AC 20-84 22 Jan 73 Appendix 3 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 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 nondestructive 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.
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.
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 Page 4 Par 8
22 Jan 73 AC 20-84 Appendix 3 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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RECORD OF CHANGES DIRECTIVE NO. AC 20-84
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