9/8/98 Longitudinal. Longitudinal magnetization is induced in a part by placing the part in a strong magnetic field, such as the center of a coil or between the poles of an electromagnetic yoke. (See figure 5-12.) When using a coil, optimum results are obtained when the following conditions are met.
The part to be examined is at least twice as long as it is wide.
The long axis of the part is parallel to the axis of the coil opening.
AC 43.13-1B The area of the coil opening is at least 10 times the cross-sectional areas of the part.
The part is positioned against the inner wall of the coil.
Three to five turns are employed for hand-held coils formed with cables.
For the 10-to-1 fill factor, the effective region of inspection is 1 coil radius on either side of the coil with 10 percent overlap. (Refer to ASTME-1444.)
•Magnetizing current Cable• -Coll -Crack May be permanent or Path of flexible cable. Current.
magnetizing Currentcurrent . Magnetic field- Coil shot - Magnetic lines of force Cable wrap Transverse crack will show Current •Yoke - Longitudinal crack will - 45° crack will show not show Longitudinal magnetic Weld field Yoke
COIL SHOT The usual way to longitudinally magnetize a part is to place the part lengthwise inside the bottom of the coil. Multiple inspections are necessary on long parts because the effective field extends only 6 to 9 inches on either side of
- Test article
CABLE WRAP Cable wrapping a coil around large or heavy parts is a common practice. Cable length must be kept as short as practical to minimize cable-resistance loss and aid in obtaining higher current amperages.
Normally, three to five turns are sufficient.
Essentially a yoke is a temporary horseshoe magnet.
It is made of low-retentivity iron that is magnetized by a small coil wound around its horizontal bar. Yokes are recommended for parts subject to arc burns and for spot inspection of large parts.
FIGURE 5-12. Longitudinal magnetization.
Par 5-47 Page 5-208
Par 5-48 Page 5-209 The intensity of the longitudinal shots is kept just below the level at which leakage fields develop across sharp changes of section, such as radii under bolt heads, threads, and other sharp angles in parts. This does not apply when checking chrome-plated parts for grinding cracks.
For example, longitudinal magnetization of a round steel bar would be produced by placing the DC coil around the bar. After application of the magnetic particles, either during or subsequent to magnetization, discontinuities perpendicular to the longitudinal axis of the bar would be disclosed.
When a yoke is used, the portion of the part between the ends of the yoke completes the path of the magnetic lines of force. This results in a magnetic field between the points of contact.
Permanent Magnets and Electromagnetic Yoke. The stability of the magnetic field generated by permanent magnets requires some agitation of the oxide particles within the field. The wet method is considered most satisfactory. Use a well-agitated plastic squirt bottle for the most effective application of the magnetic particle suspension. When the direc tion of possible cracks in a suspect area is not known, or would not necessarily be normal to the lines of for ce between the poles of the magnet, reposition the magnet to the best advantage and recheck. Usually, two shots, 90 degrees apart are required. The part must be demagnetized between each magnetization when the field direction is changed unless the next shot is at least 10 percent stronger than the previous shot, if this is the case demagnetization is not necessary.
DETERMINATION OF FIELD STRENGTH.
Factors such as part size, shape, magnetic properties of the material, and the method of magnetization will affect the field strength induced within a part by a given applied magnetizing force. The factors vary considerably, making it difficult to establish rules for magnetizing during examination.
Technique requirements are best determined on actual parts having known defects.
A magnetization indicator, such as a Quantitative Quality Indicator (QQI), shoul d be used to verify that adequate magnetic flux strength is being used. It effectively indicates the in ternally-induced field, the field direction, and the quality of particle suspension during magnetization.
The level of magnetization required for detection of service-related defects in most cases can be lower than that required for material and manufacturing control.
Contact the manufacturer for correct specifications.
NOTE: If the examination must be performed with less current than is desired because of part size or equipment limitations, the lower field strengt h can be partially accommodated by reducing the area of examination for each magnetization, or the examination can be supplemented by using electromagnetic yokes. Examine only 4 inches on either side of a coil instead of 6, or apply additional magnetization around the periphery of a hollow cylinder when using an internal conductor.
SPECIAL EXAMINATION TECHNIQUES.
Magnetic Rubber. Magnetic rubber formulations using finely divided magnetic particles in a silicone rubber base are used for the inspection of screw, bolt, or other bore holes, which are not easily accessible. The liquid silicone rubber mixture is poured into holes in magnetic parts to be inspected.
Par 5-48 Page 5-210 Curing time for silicone rubbers varies from about 30 minutes and up depending upon the particular silicone rubber, the catalyst, and the amount of catalyst used to produce the curing reaction.
Curing. While curing is taking place, the insides of the hole must be maintained in the required magnetized state. This can be accomplished using a permanent magnet, a DC yoke, an electromagnet, or some other suitable means. Whatever method of magnetization is used, the leakage fields at any discontinuities inside the holes must be maintained long enough to attract and hold in position the magnetic particles until a partial cure takes place. A two-step magnetizing procedure has been developed.
The first magnetization is accomplished for a short time in one direction followed by a second at 90 degrees to the first for the same length of time. This procedure must be repeated for whatever period of time is needed until the cure prevents particle mobility. Magnetization in two directions 90 degrees apart ensures formation of indications at discontinuities in all directions inside the holes.
After curing, the rubber plugs, which are exact replicas of the holes, are removed and visibly examined for indications which will appear as colored lines against the lighter colored background of the silicone rubber. Location of any discontinuities or other surface imperfections in the holes can be determined from the location of the indications on the plugs. The magnetic rubber inspection method is covered in detail in Air Force Technical Order 33B-1-1, section XI.
Critical Examination for Sharp Radii Parts. A critical examination is required for cracks in sharp radii;
such as threaded parts, sp lines, gear teeth roots, and abrupt changes in sections, that cause obscuring and nonrelevant indications during normal examination practices. The procedure provided herein is the most sensitive method for detecting the early beginnings of in-service fatigue cracks in the sharp, internal radii of ferro-magnetic parts. Magnetic particle examination equipment may be used; however, alternating fi elds are not reliable to provide the necessary high level of residual magnetism.
Optical aids are necessary to realize the maximum sensitivity provided by this magnetic particle procedure.
Low-power (10x-30x) binocular microscopes are recommended. As a minimum, pocket magnifiers of 7 to 10 power may be used with the following procedure.
Thoroughly clean the part at the sharp radii and fillets where soils, greases, and other contaminants tend to accumulate and at other places where they might be overlooked during a casual or hasty examination.
The residual method should be used as an aid in particular problem areas, even though it is not considered the best practice in most of the instances. The conventional wet continuous methods should be used initially for overall examination and the residual technique should be applied only for supplemental, local examination of the sharp radii. It should not be applied except in those cases where nonrelevant indications have proven to be a problem in the initial examination.
Methods of magnetization should be done according to standard procedures; however, alternating fields should not be used, and the level of magnetizing force imposed should usually be increased above the normal levels to ensure a higher residual field within the part.
Following magnetization, apply particles in liquid suspension. The application should be liberal and in a manner to cause maximum particle buildup. Immersion of
Par 5-50 Page 5-211 small parts such as rod end fittings in a container of suspension, which has just been stirred for about 30 seconds, is an excellent method.
Check for the presence of particle accumulation in the sharp radii. It is necessary that the level of magnetization and the particle application result in the formation of nonrelevant indications. Lack of indications will require remagnetization to a higher level, more care in applying the particles, or both.
Wash the parts in a clean suspension vehicle only enough to remove the weakly-held particle accumulations causing the nonrelevant indications. Particles at true cracks will be more strongly held and will persist if the washing is gently done. This can be accomplished by flowing or directing a stream of liquid vehicle over the part, or for a small component, by gently stirring in a container of the vehicle. Closely observe the removal of the nonrelevant particle accumulations in the region to be examined to avoid excessive washing. If washing is prolonged beyond the minimum needed to remove the nonrelevant indications, the small defect indications may also be washed away.
A few trials will help to develop the best method and time required for washing.
Check for crack indications with optical magnification and ample lighting. The smaller indications that are attainable by this procedure cannot be reliably seen or evaluated with the unaided eye.
DEMAGNETIZATION AND POST- EXAMINATION CLEANING. Parts should be magnetized longitudinally last before demagnetizing.
NOTE: Circular magnetism cannot be read with a field meter since it is an internal magnetic field. However, if the last shot, was a coil shot the meter can read it if a magnetic field is present.
Demagnetization. Demagnetize between successive magnetization of the same part, to allow finding defects in all directions, and whenever the residual magnetism interferes with the interpretation of the indications.
Also, demagnetize all parts and materials after completion of magnetic particle examination. Test all parts at several locations and parts for residual magnetism of complex configuration at all significant changes in geometry. Repeat demagnetization if there is any appreciable deflecti on of the field indicator needle.
AC method. Hold the part in the AC demagnetizing coils and then move the part slowly and steadily through the coils and approximately 3 to 4 feet past the coils.
Repeat this process until the part loses its residual magnetism. Rotate and tumble parts of complex configuration as they are passed through the coils.
DC method. Place the part in the same relative position as when magnetized and apply reversing DC current. Gradually reduce the current to zero and repeat the process until the residual magnetic field is depleted.
Post-Examination Cleaning.
When oil suspensions are used, solvent clean or remove the part until all magnetic particles and traces of oil are removed.
Par 5-60 Page 5-212 When parts or materials have been examined using water suspension methods, completely remove the water by any suitable means, such as an air blast, to ensure that the parts are dried immediately after cleaning. Thoroughly rinse the part with a detergent-base cleaner until all magnetic particles are removed. Then rinse in a solution of water and rust inhibitor.
For cadmium-plated parts an air-water vapor blast may be used to remove any remaining magnetic particle residue.
After final cleaning and drying, use temporary protective coatings, when necessary, to prevent corrosion.
Par 5-60 Page 5-213 After magnetic particle examination has been comple ted, restore any removed finishes according to the manufacturer’s repair manual.
NOTE: Visible penetrant is often used interchangeab ly by NDI personnel with fluorescent penetrant.
However, the chemical within most common red dye penetrants will neutralize the fluorescence of the chemicals used in that method. Therefore, a thorough cleaning of all magnetic particles is mandatory.
5-52.5-59. [RESERVED.]
Par 5-60 Page 5-214 SECTION 5. PENETRANT INSPECTION
GENERAL. Penetrant inspection is used on nonporous metal and nonmetal components to find material discontinuities that are open to the surface and may not be evident to normal visual inspection. The part must be clean before performing a penetrant inspection. The basic purpose of penetrant inspection is to increase the visible contrast between a discontinuity and its background. This is accomplished by applying a liquid of high penetrating power that enters the surface opening of a discontinuity. Excess penetrant is removed and a developer material is then applied that draws the liquid from the suspected defect to reveal the discontinuity.
The visual evidence of the suspected defect can then be seen either by a color contrast in normal visible white light or by fluorescence under black ultraviolet light. (See figure 5-13.)
The penetrant method does not depend upon ferro-magnetism like magnetic particle inspection, and the arrangement of the discontinuities is not a factor.
The penetrant method is effective for detecting surface defects in nonmagnetic metals and in a variety of nonmetallic materials. Penetrant inspection is also used to inspect items made from ferromagnetic steels and its sensitivity is generally greater than that of magnetic particle inspection. Penetrant inspection is superior to visual inspection but not as sensitive as other advanced forms of tests for detection of in-service surface cracks.
The major limitations of the penetrant inspection is that it can detect only those discontinuities that are open to the surface; some other method must be used for detecting subsurface defects. Surface roughness or porosity can limit the use of liquid penetrants. Such surfaces can produce excessi ve background indications and interfere with the inspection. Penetrant inspection can be used on most airframe parts and assemblies accessible to its application. The basic steps to perform penetrant inspections are briefly described in the following paragraphs.
EQUIPMENT USED IN THE PENETRANT INSPECTION PROCESS. Equipment varies from simple aerosol cans used in portable systems to fully automated computer-controlled systems. Whether fluorescent or visible penetrants are used, different penetrant bases are available but may require different cleaning methods. Water-washable penetrants can often be removed by a simple water washing process, whereas oil-base penetrants may require special solvents for removal. Some oilbase penetrants have emulsifiers, either added to the penetrant before it is applied or added afterwards, that allow water washing to be used. Developers used, can be applied either by a wet or dry bath. Therefore, each penetrant inspection process may require different cleaning facilities and procedures. (See table 5-3.)
BASIC STEPS TO PERFORM PENETRATION INSPECTION. Table 5-4 shows a general process, in the procedures flow sheet, for commonly used penetrant inspection processes. It is important to ensure that parts are thoroughly cleaned and dried before doing penetrant inspection. All surfaces to be inspected should be free of contaminants, paint, and other coatings that could prevent penetrant from entering discontinuities.
Table 5-5 shows applications of various methods of precleaning for penetrant inspection.
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FIGURE 5-13. Penetrant and developer action.
TABLE 5-3. Classification of penetrant inspection materials covered by MIL-I-25135E.
PENETRANT SYSTEMS
DEVELOPERS
SOLVENT REMOVERS
Type I Type II Type III
Fluorescent Dye Visible Dye Visible and Fluorescent Dye (dual mode)
Form a Form b Form c Form d Form e
Dry Powder Water Soluble Water Suspendible Nonaqueous Specific Application
Class (1)
Class (2)
Class (3)
Halogenated (chlorinated) Nonhalogenated (nonchlorinated) Specific Application
Method A Method B
Method C Method D
Water Washable Post emulsifiable, Lipophilic Solvent Removable Post Emulsifiable, Hydrophilic
Sensitivity Level 1/2 Sensitivity Level 1 Sensitivity Level 2 Sensitivity Level 3 Sensitivity Level 4
Ultralow Low Medium High Ultrahigh
TABLE 5-4. Fluorescent and visible penetrant inspection general processing procedures flowsheet.
Penetrant Inspection-General Processing Incoming parts Alkaline Steam /apor degrease Solvent wash Acid Etch
PRECLEAN Mechanical Paint stripper Ultrasonic Detergent Apply water Apply solvent Apply post PENETRATE was hable removable emulsifiable penetrant penetrant penetrant Apply Apply emulsifier remover REMOVE Water wash Solvent wipe-off • Pre-rinse Developer Developer (aqueous) (aqueous) DEVELOP Developer Developer (dry or non Developer (dry or non Dry aqueous) (dry or non aqueous aqueous) Inspect
INSPECT J Water rinse Detergent Mechanical wash
POST CLEAN Vapor degrease Solvent soak Ultrasonic clean Outgoing parts Par 5-62 Page 5-216
Par 5-62 Page 5-217 TABLE 5-5. Pre-cleaning methods for penetrant inspection.
METHOD USE Mechanical Methods
Abrasive tumbling
Removing light scale, burrs, welding flux, braze stopoff, rust, casting mold, and core material; should not be used on soft metals such as aluminum, magnesium, or titanium.
Dry abrasive grit blasting Removing light or heavy scale, flux, stopoff, rust, casting mold and core material, sprayed coatings, carbon deposits: In general, any brittle deposit. Can be fixed or portable (may peen metal over defect).
Wet abrasive grit blast Same as dry except, where deposits are light, better surface and better control of dimensions are required.
Wire brushing Removing light deposits of sca le, flux, and stopoff (may mask defect by displacing metal).
High pressure water and steam Ordinarily used with an alkaline cleaner or detergent; removing typical machine shop contamination, such as cutting o ils, polishing compounds, grease, chips, and deposits from electrical discharge machining; used when surface finish must be maintained.
Ultrasonic cleaning Ordinarily used with detergent and water or with a solvent; removing adhering shop contamination from large quantities of small parts.
Chemical Methods
Alkaline cleaning
Removing braze stopoff, rust, scale, oils, greases, polishing material, and carbon deposits; ordinarily used on large articles where hand methods are too laborious;
also used on aluminum for gross metal removal.
Acid cleaning Strong solutions for removing heavy scale; mild solutions for light scale; weak (etching) solutions for removing lightly smeared metal.
Molten salt bath cleaning Conditioning and removing heavy scale; not suitable for aluminum, magnesium, or titanium.
Solvent Methods
Solvent wiping Same as for vapor degreas ing except a hand operation; may employ nonhalogenated (nonchlorinated) solvents; used for localized low-volume cleaning.
Par 5-62 Page 5-218 CAUTION: Improper cleaning methods can cause severe damage or degradation of the item being cleaned. Personnel must select and apply cleaning processes in accordance with aircraft, engine, propeller, or appliance manufacturer’s recommendations.
CLEANERS. Contaminants are removed from parts prior to the application of penetrant inspection materials. After the inspection is completed, penetrant inspection material residues are removed. The following cleaners are commonly used during the penetrant inspection process.
Detergents. Detergent cleaners are water-based chemicals called surfactants, which surround and attach themselves to particles of contaminants allowing them to be washed away.
Solvents. Solvents dissolve cont aminants such as oils, greases, waxes, sealants, paints, and general organic matter so they can easily be wiped away or absorbed on a cloth. They are also used to remove Method C penetrant material prior to developer application.
Alkalies. Alkaline cleaners are water solutions of chemicals that remove contaminants by chemical action or displacement rather than dissolving the contaminant.
Paint Removers. The general types of removers used for conventional paint coatings are solvent, bond release, and disintegrating.
Salt Baths. Molten salt baths are used in removing heavy, tightly-held scale and oxide from low alloy steels, nickel, and cobalt-base alloys, and some types of stainless steel. They cannot be used on aluminum, magnesium, or titanium alloys.
Acids. Solutions of acids or their salts are used to remove rust, scale, corrosion products, and dry shop contamination. The type of acid used and its concentration depends on the part material and contaminants to be removed.
Etching Chemicals. Etching chemicals contain a mixture of acids or alkalies plus inhibitors. They are used to remove a thin layer of surface material, usually caused by a mechanical process, that may seal or reduce the opening of any discontinuities. The type of etching solution used depends on the part material and condition.
Penetrant Application. Apply the penetrant by spraying, brushing, or by completely submerging the part in a container of penetrant. Wait the recommended amount of time after the penetrant has been applied to allow it to enter any discontinuities
Removal of Excess Penetrant. Excess penetrant must be removed from the part’s surface to prevent a loss of contrast between indications of discontinuities and the background during the inspection. Removal may require actually washing or spraying the part with a cleansing liquid, or may simply require wiping the part clean with a solventmoistened cloth. The removal method is determined by the type of penetrant used.
Drying. If removal of the excess penetrant involves water or other cleaning liquids, drying of the part may be required prior to developer application. When drying is required, the time can be decreased by using ovens or ventilation systems.
Developer Application. Apply developer after excess penetrant is removed and, where required, the surface is dried. Apply the developer in a thin uniform layer over the surface to be inspected. Developer acts like a blotter to assist the natural capillary action bleed-out of the penetrant from discontinuities and to spread the penetrant at discontinuity surface edges to enhance bleed-out indications. After the developer is applied, allow sufficient time for the penetrant to be drawn out of any discontinuities. Follow the manufacturer’s recommendations.
Inspection for Discontinuities. After the penetrant has sufficiently developed, visu ally inspect the surface for indications from disconti nuities. Evaluate each indication observed to determine if it is within acceptable limits. Visible penetrant inspection is performed in normal visible white light, whereas fluorescent penetrant inspection is performed in black (ultraviolet) light.
Post-Cleaning. Remove inspection material residues from parts after completion
Par 5-62 Page 5-219 of penetrant inspection. This residue could interfere with subsequent part processing, or if left on some alloys, it could increase their susceptibility to hydrogen embrittlement, intergranular corrosion, and stress corrosion during service.
TECHNICAL STANDARDS. Two of the more generally accepted aerospace industry standards are the MIL-I-25135E, Inspection Materials, Penetrants (see table 5-6) and ASTM-E-1417. The penetrant materials specification (MIL-I-25135E) is used to procure penetrant materials and the process control specification (MIL-STD-6866) is used to establish minimum requirements for conducting a penetrant inspection. Table 5-6 provides a partial listing of commonly-accepted standards and specifications for penetrant inspection.