used for the angle beam shear wave inspection should be about one-half the frequency used for the straight beam
inspection. This provides approximately the same wavelength for both the longitudinal and shear waves. Refracted
longitudinal wave inspection should be at the same frequency used for straight longitudinal wave inspection.
The search unit is selected based on the requirements for mode, frequency, beam direction and beam size. The part
geometry and the limitations on accessibility to the inspection surface determine if special wedges or shoes are
required. Refer to paragraphs 184.108.40.206 through 220.127.116.11 for information on shoes and wedges.
The sound entry surface is visually examined to determine if any special preparation is required to provide a suitable
condition for ultrasonic inspection. The surface finish should be 250 microinches or smoother as explained in
paragraph 18.104.22.168. Painted surfaces can normally be inspected without removing the paint if the paint is uniform and is
tightly adhered to the part surface. Loose or uneven, patchy paint shall be stripped prior to ultrasonic inspection.
The couplant is selected based upon the surface condition; the surface orientation and the information in paragraph
ULTRASONIC INSPECTION OF BONDED STRUCTURES
ULTRASONIC INSPECTION OF BONDED STRUCTURES.
A bonded structure is one consisting of two or more components adhesively bonded together. The structure can be all
metallic or nonmetallic, or it can consist of both types of material. A bonded structure can contain honeycomb or other
type of light-weight core. Sheets of metal or nonmetal can be bonded together to provide the appropriate thickness.
Carbon/epoxy composites are bonded structures although the individual layers are only a few thousands of an inch thick
and essentially lose their individual identity in the curing process. However, separations (delaminations) do occur
between layers as a result of external impacts with foreign objects.
Variables Applicable to Bonded Structures.
Because of the many configurations and types of bonded structures, there are many variables to consider when
a. Probe-side skin material and thickness.
b. Adhesive type and thickness.
c. Underlying structure -- core material, thickness of core, cell size, and thickness of cell wall; far-side
skin material and thickness; quantity, thickness and material of doublers; attachments of closure
members; foam adhesive; steps in skins; internal ribs; and makeup of nonmetallic composite laminates
(material, number of layers and layer thickness).
d. Accessibility -- one skin or both skins.
All of these variations complicate the application of ultrasonic inspection methods. A method, which works well on
one part or in one area of the part, may not be applicable for different parts or different areas of the same part.