Page 217 - Handbook of Adhesives and Sealants

188 Chapter Five

Contact through transmission—the transmitting search unit is on
one side of the bonded structure and the receiving unit is on the
other; and
Immersion method—the assembly is immersed in a tank of water;
the water acts as a coupling mechanism for the ultrasonic signal.

Figure 5.3 illustrates these various NDT ultrasonic methods.
Pulse echo techniques are perhaps the easiest to use in production.
The sound is transmitted through the part, and reflections are ob-
tained from voids at the bond interface. The result is generally con-
sidered only qualitative because a poorly bonded joint will show as a
good joint as long as it is acoustically coupled. A thin layer of oil or
water at the interface may act as a coupling and disguise an unbonded

Figure 5.3 Ultrasonic inspection techniques. (a) Contact pulse echo with a search unit
combining a transmitter and receiver. (b) Contact through transmission. Transmitting
search unit on top and receiving search unit on bottom. (c) Immersion pulse echo with
search unit (transmitter/receiver) and part inspected under water. (d) Immersion
through transmission with both search units (transmitter and receiver) and part under
water. (e) Immersion reflector plate. Same as (c) but each unit requires a reflector plate
below the part being inspected. 4