Search units can be equipped with collimators to reduce the size of the sound beam entering the test part. The
collimator may be a solid cone (usually acrylic plastic) bonded to the face of the search unit. This type of collimator
reduces the diameter of the sound beam entering the test part to the diameter of the tip of the cone. The cone also acts
as a delay line and can result in better near surface resolution and broadens the beam spread (see 126.96.36.199.5). However,
this type of collimator reduces the energy entering the test part. Hollow cylindrical collimators may also be used in
immersion inspections in which the collimator is attached to an immersion search unit to control the beam shape.
Search Unit Wedges and Shoes.
Reasons For Use.
a. Wedges and shoes are used to adapt search units for angle beam and surface wave inspections and for
inspecting parts with curved surfaces. Normally search units are purchased with built-in wedges for
angle beam and surface wave inspections on either flat or curved surfaces. When ordering search units,
simply add appropriate requirements for the required refracted angle, the material to be inspected, the
radius of curvature of the inspection surface and the direction of the ultrasonic beam with respect to the
curvature. If necessary, straight beam search units can be adapted for use in angle beam and surface
wave inspections by using wedges fabricated in accordance with paragraph 188.8.131.52.
b. If flat probes are used on convex surfaces, the ultrasonic energy transmitted into the part is drastically
reduced, because only the center of the search unit makes good contact with the part. Flat search units
of small size (1/4-inch or less diameter or width) can be used in some cases on convex surfaces (Figure
5-35) down to 1.4-inch radius. However, loss of power results due to the smaller contact area.
c. Inspections performed with flat-faced search units on curved surfaces will be hindered by the tendency
of the transducer to rock (see Figure 5-35). This varies the angle of the incident and refracted sound
beam and causes problems in interpretation. These inspection techniques are not recommended.
Figure 5-35. Angle Beam Inspection of Curved Surface Using Flat Search Unit.