARED BY THE SAE IRON AND STEEL TECHNICAL COMMITTEE DIVISION 25NONDESTRUCTIVE TEST METHODS OF THE SAE IRON AND STEEL TECHNICAL COMMITTEESAE J425 Revised
49、 MAR91RationaleNot applicable.Relationship of SAE Standard to ISO StandardNot applicable.ApplicationThe purpose of this SAE Information Report is to provide general information relative to thenature and use of eddy current techniques for nondestructive testing. The document is not intended toprovide detailed technical information but to serve as an introduction to the principles and capabilities ofeddy current testing, and as a guide to more extensive references listed in Section 2.Reference SectionNondestructive Testing Handbook, Second Edition, Vol. 4, Electromagnetic T
