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AC 43.13-1B 9/8/98 operated near a surface edge. Edge effects on vibration paths give a test reading that may be misinterpreted. To avoid edge effects, the test probe should be moved so that the inspection path follows the surface edge, giving a con- stant edge for the test probe to inspect. Edge effects are more pronounced in thicker mate- rial. To interpret meter readings correctly, the operator should determine whether there are variations in the thickness of the material. c. Probe Sending Signal. With the ex- ception of the Sondicator models, the test probes of the testers emit a sending signal that radiates in a full circle. The sending signal of the Sondicator probe travels from one trans- ducer tip to the other. For this reason, the test probeshouldbeheldsothatthetransducertips are at right angles to the inspection path. When inspecting honeycomb panels with a Sondicator model, the transducer tips should be moved consistently in the direction of the ribbon of the honeycomb or at right angles to the ribbon so that a constant subsurface is pre- sented. 5-96. THICKNESS MEASUREMENTS. Ultrasonic inspection methods can be used for measurement of material thickness in aircraft parts and structures. a. Applications. Ultrasonic thickness measurements are used for many applications, such as: checking part thickness when access to the back side is not available; checking large panels in interior areas where a conventional micrometer cannot reach; and in maintenance inspections for checking thickness loss due to wear and/or corrosion. b. Pulse-Echo Method. The most com- monly used ultrasonic thickness measurement method. The ultrasonic instrument measures time between the initial front and back surface signals or subsequent multiple back reflection signals. Since the velocity for a given material is a constant, the time between these signals is directly proportional to the thickness. Cali- bration procedures are used to obtain direct readout of test part thickness. c. Thickness Measurement Instrument Types. Pulse-echo instruments designed ex- clusively for thickness measurements are gen- erally used in lieu of conventional pulse-echo instruments; however, some conventional pulse-echo instruments also have direct thick- ness measurement capabilities. Conventional pulse-echo instruments without direct thick- ness measuring capabilities can also be used for measuring thickness by using special pro- cedures. d. Thickness Measurement Ranges. Dependent upon the instrument used and the material under test, material thickness from 0.005 inches to 20 inches (or more) can be measured with pulse-echo instruments de- signed specifically for thickness measuring. 5-97. LEAK TESTING. The flow of a pressurized gas through a leak produces sound of both sonic and ultrasonic frequencies. If the gas leak is large, the sonic frequency sound it produces can probably be detected with the ear or with such instruments as stethoscopes or microphones; however, the ear and these in- struments have limited ability to detect and lo- cate small leaks. Ultrasonic leak detectors are frequently used to detect leaks that cannot be detected by the above methods, because they are very sensitive to ultrasonic energy and, un- der most conditions, background noise at other frequencies does not affect them. a. Standard Method. A standard method of testing for leaks using ultrasonics is pro- vided in ASTM E 1002. The method covers procedures for calibration, location, and esti- mated measurements of leakage by the ultra- sonic technique (sometimes called ultrasonic translation). Page 5-50 Par 5-95PDF Image | AFS-640
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