Figure 5-44. Straight Beam Inspection of Test Part with Curved Surface.
Glycerin, silicones, and graphite greases shall not be used as couplants unless
authorized by specific engineering approval.
A thin film of liquid couplant material is required between the search unit and the test part in order to displace air at
the interface. This allows transmission of ultrasonic energy into the test part. Any liquid that will transmit ultrasonic
energy and meets the following requirements may be used.
a. Able to wet both the face of the search unit and the test part.
b. Is not corrosive or toxic.
c. Can be applied and removed easily.
d. Is homogeneous and free of bubbles.
e. Is viscous (adheres well) enough to prevent rapid flow off the test part.
Typical couplant materials include water, oil, grease, commercial gels and penetrant emulsifier. A guide for selection
of couplant materials for use at room temperature on various surfaces is given in Table 5-4. Other couplants of similar
viscosities may also be used. For overhead or vertical surfaces, higher viscosity materials may be used.