T.O. 33B-1-15-2energy (reverse piezoelectric effect). (See Figure 5-1) Ultrasonic energy is transmitted between the search unit and thetest part through a coupling medium, such as oil, grease, water, penetrant emulsifier, water soluble gels, or otherapproved couplants (see Figure 5-2). The purpose of the couplant material is to eliminate air at the interface. Since airhas 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 VibrationsFigure 5-2. Coupling of Search Unit to Test Part for Transmission of Ultrasonic Energy.5.1.3 BasicUltrasonicInspection.5.1.3.1 Methods.5.1.3.1.1 ContactInspection.Contact Inspection is the method in which the search unit makes direct contact with the material, with a minimumcouplant film as (see Figure 5-2). Viscosity and surface wetting of the couplant must be sufficient to maintain goodultrasonic energy transmission into the part.
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