1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-05a71 Center Point of Contact: MSFCa71 Submitted by: Wilson HarkinsSubject: Eddy Current Testing of Aerospace Materials Practice: Eddy Current Testing (ECT) can be used on electrically conductive material for detecting and
2、 characterizing defects such as surface and near surface cracks, gouges, and voids. It can also be used to verify a materials heat treat condition. In addition, wall thickness of thin wall tubing, and thickness of conductive and nonconductive coating on materials can be determined using ECT.Programs
3、 that Certify Usage: This practice has been used on Solid Rocket Motor (SRM) and Redesigned Solid Rocket Motor (RSRM).Center to Contact for Information: MSFCImplementation Method: This Lesson Learned is based on Reliability Practice number PT-TE-1421 from NASA Technical Memorandum 4322A, NASA Reliab
4、ility Preferred Practices for Design and Test.Eddy Current Testing (ECT) is a fast, reliable, and cost effective nondestructive testing (NDT) method for inspecting round, flat, and irregularly shaped conductive materials. Specific processes have been developed to determine the usability and integrit
5、y of threaded fasteners. In addition, ECT has the capability of being automated. With proper equipment and skilled test technicians readout is instantaneous.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation:Alternating Current (AC) flowi
6、ng through a coil produces an alternating magnetic field about the coil. When the coil is positioned near to, or placed on, material that is capable of conducting electrical current, the magnetic field passes into the material and circular (eddy) currents are induced in the material near the coil. T
7、he flow of eddy currents in the material causes the excitation of a fluctuating magnetic field of its own. This magnetic field is always in opposition to the coils magnetic field as illustrated in Figure 1. When the coil is placed on conductive material, the strength of the coils magnetic field is r
8、educed. This change in the magnetic field causes a change in the impedance of the coil, which causes a change in the current flowing through the coil. These changes are detected by an instrument placed in the circuit.refer to D descriptionD The flow of eddy current within the material is disrupted b
9、y the presence of discontinuities, such as, cracks, porosity, or inclusions. Discontinuities cause a decrease in the flow of current in the material by increasing the length of the path along which the current must flow as shown in Figure 2. This results in a reduction of current flow which causes a
10、 change in the impedance of the test probe coil.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD There are three major factors that affect ECT. These are material conductivity, geometry, and permeability of the material being t
11、ested. In addition, there are contributors that affect the three major factors. These are shown in Table 1.Table 1. Factors Affecting Eddy Current TestingConductivity Geometry PermeabilityAlloy Hardness Temperatur Residual stress CoatingsThickness Discontinuities Coil-to-Material Seperation (liftoff
12、)Ferromagnetic* Material capable of being magnetized The ECT signal is strongly related to the geometrical shape of the coil, i.e., the size, shape, and positioning of the coil; the relationship between the coil windings and suspected discontinuities; the effect of changes in liftoff or fill factors
13、; the depth of penetration; and the edge effect.Identifying the various factors causing impedance changes depends upon the knowledge and skill of the ECT technician. Thus, selecting the appropriate eddy current probe is an important part of eddy current testing.Various types of test instruments avai
14、lable for ECT are: 1) conductivity testers, 2) crack detectors, 3) resistance and reactance measuring testers, 4) coating thickness testers, and 5) oscilloscopes and output devices such as strip chart recorders, printers, etc. when used as part of a test setup.A standard test specimen with known fla
15、w sizes must be fabricated for use in adjusting the sensitivity setting of the test instrument for accurate interpretation of the test results. The standard Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-test sample should be sound and of the same a
16、lloy, temper, and geometry as the part to be tested. Flaws may be produced in the standard test specimen by drilling, electrical discharge machining, milling, or any other means that will not distort the standard. Any flaw size outside the predetermined acceptable flaw size for the object being test
17、ed shall be a noted defect for corrective action or rejection.Table 2 lists advantages and disadvantages to be considered when selecting ECT. The most important elements required to maximize successful eddy current operations are dedicated personnel, training, proper equipment, and adequate standard
18、s. Also, at reasonable time intervals during ECT, routine checks should be made with the standard test specimen to insure equipment is operating properly.Table 2. Advantages and Limitations of Eddy Current Testing (ECT)Advantages Limitations1. High speed testing (can be automated) 2. Accurate measur
19、ing of conductivity 3. Discontinuities at or near surface can be reliably detected 4. High-sensitivity to small discontinuities 5. Accurate coating thickness measurements 6. Direct Go/No Go answers can be quickly obtained 7. No physical contact required 8. Low cost 9. Portable1. Limited penetration
20、into test article 2. Several variables simultaneously affect output indication 3. Discontinuities are qualitative not quantitative indications 4. Material must be conductive 5. Requires skill when many variables are involved 6. False indications can result from edge effects and parts geometryECT is
21、useful in the areas of material heat treat determination, coating thickness measurements, and flaw detection. A list of typical applications is shown in Table 3.Table 3. Typical Applications of Eddy Current TestingProvided by IHSNot for ResaleNo reproduction or networking permitted without license f
22、rom IHS-,-,-Material Property DeterminationsThickness Measurements Flaw DetectionHeat treatment evaluations Hardness Fire damage Impurities Chemical compositions Corrosion damage Conductivity of ionized gasThin sheet metal Foil Paints Anodic coatings Lacquers Thin insulation Rocket motor liningsShee
23、t metal Foil Wire Bars Tubes Bolt holes Fasteners Welds Ball bearingsTechnical Rationale:For the past ten years, eddy current testing has been a necessary tool at MSFC for the inspection of bolts, (heads, grip length, and threads), nuts, and holes for defects. Standards are fabricated as required an
24、d special adapters have been fabricated which allows the probe (coil) to be positioned at the correct angle and distance from surface. ECT is a fast, accurate, and highly reliable method for determining defects in surface and near surface areas of aerospace materials.References1. MIL-HDBK-728/2: “Mi
25、litary Handbook Eddy Current Testing,“ December 1985.2. MIL-HDBK-727/1: “Military Handbook Nondestructive Testing,“ December 1985.3. Bray, Don E., and Don McBride: “Nondestructive Testing Techniques,“ John Wiley & Sons, Inc., 1992.4. CT-6-5: “Nondestructive Testing Eddy Current,“ Classroom Training
26、Handbook, Second Edition, Convair Division of General Dynamics, 1979.5. PT-4-5: “Nondestructive Testing, Eddy Current Testing,“ Programmed Instruction Handbook, Convair Division of General Dynamics, San Diego, CA, 1967.6. Bray, Don E., and Roderick Stanley: “Nondestructive Evaluation,“ McGraw-Hill,
27、Inc., 1989.7. Kutz, Myer: “Mechanical Engineers Handbook,“ John Wiley & Sons, Inc., 1986.8. ASTM E426-92: “Standard Practice for Electromagnetic (Eddy Current) Examination of Seamless and Welded Tubular Products, Austenitic Stainless Steel and Similar Alloys.“9. Birnbaum, George, and George Free: “E
28、ddy Current Characterization of Materials and Structures,“ ASTM Special Technical Publication 722, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA, 19103, September 1979.10. McMaster, R.C., P. McIntire, and M. L. Mester: “Nondestructive Testing Handbook,“ Second Editio
29、n, American Society for Nondestructive Testing, Inc., 1986.11. Metals Handbook, Volume 1: “Nondestructive Inspection and Quality Control“ ASM International Metals, Park, OH, 1989.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Impact of Non-Practice:
30、 Failure to detect flaws in the surface or near surface areas of components could result in failure of the components and possibly failure of the mission. Failure to use ECT when applicable to aerospace materials could result in the use of a slower, less cost effective, and less reliable method for
31、detecting surface and near surface flaws in aerospace materials.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-05a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
