Par 5-50 Page 5-30 indications. Lack of indications will require remagnetization to a higher level, more care in applying the particles, or both.
(6) 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.
(7) 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.
5-51. DEMAGNETIZATION AND POST-EXAMINATION CLEANING. Parts should be magnetized longitudinally last before demagnetizing. 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.
a. 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 deflection of the field indicator needle.
(1) 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.
(2) 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.
b. Post-Examination Cleaning.
(1) When oil suspensions are used, solvent clean or remove the part until all magnetic particles and traces of oil are removed.
(2) 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.
Par 5-51 Page 5-31
(3) For cadmium-plated parts an air water vapor blast may be used to remove any remaining magnetic particle residue.
(4) After final cleaning and drying, use temporary protective coatings, when necessary, to prevent corrosion.
(5) After magnetic particle examination has been completed, restore any removed finishes according to the manufacturer’s repair manual.
5-52.–5-59. [RESERVED.]
Par 5-60 Page 5-32 SECTION 5. PENETRANT INSPECTION
5-60. GENERAL.
a. 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.)
b. The penetrant method does not depend upon ferromagnetism 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.
NOTE: Visible penetrant is often used interchangeably with fluorescent penetrant. However the chemicals within most common red dye penetrants will neutralize the fluorescence of the chemicals used in that method and possibly mask a defect. Fluorescent Penetrant Inspection (FPI) should not be accomplished on parts that have been previously inspected with visible dye.
Type II (visible dye) should not be used for final acceptance of aerospace products and should not be used prior to a type I (FPI) inspection of the same surface.
c. 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 excessive 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.
Par 5-60 Page 5-33 FIGURE 5-13. PENETRANT AND DEVELOPER ACTION 5-
61. 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 oil-base 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.)
Par 5-61 Page 5-34 TABLE 5-3. CLASSIFICATION OF PENETRANT INSPECTION MATERIALS COVERED BY MIL-I-25135E
PENETRANT SYSTEMS DEVELOPERS SOLVENT REMOVERS Type I Fluorescent Dye Form a Dry Powder Class (1) Halogenated (chlorinated) Type II Visible Dye Form b Water Soluble Class (2) Nonhalogenated (nonchlorinated) Type III Visible and Fluorescent Dye (dual mode) Form c Water Suspendible Class (3) Specific Application Form d Nonaqueous Form e Specific Application
Method A Water Washable
Method B Post Emulsifiable, Lipophilic
Method C Solvent Removable
Method D Post Emulsifiable, Hydrophilic
Sensitivity Level ½ Ultralow Sensitivity Level 1 Low Sensitivity Level 2 Medium Sensitivity Level 3 High Sensitivity Level 4 Ultrahigh
5-62. 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.
TABLE 5-4. FLUORESCENT AND VISIBLE PENETRANT INSPECTION GENERAL
PROCESSING PROCEDURES FLOWSHEET Penetrant Inspection-General Processing Incoming parts Alkaline Steam Vapor degrease Solvent wash Acid Etch
PRECLEAN Machanical Paint stripper JItrasonic Detergent Apply water Apply solvent Apply post PENETRATE washable removable emulsifiable penetrant penetrant penetram Apply Apply emulsifier remover REMOVE Water wash Solvent w pe-off Pre-rinse Dry (aqueous) Developer (aqueous) Developer DEVELOP Developer Developer (dry or non Developer (dry or non Dry aqueous (dry or non aqueous aqueous) Inspect INSPECT Г Water Detergen Mechanical wash
POST CLEAN Vapor degrease Solvent soak Ultrasonic clean Outgoing parts Par 5-62 Page 5-35
Par 5-62 Page 5-36 TABLE 5-5. PRE-CLEANING METHODS FOR PENETRANT INSPECTION
METHOD USE Mechanical Methods Abrasive tumbling Removing light scale, burrs, welding flux, braze stop-off, 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, stop-off, 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 scale, flux, and stop-off (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 oils, 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 stop-off, 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 degreasing except a hand operation; may employ nonhalogenated (nonchlorinated) solvents; used for localized low-volume cleaning.
Par 5-62 Page 5-37 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.
5-63. CLEANERS AND APPLICATIONS. Use a cleaner to remove contaminants 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.
a. Detergents. Detergent cleaners are water-based chemicals called surfactants, which surround and attach themselves to particles of contaminants allowing them to be washed away.
b. Solvents. Solvents dissolve contaminants 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.
c. Alkalis. Alkaline cleaners are water solutions of chemicals that remove contaminants by chemical action or displacement rather than dissolving the contaminant.
d. Paint Removers. The general types of removers used for conventional paint coatings are solvent, bond release, and disintegrating.
e. Salt Baths. Molten salt baths are used in removing heavy, tightly-held scale and oxide from low alloy steels, nickel, and cobalt-based alloys, and some types of stainless steel. They cannot be used on aluminum, magnesium, or titanium alloys.
f. 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.
g. Etching Chemicals. Etching chemicals contain a mixture of acids or alkalis 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.
h. 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.
(1) 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 solvent-moistened cloth. The removal method is determined by the type of penetrant used.
Par 5-63 Page 5-38
(2) 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.
i. 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.
j. Inspection for Discontinuities. After the penetrant has sufficiently developed, visually inspect the surface for indications from discontinuities. Evaluate each indication observed to determine if it is within acceptable limits. Visible penetrant inspection is performed in normal visible white light, whereas FPI is performed in black (ultraviolet) light.
k. Post-Cleaning. Remove inspection material residues from parts after completion 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.
5-64. 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 E1417. 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.
TABLE 5-6. LISTING OF COMMONLY ACCEPTED STANDARDS AND
SPECIFICATIONS FOR PENETRANT INSPECTION
ASTM STANDARDS TITLE ASTM E165 Standard Practice for Liquid Penetrant Inspection Testing for General Industry.
ASTM E1135 Standard Test Method for Comparing the Brightness of Fluorescent Penetrants.
ASTM E1208 Standard Test Method for Fluorescent Liquid Penetrant Examination Using the Lipophilic Post-Emulsification Process.
ASTM E1209 Standard Practice for Fluorescent Liquid Penetrant Testing Using the Water-Washable Process.
ASTM E1210 Standard Practice for Fluorescent Liquid Penetrant Testing Using the Hydrophilic Post-Emulsification Process.
Par 5-64 Page 5-39 ASTM E1219 Standard Process for Fluorescent Liquid Penetrant Testing Using the Solvent-Removable Process.
ASTM E1220 Standard Practice for Visible Penetrant Testing Using the Solvent-Removable Method.
ASTM E1316 Standard Terminology for Nondestructive Examinations.
ASTM E1417/E1417M Standard Practice for Liquid Penetrant Testing.
ASTM D2512 Standard Test Method for Compatibility of Materials with Liquid Oxygen (Impact Sensitivity Threshold and Pass-Fail Techniques).
SAE-AMS
SPECIFICATIONS TITLE AMS 2647 Fluorescent Penetrant Inspection Aircraft Structures and Engine Component Maintenance.
US GOVERNMENT SPECIFICATIONS MIL-STD-271 Requirements for Nondestructive Testing Methods.
MIL-STD-410 Nondestructive Testing Personnel Qualifications and Certifications.
MIL-STD-1907 Inspection, Liquid Penetrant and Magnetic Particle, Soundness Requirements for Materials, Parts and Welds.
MIL-STD-6866 Inspection, Liquid Penetrant.
MIL-HDBK-728/1 Nondestructive Testing.
MIL-HDBK-728/3 Liquid Penetrant Testing.
MIL-I-25135E Inspection Materials, Penetrants.
Qualified Products List (QPL)-25135 Inspection Materials, Penetrants.
T. O. 33B-1-1 Technical Manual: Nondestructive Inspection Methods, Basic Theory.
OTHER PUBLICATIONS SNT-TC-1A Personnel Qualification and Certification in Nondestructive Testing.
ATA Spec 105 Guidelines for Training and Qualifying Personnel in Nondestructive Methods.
Metals Handbook, Ninth Edition, Volume 17 Nondestructive Evaluation and Quality Control.
5-65.–5-72. [RESERVED.]
Par 5-73 Page 5-40 SECTION 6. RADIOGRAPHY (X-RAY) INSPECTION
5-73. GENERAL. Radiography (x-ray) is an NDI method used to inspect material and components, using the concept of differential adsorption of penetrating radiation. Each specimen under evaluation will have differences in density, thickness, shapes, sizes, or absorption characteristics, thus absorbing different amounts of radiation. The unabsorbed radiation that passes through the part is recorded on film, fluorescent screens, or other radiation monitors.
Indications of internal and external conditions will appear as variants of black/white/gray contrasts on exposed film, or variants of color on fluorescent screens. (See Figure 5-14.)
FIGURE 5-14. RADIOGRAPHY 5-
74. LIMITATIONS. Compared to other nondestructive methods of inspection, radiography is expensive. Relatively large costs and space allocations are required for a radiographic laboratory. Costs can be reduced considerably when portable x-ray or gamma-ray sources are used in film radiography and space is required only for film processing and interpretation.
Operating costs can be high because sometimes as much as 60 percent of the total inspection time is spent in setting up for radiography. With real-time radiography, operating costs are usually much lower, because setup times are shorter and there are no extra costs for processing or interpretation of film.
5-75. FILM OR PAPER RADIOGRAPHY. In film or paper radiography, a two dimensional (2D) latent image from the projected radiation is produced on a sheet of film or paper that has been exposed to the unabsorbed radiation passing through the test piece. This technique requires subsequent development of the exposed film or paper so that the latent image becomes visible for viewing.
Par 5-76 Page 5-41
5-76. REAL-TIME RADIOGRAPHY. A 2D image that can be immediately displayed on a viewing screen or television monitor. This technique converts unabsorbed radiation into an optical or electronic signal which can be viewed immediately or can be processed with electronic or video equipment.
5-77. ADVANTAGE OF REAL-TIME RADIOGRAPHY OVER FILM RADIOGRAPHY. The principal advantage of real-time radiography over film radiography is the opportunity to manipulate the test piece during radiographic inspection. This capability allows the inspection of internal mechanisms and enhances the detection of cracks and planar defects by allowing manipulation of the part to achieve the best orientation for flaw detection.
Part manipulation during inspection also simplifies three dimensional (3D) dynamic imaging for the determination of flaw location and size. In film radiography, depth parallel to the radiation beam is not recorded. Consequently, the position of a flaw within the volume of a test piece cannot be determined exactly with a single radiograph. To determine flaw location and size more exactly with film radiography, other film techniques such as stereo-radiography, triangulation, or simply making two or more film exposures with the radiation beam being directed at the test piece from a different angle for each exposure, must be used.
5-78. COMPUTED TOMOGRAPHY (CT). CT is another important radiological technique with enhanced flaw detection and location capabilities. Unlike film and real-time radiography, CT involves the generation of cross-sectional views instead of a planar projection. The CT image is comparable to that obtained by making a radiograph of a physically sectioned thin planar slab from an object. This cross-sectional image is not obscured by overlying and underlying structures and is highly sensitive to small differences in relative density. CT images are also easier to interpret than radiographs.
5-79. USES OF RADIOGRAPHY. Radiography is used to detect the features of a component or assembly that exhibit a difference in thickness or density as compared to surrounding material.
Large differences are more easily detected than small ones. In general, radiography can detect only those features that have an appreciable thickness in a direction along the axis of the radiation beam. Therefore, the ability of radiography to detect planar discontinuities, such as cracks, depends on proper orientation of the test piece during inspection. Discontinuities which have measurable thickness in all directions, such as voids and inclusions, can be detected as long as they are not too small in relation to section thickness. In general, features that exhibit a 2 percent or more difference in radiation adsorption compared to the surrounding material can be detected.
5-80. COMPARISON WITH OTHER NDI METHODS. Radiography and ultrasonic are the two generally-used, NDI methods that can satisfactorily detect flaws that are completely internal and located well below the surface of the test part. Neither method is limited to the detection of specific types of internal flaws. However, radiography is more effective when the flaws are not planar, while ultrasonic is more effective when flaws are planar. In comparison to other generally-used NDI methods (e.g., magnetic particle, liquid penetrant, and eddy current inspection), radiography has the following advantages:
• The ability to inspect for both internal and external flaws.
• The ability to inspect covered or hidden parts or structures.