energy (reverse piezoelectric effect). (See Figure 5-1) Ultrasonic energy is transmitted between the search unit and the
test part through a coupling medium, such as oil, grease, water, penetrant emulsifier, water soluble gels, or other
approved couplants (see Figure 5-2). The purpose of the couplant material is to eliminate air at the interface. Since air
has a very high acoustic impedance, it is a poor transmitter of ultrasound, and therefore is not a good coupling medium.
The couplant material shall not be injurious to the material to be inspected.
Figure 5-1. Generation of Ultrasonic Vibrations
Figure 5-2. Coupling of Search Unit to Test Part for Transmission of Ultrasonic Energy.
Basic Ultrasonic Inspection.
Contact Inspection is the method in which the search unit makes direct contact with the material, with a minimum
couplant film as (see Figure 5-2). Viscosity and surface wetting of the couplant must be sufficient to maintain good
ultrasonic energy transmission into the part.