1、Designation: E2884 13Standard Guide forEddy Current Testing of Electrically Conducting MaterialsUsing Conformable Sensor Arrays1This standard is issued under the fixed designation E2884; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers the use of conformable eddy currentsensor arrays for nondestructive examination of e
3、lectricallyconducting materials for discontinuities and material quality.The discontinuities include surface breaking and subsurfacecracks and pitting as well as near-surface and hidden-surfacematerial loss. The material quality includes coating thickness,electrical conductivity, magnetic permeabili
4、ty, surface rough-ness and other properties that vary with the electrical conduc-tivity or magnetic permeability.1.2 This guide is intended for use on nonmagnetic andmagnetic metals as well as composite materials with anelectrically conducting component, such as reinforced carbon-carbon composite or
5、 polymer matrix composites with carbonfibers.1.3 This guide applies to planar as well as non-planarmaterials with and without insulating coating layers.1.4 UnitsThe values stated in SI units are to be regardedas standard. The values given in parentheses are mathematicalconversions to inch-pound unit
6、s that are provided for informa-tion only and are not considered standard.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine
7、 the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E543 Specification for Agencies Performing NondestructiveTestingE1316 Terminology for Nondestructive ExaminationsE2238 Guide for Evacuation Route Diagrams2.2 ASNT Documents:3SNT-TC-1A Recommended Pr
8、actice for Personnel Qualifi-cation and Certification in Nondestructive TestingANSI/ASNT-CP-189 Standard for Qualification and Certifi-cation of NDT Personnel2.3 AIA Standard:NAS 410 Certification and Qualification of NondestructiveTesting Personnel42.4 Department of Defense Handbook:MIL-HDBK1823A N
9、ondestructive Evaluation System Re-liability Assessment3. Terminology3.1 DefinitionsFor definitions of terms relating to thisguide refer to Terminology E1316.3.2 Definitions of Terms Specific to This Standard:3.2.1 B-Scana method of data presentation utilizing ahorizontal base line that indicates di
10、stance along the surface ofa material and a vertical deflection that represents a measure-ment response for the material being examined.3.2.2 C-Scana method of data presentation which pro-vides measurement responses for the material being examinedin two-dimensions over the surface of the material.3.
11、2.3 conformablerefers to an ability of sensors or sensorarrays to conform to non-planar surfaces without significanteffects on the measurement results, or with effects that arelimited to a quantifiable bound.3.2.4 depth of sensitivitydepth to which the sensor re-sponse to features or properties of i
12、nterest exceeds a noisethreshold.3.2.4.1 DiscussionThe depth of sensitivity is generallysmaller than the depth of penetration since it incorporates acomparison between the signal obtained from a feature as well1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and
13、 is the direct responsibility of Subcommittee E07.07 on Electro-magnetic Method.Current edition approved June 1, 2013. Published June 2013. DOI: 10.1520/E2884-13.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book
14、of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.4Available from Aerospace Industries Association of A
15、merica, Inc. (AIA), 1000Wilson Blvd., Suite 1700,Arlington, VA22209-3928, http:/www.aia-aerospace.org.(Replacement standard for MIL-STD-410.)Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1as measurement noise, whereas the depth of pe
16、netration refersto the decrease in field intensity with distance away from a testcoil.3.2.5 discontinuity-containing reference standarda regionof the material under examination or a material having elec-tromagnetic properties similar to the material under examina-tion for which a discontinuity havin
17、g known characteristics ispresent.3.2.6 discontinuity-free reference standarda region of thematerial under examination or a material having electromag-netic properties similar to the material under examination forwhich no discontinuities are present.3.2.7 drive windinga conductor pattern or coil tha
18、t pro-duces a magnetic field that couples to the material beingexamined.3.2.7.1 DiscussionThe drive winding can have variousgeometries, including: 1) a simple linear conductor that isplaced adjacent to a one-dimensional array of sensing ele-ments; 2) one or multiple conducting loops driven to create
19、 acomplex field pattern; and 3) multiple conducting loops with aseparate loop for each sensing element.3.2.8 insulating shimsconformable and substantially non-conducting or insulating foils that are used to measure effectsof small lift-off excursions on sensor response.3.2.9 lift offnormal distance
20、from the plane of the con-formable sensor winding conductors to the surface of theconducting material under examination.3.2.10 model for sensor responsea relation between theresponse of the sensor (for example, impedance magnitude andphase or real and imaginary parts) and properties of interest(for
21、example, electrical conductivity, magnetic permeability,lift-off, and material thickness) for at least one sensing elementand at least one drive winding.3.2.10.1 DiscussionThese model responses may be ob-tained from database tables and may be analysis-based orempirical.3.2.11 sensing elementa means
22、for measuring the mag-netic field intensity or rate of change of magnetic fieldintensity, such as an inductive coil or a solid-state device.3.2.11.1 DiscussionThe sensing elements can be arrangedin one or two-dimensional arrays. They can provide either anabsolute signal related to the magnetic field
23、 in the vicinity ofthe sense element or a differential signal.3.2.12 spatial half-wavelengthspacing between the con-ductors of a linear drive winding with current flow in oppositedirections.3.2.12.1 DiscussionThis spacing affects the depth of sen-sitivity. The spatial wavelength equals two times thi
24、s spacing.For a circular drive winding, the effective spatial half-wavelength is equal to the drive winding diameter.3.2.13 system performance verificationthe use of a mea-surement of one or more response values, typically physicalproperty values, for a reference part to confirm that theresponse val
25、ues are within specified tolerances to validate thesystem standardization and verify proper instrument operation.4. Summary of Guide4.1 The examination is performed by scanning a conform-able eddy current sensor array over the surface of the materialof interest, with the sensor array energized with
26、alternatingcurrent of one or more frequencies. The electrical responsefrom each sensing element of the eddy current sensor array ismodified by the proximity and local condition of the materialbeing examined. The extent of this modification is determinedby the distance between the eddy current sensor
27、 array and thematerial being examined, as well as the dimensions andelectrical properties (electrical conductivity and magnetic per-meability) of the material. The presence of metallurgical ormechanical discontinuities in the material alters the measuredimpedance of the eddy current sense elements.
28、While scanningover the material, the position at each measurement locationshould be recorded along with the response of each sensingelement in the sensor array. The measured responses andlocation information can then be used, typically in the form ofa displayed image (C-scan (3.2.2) or in the form o
29、f a plot(B-scan (3.2.1), to determine the presence and characteristicsof material property variations or discontinuities.4.2 The eddy current sensor arrays used for the examinationare flexible and, with a suitable backing layer, can conform toboth flat and curved surfaces, including fillets, cylindr
30、icalsurfaces, etc. The sensor array can have a variety of configu-rations. These include: 1) a linear drive conductor that isenergized by the instrument alternating current and a lineararray of absolute sense elements positioned parallel to the driveconductor; 2) a complex drive conductor that produ
31、ces adesired field pattern at each sensing element; and 3) individualdrive conductors associated with each sensing element. Asso-ciated with each sense element are one or more measurementresponses that reflect the local material condition at eachlocation over the surface. The sensor arrays may be us
32、ed withmodels for the sensor response and appropriate algorithms toconvert measured responses for each sensing element intophysical properties, such as lift-off, electrical conductivity,magnetic permeability, coating thickness, and/or substratethickness. Baseline values for these measurement respons
33、es orphysical properties are used to ensure proper operation duringthe examination while local variations in one or more of theseproperties can be used to detect and characterize the disconti-nuity. For example, although, an impedance magnitude or othersensing element response can be used without a
34、model todetermine the presence of a flaw, a measurement of the lift-offat each sensing element location ensures that the sensor isconforming properly to the surface. Also, a position measure-ment capability, such as a rolling position encoder, can be usedto measure location in the scan direction and
35、 ensure thatsufficient data resolution is achieved. Visual or audio signalingdevices may be used to indicate the position of the disconti-nuity.5. Significance and Use5.1 Eddy current methods are used for nondestructivelylocating and characterizing discontinuities in magnetic ornonmagnetic electrica
36、lly conducting materials. Conformableeddy current sensor arrays permit examination of planar andE2884 132non-planar materials but usually require suitable fixtures tohold the sensor array near the surface of the material ofinterest, such as a layer of foam behind the sensor array alongwith a rigid s
37、upport structure.5.2 In operation, the sensor arrays are standardized withmeasurements in air and/or a reference part. Responses mea-sured from the sensor array may be converted into physicalproperty values, such as lift-off, electrical conductivity, and/ormagnetic permeability. Proper instrument op
38、eration is verifiedby ensuring that these measurement responses or propertyvalues are within a prescribed range. Performance verificationon reference standards with known discontinuities is performedperiodically.5.3 The sensor array dimensions, including the size andnumber of sense elements, and the
39、 operating frequency areselected based on the type of examination being performed.The depth of penetration of eddy currents into the materialunder examination depends upon the frequency of the signal,the electrical conductivity and magnetic permeability of thematerial, and some dimensions of the sen
40、sor array. The depthof penetration is equal to the conventional skin depth at highfrequencies but is also related to the sensor array dimensions atlow frequencies, such as the size of the drive winding and thegap distance between the drive winding and sense elementarray. For surface-breaking discont
41、inuities on the surfaceadjacent to the sensor array, high frequencies should be usedwhere the penetration depth is less than the thickness of thematerial under examination. For subsurface discontinuities orwall thickness measurements, lower frequencies and largersensor dimensions should be used so t
42、hat the depth ofpenetration is comparable to the material thickness.5.4 Insulating layers or coatings may be present between thesensor array and the surface of the electrically conductingmaterial under examination. The sensitivity of a measurementto a discontinuity generally decreases as the coating
43、 thicknessand/or lift-off increases. For eddy current sensor arrays havinga linear drive conductor and a linear array of sense elements,the spacing between the drive conductor and the array of senseelements should be smaller than or comparable to the thicknessof the insulating coating. For other arr
44、ay formats the depth ofsensitivity should be verified empirically.5.5 Models for the sensor response may be used to convertresponses measured from the sensor array into physical prop-erty values, such as lift-off, electrical conductivity, magneticpermeability, coating thickness, and/or substrate thi
45、ckness. Fordetermining two property values, one operational frequencycan be used. For nonmagnetic materials and examination forcrack-like discontinuities, the lift-off and electrical conductiv-ity should be determined. For magnetic materials, when theelectrical conductivity can be measured or assume
46、d constant,then the lift-off and magnetic permeability should be deter-mined. The thickness can only be determined if a sufficientlylow excitation frequency is used where the depth of sensitivityis greater than the material thickness of interest. For determin-ing more than two property values, measu
47、rements at operatingconditions having at least two depths of penetration should beused; these different depths of penetration can be achieved byusing multiple operational frequencies or multiple spatialwavelengths.5.6 Processing of the measurement response or propertyvalue data may be performed to h
48、ighlight the presence ofdiscontinuities, to reduce background noise, and to characterizedetected discontinuities. As an example, a correlation filter canbe applied in which a reference signature response for adiscontinuity is compared to the measured responses for eachsensor array element to highlig
49、ht discontinuity-like defects.Care must be taken to properly account for the effect ofinterferences such as edges and coatings on such signatures.6. Basis of Application6.1 The following items are subject to contractual agree-ment between the parties using or referencing this standard.6.2 Personnel QualificationIf specified in the contractualagreement, personnel performing examinations to this standardshall be qualified in accordance with a nationally or interna-tionally recognized NDT personnel qualification practice orstandard such as ANSI/ASNT-CP-189, SNT-
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