Volume 17 - Nondestructive Evaluation and Quality Control Part 4 ppt

Fig. 57 Current and flux density curves during demagnetization, projected from the hysteresis loop. See text for discussion. In using this principle, the magnetizing force must be high enough at the start to overcome the coercive force and to reverse the residual field initially in the part. Also, the incremental decrease between successive reductions in current must be small enough so that the reverse magnetizing force will be able, on each cycle, to reverse the field remaining in the part from the last previous reversal. Demagnetization With Alternating Current. A common method of demagnetizing small to moderate-size parts is by passing them through a coil through which alternating current at line frequency is passing (usually 50 to 60 Hz). Alternatively, the 60-Hz alternating current is passed through a coil with the part inside the coil, and the current is gradually reduced to zero. In the first method, the strength of the reversing field is reduced by axially withdrawing the part from the coil (or the coil from the part) and for some distance beyond the end of the coil (or part) along that axial line. In the second method, gradual decay of the current in the coil accomplishes the same result. Passing a part through an ac coil is usually the faster, preferred method. Small parts should not be loaded into baskets and the baskets passed through the coil as a unit, because alternating current will not penetrate into such a mass of parts and because only a few parts on the outside edges will be demagnetized (and these possibly only partially demagnetized). Small parts can be demagnetized in multiple lots only if they are placed in a single layer on a tray that holds them apart and in a fixed position with their long axes parallel to the axis of the coil. Large parts are not effectively demagnetized with 60-Hz alternating current, because of its inability to penetrate. Alternating current with 25-Hz frequency is more effective. Machines that provide decaying alternating current have a built-in means for automatically reducing the alternating current to zero by the use of step-down switches, variable transformers, or saturable-core reactors. When decaying alternating current is used, the current can be passed directly through the part instead of through a coil. Passing the current through the part is more effective on long, circularly magnetized parts than the coil method, but does not overcome the lack of penetration because of the skin effect, unless frequencies much lower than 60 Hz are used. High field strength ac demagnetizing coils are available with power factor correction, resulting in lower line current. Demagnetization With Direct Current. Methods of demagnetizing with direct current are essentially identical in principle to the methods just described for alternating current. By using reversing and decreasing direct current, low- frequency reversals are possible, resulting in more complete penetration of even large cross sections. A commonly used frequency is one reversal per second. It is a successful means of removing circular magnetic fields, especially when the current is passed directly through the part and can be used to demagnetize large parts. When a part in a coil is demagnetized using direct current at one reversal per second, the part remains in the coil for the duration of the entire cycle. Oscillating circuits are a means of obtaining a reversing decaying current for demagnetizing purposes. By connecting a large capacitance of the correct value across the demagnetizing coil, the coil becomes part of an oscillatory circuit. The coil is energized with direct current; when the source of current is cut off, the resonant resistance-inductance-capacitance circuit oscillates at its own resonant frequency, and the current gradually diminishes to zero. Yokes, either direct or alternating current, provide a portable means for demagnetizing parts. The space between the poles of the yoke should be such that the parts to be demagnetized will pass between them as snugly as possible. With alternating current flowing in the coil of the yoke, parts are passed between the poles and withdrawn. Yokes can be used on large parts for local demagnetization by placing the poles on the surface, moving them around the area, and then withdrawing the yoke while it is still energized. Yokes using low-frequency reversing direct current, instead of alternating current, are more effective in penetrating larger cross sections. The applicability of demagnetizing methods, based on part size, metal hardness, and production rate, is given in Table 2. Table 2 Applicability of demagnetizing methods on the basis of part size, metal hardness, and production rate Part size (a) Metal hardness (a) Production rate (a) Method Small Medium Large Soft Medium Hard Low Medium High Coil, 60-Hz ac A A N A A N A A A Coil, dc, 30-point reversing step down N A A A A A A N N Through current, ac, 30-point step down N A A A A A A A N Through current, ac, reactor decay N A A A A A A A N Through current, dc, 30-point reversing step down N A A A A A A N N Yoke, ac A (b) N A A N A N N Yoke, reversing dc A (b) N A A A A N N (a) A, applicable; N, not applicable. (b) Used for local areas only Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Appendix: Proprietary Methods of Magnetic Particle Inspection Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Magnetic Rubber Inspection Henry J. Weltman, Jack D. Reynolds, John E. Halkias, and William T. Kaarlela, General Dynamics Corporation Several proprietary methods of magnetic particle inspection have been developed for specific applications. Three of these methods, which are described in this section, are magnetic rubber inspection, magnetic printing, and magnetic painting. Magnetic rubber inspection is a nondestructive inspection method for detecting discontinuities on or near the surfaces of parts made of ferromagnetic metals. In this method, finely divided magnetic particles, dispersed in specially formulated room temperature curing rubber, are applied to a test surface, which is subsequently magnetized. The particles are attracted to the flux fields associated with discontinuities. Following cure of the rubber (about 1 h), the solid replica casting is removed from the part and examined, either visually or with a low-power microscope, for concentrations of magnetic particles that are indications of discontinuities on or just below the surface of the testpiece. Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Method Advantages and Limitations Advantages. Magnetic rubber inspection extends and complements the capabilities of other nondestructive inspection methods in certain problem areas. These include: • Regions with limited visual accessibility • Coated surfaces • Regions having difficult-to-inspect shapes and sizes • Indications requiring magnification for detection or interpretation The replica castings furnish evidence of machining quality, physical dimensions, and surface conditions. The replicas can also be used to detect and record the initiation and growth of fatigue cracks at selected intervals during a fatigue test. The replicas provide a permanent record of the inspection; however, because the replicas shrink slightly during storage, critical measurements should be made within 72 h of casting. Replicas stored for extended periods may require a light wipe with solvent to remove any secreted fluid. Limitations. The process is limited to the detection of discontinuities on or near the surfaces of parts made of ferromagnetic metals. It can be used on nonmagnetic metals for surface topography testing only. In this application, surface conditions, tool marks, and physical dimensions will be recorded, but there will be no migration of magnetic particles. Magnetic rubber inspection is not as fast as other inspection methods, because of the time required to cure the rubber. This is of little disadvantage, however, when a large number of parts are being inspected. By the time all the regions being inspected have been prepared, poured, and magnetized, the first replicas are usually cured and ready for removal and examination. Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Procedure The conventional procedure used in magnetic rubber inspection can be divided into three steps: • Preinspection preparation of parts • Catalyzing, pouring, and magnetizing • Review and interpretation of cured replicas Preinspection preparation consists of cleaning the part of loose dirt or other contamination. It is often unnecessary to remove paint, plating, or flame-sprayed metal coating, but the removal of such coatings will often intensify any magnetic indications. Coatings thicker than 0.25 mm (0.01 in.) should always be removed. The next step is to prepare a reservoir to hold the liquid rubber on the inspection area. This is accomplished with the use of aluminum foil, aluminum or plastic tubing, and plastic tape and putty to sea] the reservoirs against leakage. Catalyzing and Pouring. The rubber inspection material must be thoroughly mixed before use to ensure a homogeneous dispersion. Black-oxide particles are included in the inspection material. A measured quantity of curing agents is stirred into the rubber, which is then transferred to the prepared reservoir. Magnetizing. Continuous or residual magnetism is then induced into the part by using permanent magnets, direct current flowing through the part, or dc yokes, coils, prods, or central conductors. Direct current yokes are preferred for most applications. Because the magnetic particles in the suspension must migrate through the rubber, the duration of magnetism is usually longer than that of the standard magnetic particle method. The minimum flux density along the surface of the test specimen is 2 mT (20 G); the higher the flux density, the shorter the required duration. Optimum durations of magnetization vary with each inspection task. Some typical examples of flux densities and durations of magnetization are given in Table 3. Table 3 Flux density and duration of magnetization for various applications of magnetic rubber inspection Flux density Type of area inspected mT G Duration of magnetization, min 5-10 50-100 Uncoated holes 2.5-5 25-50 1 Coated holes 10-60 (a) 100-600 -1 (a) 15 150 1 10 100 3 5 50 10 Uncoated surfaces 2 20 30 Coated surfaces 5-60 50-600 1-60 (a) (a) Flux density and time depend on the thickness of the coating. As in the standard magnetic particle method, cracks and other discontinuities are displayed more strongly when they lie perpendicular to the magnetic lines of force. Therefore, the magnetizing current should be applied from two directions to increase reliability. This is accomplished by magnetizing in one direction, then moving the magnetizing unit to change the field 90° and remagnetizing on the same replica. Experiments have shown that the second magnetization does not disturb particles drawn to discontinuities during the first magnetization. Review and Interpretation. Following cure, the replicas are removed from the part and examined for concentration of magnetic particles, which indicates the presence of discontinuities. This examination is best conducted with a low- power microscope (about seven to ten diameters) and a high-intensity light. During this examination, the topography of the replica is noted; tool marks, scratches, or gouges in the testpiece are revealed. Indications on a replica removed from a 16 mm ( in.) diam through hole in 24 mm ( in.) thick D-6ac low-alloy ultrahigh-strength steel plate are shown in Fig. 58. Fig. 58 Indications of discontinuities (arrows) on a magnetic rubber replica removed from a 16 mm ( in.) diam through hole in 24 mm ( in.) thick D-6ac steel plate Alternative Procedure. Another procedure used in magnetic rubber inspection involves placing a thin plastic film between the test surface and the rubber. This can be accomplished by stretching a sheet of polyvinylidene chloride over the test area and painting a thin layer of catalyzed or uncatalyzed rubber over it. The film can then be removed and examined for indications immediately following magnetization, eliminating the need to wait for the rubber to cure. In addition to providing immediate inspection results, this technique has other advantages: • No damming is required • Postinspection cleanup is easier because the rubber never directly comes into contact with the part • Uncatalyzed rubber can be reused • Catalyzed rubber can be used if a permanent record is desired The technique, however, is less sensitive than the conventional magnetic rubber inspection method and is difficult to apply to irregularly shaped surfaces. Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Use on Areas of Limited Visual Accessibility Examples of areas of limited visual accessibility that can be magnetic rubber inspected are holes and the inside surfaces of tubular components. Holes with small diameters, especially if they are threaded, are very difficult to inspect by other nondestructive methods. The deeper the hole and the smaller the diameter, the greater the problem. Liquid penetrant and magnetic particle methods are each highly dependent on the visual accessibility of the part itself; therefore, they are limited in such applications. With the use of magnetic rubber inspection, however, the visibility restriction is removed because replica castings can be taken from the inaccessible areas and examined elsewhere under ideal conditions without any visual limitations. An application for the inspection of small-diameter holes is illustrated in Fig. 59. The testpiece is a 4.0 mm ( in.) thick D-6ac steel aircraft longeron containing several groups of three nutplate holes (Fig. 59a). Each group consisted of two rivet holes 2.4 mm ( in.) in diameter and a main hole 6.4 mm ( in.) in diameter. Examination of a replica of one group of nutplate holes (Fig. 59b) revealed indications of cracks in one of the rivet holes and in the main hole. Fig. 59 Aircraft longeron (a), of 4.0 mm ( in.) thick D-6 ac steel, showing nutplate holes that were magnetic rubber inspected. (b) Cured magnetic rubber replica with indications (arrows) of cracks in the 6.4 mm ( in.) diam main hole and a 2.4 mm ( in.) diam rivet hole Blind holes present a problem in conventional magnetic particle inspection or in liquid penetrant inspection. If the part is stationary, the inspection fluid will accumulate at the bottom of the hole, preventing inspection of that area. Another problem is directing adequate light into a blind hole for viewing. Similar visibility problems restrict inspection of the inside surfaces of tubular components. The longer the component and the smaller its diameter, the more difficult it becomes to illuminate the inside surface and to see the area of interest. Magnetic particle, liquid penetrant, and borescope techniques have limited value in this type of application. Grooves, lands, and radical section changes also limit the use of ultrasonic and radiographic methods for the inspection of inside surfaces. The magnetic rubber technique, however, provides replica castings of such surfaces for examination after the replicas have been removed from the components. Some examples of this application include mortar and gun barrels, pipe, tubing, and other hollow shafts. Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Use on Coated Surfaces Coatings such as paint, plating, and flame- or plasma-sprayed metals have always presented difficulties in conventional nondestructive inspection. Liquid penetrants are unsuccessful unless discontinuities in the substrate have also broken the surface of the coating. Even then, it is difficult to determine whether a liquid penetrant indication resulted from cracks in the coating or cracks in the coating plus the substrate. Production ultrasonic techniques have been successfully used to locate discontinuities in coated flat surfaces; however, their ability to detect small cracks less than 2.54 mm (0.100 in.) long by 0.0025 mm (0.0001 in.) wide in bare or coated material is poor to marginal. Because most coatings are nonmagnetic, it is possible to use magnetic particle and magnetic rubber techniques to inspect ferromagnetic materials through the coatings. Experience has shown that conventional magnetic particle techniques also become marginal if the coating is 0.10 mm (0.004 in.) thick or greater. However, magnetic rubber inspection has the capability of producing indications through much thicker coatings. Because of the weak leakage field at the surface, the particles used in the conventional magnetic particle method are lightly attracted to the region of the discontinuity. In the magnetic rubber technique, the reduced particle attraction is compensated for by increasing the time of magnetization, up to several minutes, to ensure sufficient particle accumulation. The attracted particles remain undisturbed until the rubber is cured. Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Use on Difficult-to-Inspect Shapes or Sizes Complex structures exhibiting varying contours, radical section changes, and surface roughness present conditions that make interpretation of data obtained by radiographic, magnetic particle, liquid penetrant, or ultrasonic inspection difficult because of changing film densities, accumulation of excess fluids, and high background levels. As a result, discontinuities in such structures frequently remain undetected. The magnetic rubber process minimizes background levels on the cured replicas with little change in the intensity of any crack indications. Typical items to which magnetic rubber inspection is applicable are multiple gears, internal and external threads, and rifling grooves in gun barrels. When magnetic particle fluid is applied to a threaded area, some of the liquid is held by surface tension in the thread roots (the most likely area for cracks). This excess fluid masks defect indications, especially when the fluorescent method is used. With the magnetic rubber method, thread root cracks are displayed with little or no interfering background. Example 6: Magnetic Rubber Inspection of Spline Teeth in an Aircraft-Flap Actuator. The process applied to internal spline teeth is illustrated in Fig. 60(a), which shows an aircraft-flap actuator bracket with the magnetic rubber replica. A macroscopic view of this replica (Fig. 60b) reveals several cracks in the roots of the spline teeth. The bracket was made of 4330 steel. The spline teeth were 16 mm ( in.) long, with 24/48 pitch. Fig. 60 Magnetic rubber inspection of spline teeth in a 4330 steel bracket for an aircraft- flap actuator. (a) View of bracket with rubber replica removed. (b) Macrograph of replica showing crack indications in roots of teeth Magnetic Particle Inspection Revised by Art Lindgren, Magnaflux Corporation Magnification of Indications The examination of cast replicas under magnification permits detection of cracks as short as 0.05 mm (0.002 in.). Detection of these small cracks is often important to permit easier rework of the part prior to crack propagation. These cracks are also of interest during fatigue test monitoring. Discontinuity indications in magnetic particle inspection often result from deep scratches or tool marks on the part surface, and it is difficult to distinguish them from cracks. When the magnetic rubber replicas are viewed under magnification, the topography of the surface is easily seen, and indications from scratches and tool marks can be distinguished from crack indications. This distinction may prevent the unnecessary rejection of parts from service. [...]... Battalion Center, 1980 42 J.R Barton, J Lankford, Jr., and P.L Hampton, Advanced Nondestructive Testing Methods for Bearing Inspection, Trans SAE, Vol 81, 1972, p 681 43 F.N Kusenberger and J.R Barton, "Development of Diagnostic Test Equipment for Inspection Antifriction Bearings," AMMRL CTR 7 7-1 3, Final Report, Contract Nos DAAG4 6-7 4- C-0012 and DAAG4 6-7 5-C0001, U.S Army Materials and Mechanics Research... Lankford, Jr., and P.L Hampton, Advanced Nondestructive Testing Methods for Bearing Inspection, Trans SAE, Vol 81, 1972, p 681 F.N Kusenberger and J.R Barton, "Development of Diagnostic Test Equipment for Inspection Antifriction Bearings," AMMRL CTR 7 7-1 3, Final Report, Contract Nos DAAG4 6-7 4- C-0012 and DAAG4 6-7 5-C-0001, U.S Army Materials and Mechanics Research Center, March 1977 J.R Barton and J Lankford,... McMaster, Nondestructive Testing Handbook, Vol II, Section 30, The Ronald Press Company, 1959 H.J Bezer, Magnetic Methods of Nondestructive Testing, Part 1, Br J NDT, Sept 19 64, p 8 5-9 3; Part 2, Dec 19 64, p 10 9-1 22 F.W Dunn, Magnetic Particle Inspection Fundamentals, Mater Eval., Dec 1977, p 4 2 -4 7 C.N Owston, The Magnetic Leakage Field Technique of NDT, Br J NDT, Vol 16, 19 74, p 162 F Förster, Non-Destructive... McMaster, Nondestructive Testing Handbook, Vol II, Section 30, The Ronald Press Company, 1959 11 H.J Bezer, Magnetic Methods of Nondestructive Testing, Part 1, Br J NDT, Sept 19 64, p 8 5-9 3; Part 2, Dec 19 64, p 10 9-1 22 12 F.W Dunn, Magnetic Particle Inspection Fundamentals, Mater Eval., Dec 1977, p 4 2 -4 7 13 C.N Owston, The Magnetic Leakage Field Technique of NDT, Br J NDT, Vol 16, 19 74, p 162 14 F Förster,... Report, Contract DAAG4 6-7 6-C-0075, Army Materials and Mechanics Research Center, Sept 1977 40 M.C Moyer and B.A Dale, An Automated Thread Inspection Device for the Drill String, in Proceedings of the First National Seminar on Nondestructive Inspection of Ferromagnetic Materials, Dresser Atlas, 19 84 41 H.H Haynes and L.D Underbakke, "Nondestructive Test Equipment for Wire Rope," Report TN-15 94, Civil Engineering... Khalileev and P.A Grigor'ev, Methods of Testing the Condition of Underground Pipes in Main Pipelines (Review), Sov J NDT, Vol 10 (No 4) , July-Aug 19 74, p 43 8 -4 59 W.M Rogers, New Methods for In Place Inspection of Pipelines, in Proceedings of the 16th Mechanical Working and Steel Processing Conference (Dolton, IL), Iron and Steel Society of AIME, 19 74, p 47 1 -4 79 J.A Birdwell, F.N Kusenberger, and J.R... Khalileev and P.A Grigor'ev, Methods of Testing the Condition of Underground Pipes in Main Pipelines (Review), Sov J NDT, Vol 10 (No 4) , July-Aug 19 74, p 43 8 -4 59 36 W.M Rogers, New Methods for In Place Inspection of Pipelines, in Proceedings of the 16th Mechanical Working and Steel Processing Conference (Dolton, IL), Iron and Steel Society of AIME, 19 74, p 47 1 -4 79 37 J.A Birdwell, F.N Kusenberger, and J.R... From Prolate and Oblate Spheroidal 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Inclusions, J Appl Phys., Vol 53, 1982, p 843 7 J.H Hwang and W Lord, Magnetic Leakage Field Signatures of Material Discontinuities, in Proceedings of the Tenth Symposium on NDE (San Antonio, TX), Southwest Research Institute, 1975 J.F Bussière, On Line Measurement of the Microstructure and Mechanical... 1978 17 F Förster, Development in the Magnetography of Tubes and Tube Welds, Non-Destr Test., Dec 1975, p 30 4- 3 08 18 W Stumm, Tube Testing by Electromagnetic NDE Methods-1, Non-Destr Test., Oct 19 74, p 25 1-2 56 19 R.E Beissner, G.L Burkhardt, M.D Kilman, and R.K Swanson, Magnetic Leakage Field Calculations for Spheroidal Inclusions, in Proceedings of the Second National Seminar on Nondestructive Evaluation. .. Report, Contract DAAG4 6-7 6-C-0075, Army Materials and Mechanics Research Center, Sept 1977 M.C Moyer and B.A Dale, An Automated Thread Inspection Device for the Drill String, in Proceedings of the First National Seminar on Nondestructive Inspection of Ferromagnetic Materials, Dresser Atlas, 19 84 H.H Haynes and L.D Underbakke, "Nondestructive Test Equipment for Wire Rope," Report TN-15 94, Civil Engineering . 5-1 0 5 0-1 00 Uncoated holes 2. 5-5 2 5-5 0 1 Coated holes 1 0-6 0 (a) 10 0-6 00 -1 (a) 15 150 1 10 100 3 5 50 10 Uncoated surfaces 2 20 30 Coated surfaces 5-6 0 5 0-6 00 1-6 0 (a) . penetrate into such a mass of parts and because only a few parts on the outside edges will be demagnetized (and these possibly only partially demagnetized). Small parts can be demagnetized in. covers dark- and light-colored test surfaces equally well. Consequently, the contrast between indication and background is independent of the test-surface color. In contrast to dry magnetic particles,