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.
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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.
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b. Application of Ultrasonic Inspection. Application to bonded structures must be examined in detail because of the many inspection methods and structural configurations. The advantages and limitations of each inspection method should be considered, and reference standards (representative of the structure to be inspected) should be ultrasonically inspected to verify proposed techniques.
c. Internal Configuration. Complete information on the internal configuration of the bonded test part must be obtained by the inspector. Drawings should be reviewed, and when necessary, radiographs of the test part should be taken. Knowledge of details such as the location and boundaries of doublers, ribs, etc., is required for valid interpretation of ultrasonic inspection results. The boundaries of internal details should be marked on the test part using a grease pen or other easily removable marking.
d. Reference Standards. Standards can be a duplicate of the test part except for the controlled areas of unbond. As an option, simple test specimens, which represent the varied areas of the test part and contain controlled areas of unbond, can be used. Reference standards must meet the following requirements:
(1) The reference standard must be similar to the test part regarding material, geometry, and thickness. This includes containing closure members, core splices, stepped skins, and internal ribs similar to the test part if bonded areas over or surrounding these details are to be inspected.
(2) The reference standard must contain bonds of good quality except for controlled areas of unbond fabricated as explained below.
(3) The reference standard must be bonded using the adhesive and cure cycle prescribed for the test part.
e. Types of Defects. Defects can be separated into five general types to represent the various areas of bonded sandwich and laminate structures as follows:
(1) Type I. Unbonds or voids in an outer skin-to-adhesive interface.
(2) Type II. Unbonds or voids at the adhesive-to-core interface.
(3) Type III. Voids between layers of a laminate.
(4) Type IV. Voids in foam adhesive or unbonds between the adhesive and a closure member at core-to-closure member joints.
(5) Type V. Water in the core.
f. Fabrication of NDI Reference Standards. Every ultrasonic test unit should have sample materials that contain unbonds equal to the sizes of the minimum rejectable unbonds for the test parts. Information on minimum rejectable unbond sizes for test parts should be obtained from the OEM’s manuals, FAA requirements, or the cognizant FAA ACO engineer. One or more
Par 5-94 Page 5-49 of the following techniques can be used in fabricating reference defects; however, since bonding materials vary, some of the methods may not work with certain materials.
(1) Standards for Types I, II, III, and IV unbonds can be prepared by placing discs of
0.006 inch thick (maximum) Teflon sheets over the adhesive in the areas selected for unbonds.
For Type II unbonds, the Teflon is placed between the core and adhesive. The components of the standard are assembled and the assembly is then cured.
(2) Types I, II, and III standards can also be produced by cutting flat-bottomed holes of a diameter equal to the diameter of the unbonds to be produced. The holes are cut from the back sides of bonded specimens, and the depths are controlled to produce air gaps at the applicable interfaces. When using this method, patch plates can be bonded to the rear of the reference standard to cover and seal each hole.
(3) Type II standards can be produced by locally undercutting (before assembly) the surface of the core to the desired size unbond. The depth of the undercut should be sufficient to prevent adhesive flow causing bonds between the undercut core and the skin.
(4) Type IV standards can be produced by removing the adhesive in selected areas prior to assembly.
(5) Type V standards can be produced by drilling small holes in the back of the standard and injecting varying amounts of water into the cells with a hypodermic needle. The small holes can then be sealed with a small amount of water-resistant adhesive.
g. Inspection Coverage. Examples of several different configurations of bonded structure along with suggested inspection coverages with standard ultrasonic test instruments are shown in Figure 5-21. In many cases, access limitations will not permit application of the suggested inspections in all of the areas shown. The inspection coverages and suggested methods contained in Figure 5-21 and Table 5-7 are for reference only. Details of the inspection coverage and inspections for a particular assembly should be obtained from the OEM’s manuals, or other FAA-approved requirements.
h. Inspection Methods. Table 5-8 lists the various inspection methods for bonded structures along with advantages and disadvantages of each inspection method.
FIGURE 5-21. EXAMPLES OF BONDED STRUCTURE CONFIGURATIONS AND
SUGGESTED INSPECTION COVERAGE CORE WITH "Z" CLOSURE MEMBER CORE AND SPLICE JOINT CORE (A) (B) (C) CORE WITH "Z" CLOSURE CORE AND CLOSURE CORE AND INTERNAL RIBS WITH
MEMBER AND LAND IN SKIN MEMBER FLIM ADHESIVE BOND TO TOP OF CORE (D) (E)
TEST PER DETAIL CORE AND INTERNAL RIBS
CORE AND WHEN GAP IS CORE, CLOSURE WITH FOAM ADHESIVE BOND CLOSURE 1 INCH OR WIDER MEMBER, AND TO TOP OF CORE
MEMBER LANDS IN SKINS (G)
CORE AND CLOSE OUT CORE AND CLOSURE CORE, CLOSURE MEMBER
MEMBER WITH SHEAR AND DOUBLER PLATES (K) "Z" OR HAT LAMINATED "T," DOUBLER, TRIPLER, COMPOSITE COMPOSITE
LAMINATES WEB, OR CHANNEL OR COMPOSITES SPLICE JOINT SPLICE JOINT (N) (P) (Q) (R) (S) NOTES:
1. THE NUMBERED LINES SURROUNDING EACH VIEW INDICATE THE SCAN PLANES. THE
NUMBER ON EACH LINE IS USED TO DETERMINE THE ACCEPTABLE INSPECTION METHODS BY REFERRING TO TABLES.
2. WHERE SURFACES ARE SYMMETRICAL, THE COVERAGE ILLUSTRATED SHALL BE CONSIDERED TYPICAL FOR BOTH SIDES.
3. SHADED AREAS REPRESENT FOAM ADHESIVE. I
4. WHEN THE SAME METHOD(S) ARE SPECIFIED IN MORE THAN ONE SCAN PLANE, CALI- BRATION SHALL BE VERIFIED FOR EACH PLANE.
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Par 5-94 Page 5-51 TABLE 5-7. ACCEPTABLE ULTRASONIC INSPECTION METHODS ASSOCIATED
WITH THE EXAMPLE BONDED STRUCTURE CONFIGURATIONS SHOWN IN FIGURE 5-21
NUMBER ACCEPTABLE METHODS 1 Either
(a) Pulse-echo straight or angle beam, on each side;
or
(b) Through-transmission.
2 Pulse-echo, straight beam, on each skin.
3 Refer to 1 for methods. If all these methods fail to have sufficient penetration power to detect reference defects in the reference standard, then the ringing method, applied from both sides, should be used. Otherwise, the ringing method is unacceptable.
4 Either
(a) Ringing, on each skin of the doubler;
or
(b) Through-transmission;
or
(c) Damping.
5 Ringing.
6 Through-transmission. Dotted Line represents beam direction.
7 Either
(a) Through-transmission;
or
(b) Damping.
NOTE:
A variety of ultrasonic testing methods and instruments are available for adaptation to specific inspection problems. Other bond inspection instruments can be used if detailed procedures are developed and proven on the applicable reference standards for each configuration of interest.
Some representative instruments, which can be used for the inspection of bonded structures are the Sondicator, Harmonic Bond Tester, Acoustic Flaw Detector, Audible Bond Tester, Fokker Bond Tester, 210 Bond Tester, and Bondascope 2100.