AC 20-83 17 Jan 73 Appendix, 3
(3) Visual examination of the surfaces for indications of fatigue cracks. These indications are provided by the particles being attracted to the locations of the cracks (or other defects) due to local variations in the magnetic field that are produced.
c. Two classes of magnetic particles are available. The wet method particles use a liquid vehicle; the dry method particles are borne by air. These particles are usually:
(1) Colored to give contrast with the surface being inspected, or
(2) Coated with fluorescent material to make them readily visible under black light. Parts inspected by magnetic particle methods must be cleaned.
7. RADIOGRAPHY. Radiography is a method of nondestructive testing which uses X-ray, gamma, beta, or neutron radiation. it is based on the ability i these radioactive sources to penetrate materials. The intensity of the penetrating radioactivity is modified by passage through materials and by defects in the material. These intensity changes are recorded on film as areas of varying density (or darkness) which permits distinguishing flaws and cracks. Obviously, maxinum sensitivity occurs when the cracks are oriented such that its longest dimension is parallel to the direction of radiation.
X-ray radiography has two main advantages; (1) versatility, and
(2) sensitivity. The X-ray energy source can be easily adjusted for variations in thickness. It is also adaptable to fluoroscopy and television systems.
The advantages of gamma radiography are; (1) portability, and (2) a relatively. low cost. Portability comes from the fact that the source is small. This permits its effective use in the field particularly in remote areas. One of the difficulties with the gamma radioactive source is that the source cannot be varied or turned off so that safety precautions must be observed at all times. (X-ray constitutes a health hazard only during operation of the X-ray equipment).
c. Conventional radiography is firmly established and reasonably easy to understand. One of the original drawbacks was the long time involved in the developing and processing of film. This has been overcome by modern automatic film processing techniques.
d. Interpretation of the processed film is the most important phase of radiography. Adequate tools such as a film illuminator lens and good working conditions should be available to assist the interpreter in detecting cracks in the part displayed on the film.
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AC 20-83 17 Jan 73 Appendix 3
8. ULTRASONICS. Ultrasonic methods have received wide acceptance in the aviation industry and are particularly useful for determining the integrity of a member of a structure. Basically, sound energy above the audible range is transmitted into a part, and a signal is received and analyzed. The ultrasonic wave is transmitted and received through transducers, which are placed upon the part to be inspected. The transducer must be properly coupled to the part and is the most critical aspect of the inspection. Good coupling can be achieved by using liquids at the transducer part interface.
a. The ultrasonic wave (or beam) may be evaluated in terms of either through transmission or reflection. The receiving transducer may be a separate unit (through transmission) or it may be the same transducer that sent the signal (reflection and resonance). For crack detection, the reflection technique is most commonly used. It permits the determination of the location of the crack wherever it might be within the part and only one transducer on one side of the part is needed. When ultrasonic wave (pulse) is sent into the part, a discontinuity (e.g. a crack) in its path on which it impinges will both absorb and reflect energy. A defect can be recognized by the relative time for return of the reflected energy to the transducer.
The ultrasonic methods now available are rapid, economical, sensitive, and can have good accuracy for determining crack extent and position.
Equipment is light and portable so that on-site inspections are possible. There are conditions which can limit the usefulness of ultrasonic inspections. These include unfavorable part geometry (such as complexity, contour, and size) orientation of the cracks, and misleading responses which may occasionally be obtained. Also, ultrasonic inspection, as presently practiced, depends upon experience, skill, and judgment of the inspector. He must interpret the crack size and location by the indirect evidence presented by the electronic equipment (oscilloscope). He must be able to distinguish between significant signals and spurious ones.
9. EDDY CURRENT. The eddy current method is a comparatively recent nondes- " tructive testing technique. It is being frequently used for nonmagnetic materials. (When used for magnetic materials, it requires more complex systems). The principle involved is simple. A coil that is carrying a high-frequency alternating current is brought near an electrical conductor and eddy currents are generated in the conductor. The eddy (or induced) current creates a magnetic field. Flaws, cracks, etc., cause resistance changes within the part. This affects the induced currents, and thus, the magnetic field produced by them. Detection and measurement of the magnetic field form the basis of the nondestructive testing.
a. Two types of coils are in common use. One is a circumferential coil through which a part passes. The second is called a probe coil which is placed on the surface of the part to be inspected. Each type coil can be made in a number of designs, depending on the application.
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17 Jan 73 AC 20-83 Appendix 3 b • Eddy current instruments have various degrees of versatility. Many are portable. Once proven for a specific application, the inspection process is very rapid. Eddy current methods have been used very successfully for fatigue crack detection. However, eddy current methods are sensitive to many variables that influence the results obtained. Also, signals obtained are sometimes of a comparative nature, and reference standards are needed for interpretation.
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