SECTION IX ADVANCES IN ELECTROMAGNETIC TEST METHODS

Impedance Effects of Nonconductive Coatings

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 4-91 SECTION IX ADVANCES IN ELECTROMAGNETIC TEST METHODS 4.9 ADVANCES IN ELECTROMAGNETIC TEST METHODS. 4.9.1 General Improvements. 4.9.1.1 Impedance Plane Eddy Current Test Equipment. A significant increase in testing capability has been realized by the upgrading of existing test techniques with newer instrumentation. The use of modern impedance plane equipment has greatly increased the flaw analysis capability of the inspection process. 4.9.1.2 Digital Equipment. The use of digital test equipment, along with digital computers to process and analyze data, has provided significant reduction in the noise levels. This has effectively increased the sensitivity of the flaw detection process. 4.9.1.3 Mechanical Scanning. Increased use of mechanical scanners to control probe movement has increased the detection capability of many test methods. Repeatability of testing is also enhanced by mechanical scanning. A mechanical scanner can provide testing of difficult to reach areas of parts. Remote video cameras can also be incorporated with a mechanical scanner to provide visual coverage during the testing of inaccessible areas. 4.9.1.4 Data Management. Mechanical scanners controlled by computers or other microprocessors provide data management and increased assurance of proper coverage of the part being tested. Fastener hole inspection is a specific example of a test method that has been significantly improved by the use of mechanical scanning and computer data management. Improved record keeping along with the ability to analyze data are among the benefits to be realized by better data management. 4.9.1.5 Improved Calibration Standards. Improved calibration standards are required to meet the need for increased sensitivity and improved flaw discrimination. Improved test methods are capable of discriminating between actual flaws and fabricated discontinuities. Artificial flaws such as drilled holes and EDM notches are not sufficiently "real". In some cases the artificial flaw may not respond with the same phase as an actual flaw. More and more testing procedures require the calibration standard to contain actual fatigue cracks rather than EDM notches or other artificial flaws. 4.9.2 Techniques Available For use. 4.9.2.1 Mulitfrequency Testing Techniques. Multifrequency techniques have found a variety of applications in which several material properties are changing at the same time. A single frequency test signal is composed of phase and amplitude; therefore, only two variables such as the phase and amplitude of a signal response from a crack can be measured. If the wall thickness of a part is also changing, this variation could affect the phase or amplitude of the crack signal. By the use of multiple frequency techniques, multiple variables can be selectively detected and analyzed during the same test. For example, this allows dimensional and/or permeability variations to be filtered out during the testing process. 4.9.2.2 Dual Frequency Testing. If only two frequencies are used, one frequency channel can operate in the differential probe mode and the other frequency channel can operate in the absolute mode. With this setup the differential mode can be used to detect discrete indications such as small cracks and holes, and the absolute mode can be used simultaneously to record wall thickness or other dimensional changes in the test part.

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