Inspection Concepts & Techniques
What is the purpose of the couplant used during ultrasonic inspection?
Answer
It excludes air between the transducer and the part so sound energy can pass into the material
Ultrasonic energy does not transmit efficiently across an air gap, so a couplant (such as oil, gel, or water) is applied to exclude air between the transducer and the test surface, allowing sound to pass into the part. Developing an indication under UV light describes fluorescent penetrant, inducing eddy currents describes eddy current inspection, and cooling the transducer is not the couplant's function, so those are cross-method distractors.
Source: FAA-H-8083-30B, Ch. 10 (Inspection Concepts and Techniques), 'Ultrasonic Inspection'.
Not the answer
- It cools the transducer to keep it from overheating during the test
- It induces eddy currents in the surface of the part being tested
- It chemically develops the indication so it can be seen under ultraviolet light
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