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.
Par 5-80 Page 5-42
• The ability to detect significant variations in composition.
• Provides a permanent recording of raw inspection data.
5-81. FLAWS. Certain types of flaws are difficult to detect by radiography. Cracks cannot be detected unless they are essentially along the axis of the radiation beam. Tight cracks in thick sections may not be detected at all, even when properly oriented. Minute discontinuities such as:
inclusions in wrought material, flakes, microporosity, and microfissures may not be detected unless they are sufficiently segregated to yield a detectable gross effect. Delaminations are nearly impossible to detect with radiography. Because of their unfavorable orientation, delaminations do not yield differences in adsorption that enable laminated areas to be distinguished from delaminated areas.
5-82. FIELD INSPECTION. The field inspection of thick sections can be a time consuming process, because the effective radiation output of portable sources may require long exposure times of the radiographic film. This limits field usage to sources of lower activity that can be transported. The output of portable x-ray sources may also limit field inspection of thick sections, particularly if a portable x-ray tube is used. Portable x-ray tubes emit relatively low-energy (300 kiloelectron volt (keV)) radiation and are limited in the radiation output. Both of these characteristics of portable x-ray tubes combine to limit their application to the inspection of sections having the adsorption equivalent of 75 millimeters (mm) (3 inches) of steel maximum. Portable linear accelerators and betatrons that provide high-energy (> 1 megaelectron volt (MeV)) X-rays can be used for the radiographic field inspection of thicker sections.
5-83. SAFETY. Radiographic safety requirements can be obtained from; the Original Equipment Manufacturer’s (OEM) manual, FAA requirements, cognizant FAA Aircraft Certification Office (ACO) engineers, and radiation safety organizations such as the Nuclear Regulatory Commission (NRC). Information in radiation safety publications can be used as a guide to ensure that radiation exposure of personnel involved in radiographic operations is limited to safe levels, and to afford protection for the general public.
5-84.–5-88. [RESERVED.]
Par 5-89 Page 5-43 SECTION 7. ULTRASONIC INSPECTION
5-89. GENERAL. Ultrasonic inspection is an NDI technique that uses sound energy moving through the test specimen to detect flaws. The sound energy passing through the specimen will be displayed on a cathode ray tube (CRT), a liquid crystal display (LCD) computer data program, or video/camera medium. Indications of the front and back surface and internal/external conditions will appear as vertical signals on the CRT screen or nodes of data in the computer test program. There are three types of display patterns: “A” scan, “B” scan, and “C” scan. Each scan provides a different picture or view of the specimen being tested. (See Figure 5-15.)
FIGURE 5-15. ULTRASOUND 5-
90. SOUND REFLECTION. The amount of reflection that occurs when a sound wave strikes an interface depends largely on the physical state of the materials forming the interface and to a lesser extent on the specific physical properties of the material. For example: sound waves are almost completely reflected at metal/gas interfaces; and partial reflection occurs at metal/liquid or metal/solid interfaces.
Par 5-91 Page 5-44
5-91. ULTRASONIC INSPECTION TECHNIQUES. Two basic ultrasonic inspection techniques are employed: pulse-echo and through-transmission. (See Figure 5-16.)
a. Pulse-Echo Inspection. This process uses a transducer to both transmit and receive the ultrasonic pulse. The received ultrasonic pulses are separated by the time it takes the sound to reach the different surfaces from which it is reflected. The size (amplitude) of a reflection is related to the size of the reflecting surface. The pulse-echo ultrasonic response pattern is analyzed on the basis of signal amplitude and separation.
b. Through-Transmission Inspection. This inspection employs two transducers, one to generate and a second to receive the ultrasound. A defect in the sound path between the two transducers will interrupt the sound transmission. The magnitude (the change in the sound pulse amplitude) of the interruption is used to evaluate test results. Through-transmission inspection is less sensitive to small defects than is pulse-echo inspection.
FIGURE 5-16. PULSE-ECHO AND THROUGH-TRANSMISSION ULTRASONIC
INSPECTION TECHNIQUES 5-
92. FLAW DETECTION. Ultrasonic inspection can easily detect flaws that produce reflective interfaces. Ultrasonic inspection is used to detect surface and subsurface discontinuities, such as: cracks, shrinkage cavities, bursts, flakes, pores, delaminations, and porosity. It is also used to measure material thickness and to inspect bonded structure for bonding voids. Ultrasonic inspection can be performed on raw material, billets, finished, and semi-finished materials, welds, and in-service assembled or disassembled parts. Inclusions and other nonhomogeneous areas can also be detected if they cause partial reflection or scattering of the ultrasonic sound waves or produce some other detectable effect on the ultrasonic sound waves. Ultrasonic inspection is one of the more widely-used methods of NDI.
Par 5-93 Page 5-45
5-93. BASIC EQUIPMENT. Most ultrasonic inspection systems include the following basic equipment; portable instruments (frequency range 0.5 to 15 MHz), transducers (longitudinal and shear wave), positioners, reference standards, and couplant.
a. Ultrasonic Instruments. A portable, battery-powered ultrasonic instrument is used for field inspection of airplane structure. (See Figure 5-17.) The instrument generates an ultrasonic pulse, detects and amplifies the returning echo, and displays the detected signal on a CRT or similar display. Piezoelectric transducers produce longitudinal or shear waves, which are the most commonly used wave forms for aircraft structural inspection. Or is a configuration that provides an ultrasonic response pattern representative of the test structure. The reference standard contains a simulated defect (notch) that is positioned to provide a calibration signal representative of the expected defect. The notch size is chosen to establish inspection sensitivity (response to the expected defect size). The inspection procedure gives a detailed description of the required reference standard.
FIGURE 5-17. TYPICAL PORTABLE ULTRASONIC INSPECTION INSTRUMENT
b. P ositioning Fixtures. To direct ultrasound at a particular angle, or to couple it into an irregular surface, transducer positioning fixtures and sound-coupling shoes are employed. (See Figure 5-18.) Shoes are made of a plastic material that has the necessary sound-transmitting characteristics. Positioning fixtures are used to locate the transducer at a prescribed point and can increase the sensitivity of the inspection. (See Figure 5-19.) If a transducer shoe or positioning fixture is required, the inspection procedure will give a detailed description of the shoe or fixture.
FIGURE 5-18. EXAMPLE OF POSITION FIXTURE AND SHOE
0.35
(1.40) *(1.27)* 0.50 0.50. (1.27) 129 + 0.005 (3 277) radius.
21 deg ‡ 30 min
0.70
(1.78) 323P
0.380 + 0.005 - 0.000 (0.965) diameter 0.85 (flat bottom hole) (2.16) Note:
All dimensions in inches followed by centimeters in parentheses.
FIGURE 5-19. EXAMPLE OF THE USE OF A TRANSDUCER POSITIONING
FIXTURE General:
• Hand positioning of transducer Crack • Detectable crack - 0.15 to 0.3 in Centering pin Transducer Positioning fixture Optimum:
• Transducer positioned with fixture
• Detectable crack - 0.10 to 0.15 ir
• Crack
c. Reference Standards. Reference standards are used to calibrate the ultrasonic instrument (see Figure 5-20), reference standards serve two purposes to provide an ultrasonic response pattern that is related to the part being inspected, and to establish the required inspection sensitivity. To obtain a representative response pattern, the reference standard configuration is the same as that of the test structure, Par 5-93 Page 5-46
Par 5-93 Page 5-47 FIGURE 5-20. EXAMPLE OF A TYPICAL REFERENCE STANDARD
d. C ouplants. Inspection with ultrasonics is limited to the part in contact with the transducer. A layer of couplant is required to couple the transducer to the test piece because ultrasonic energy will not travel through air. Some typical couplants used are: water, glycerin, motor oils, and grease.
5-94. INSPECTION OF BONDED STRUCTURES. Ultrasonic inspection is finding increasing application in aircraft bonded construction and repair. Detailed techniques for specific bonded structures should be obtained from the OEM’s manuals, or FAA requirements. In addition, further information on the operation of specific instruments should be obtained from the applicable equipment manufacturer manuals.
a. Types of Bonded Structures. Many configurations and types of bonded structures are in use in aircraft. All of these variations complicate the application of ultrasonic inspections. An inspection method that works well on one part or one area of the part may not be applicable for different parts or areas of the same part. Some of the variables in the types of bonded structures are as follows.
(1) Top skin material is made from different materials and thickness.
(2) Different types and thickness of adhesives are used in bonded structures.
(3) Underlying structures contain differences in; core material, cell size, thickness, and height, back skin material and thickness, doublers (material and thickness), closure member attachments, foam adhesive, steps in skins, internal ribs, and laminates (number of layers, layer thickness, and layer material).
(4) The top only or top and bottom skin of a bonded structure may be accessible.