Flash welding, which produces heat by creating an arc between the pieces to be joined, and pressure welding,
done by applying pressure to suitably prepared surfaces at temperatures lower than the melting point of the
parts, are seldom radiographically inspected. When performed, the inspection is usually made to detect
cracks produced in welding procedure.
Incomplete fusion at the interfaces between weld and parent metal has certain factors in common with a
crack. But the plane of incomplete fusion is rarely normal to the plate surface and for this reason is not
always revealed. Where such a discontinuity is suspected, additional exposures at various angles may reveal
the lack of bonding.
When materials are utilized fully as required in the design of modern aircraft, there is occasional failure due
to fatigue. These failures are results of over-stress of the material due to unusual operating conditions or
deterioration of the material. This type of material change is most difficult to detect due to the very nature of
the changes and the inaccessibility of the areas in which these changes are most likely to occur in an aircraft.
These service type changes in an aircraft are usually due to wear, corrosion, fractures or shear of material.
Radiography has been used to detect these conditions when they occur in inaccessible areas and are not
available for visual inspection.
Rivets and bolts may wear the skin, spar and frame holes so that there is not a correct fit in the holes for
adequate strength in joints or attachments of a wing section. This can occur due to continued f lexing of
components from use or because of severe stress due to unusual operating conditions in turbulent weather or
an adverse landing. This condition may also result in radial cracks from boltholes. This type of failure is
extremely difficult to detect by radiography. Any angle of exposure results in superimposition of bolt or nut
over crack. Loose bolts and rivets have been detected satisfactorily when occurring in position to be located.
Elongation of rivet holes caused by bearing failure or sheared rivets should not be confused with elongation of
holes from drilling. If fatigue is suspected in a riveted joint, the half moon indications should all be on the
same side of the rivet and the rivets in the joint should show similar indications of failure. Intermittent
indications would normally be considered fabrication tolerance.
Corrosion may occur in aircraft materials, which reduces its strength and expedites the possible failure. This
deterioration of the metal may be due to electrolytic action, moisture, chemicals or gases which attack the
metals, intergranular action due to improper heat treatment at the time of manufacture, or other factors.
This condition usually occurs on internal surfaces of such components as tubular supports or housings. Since
corrosion represents a change of material and occurs in all directions it is easily detected by a proper
radiographic exposure. If corrosion has proceeded to this point, the support is appreciably reduced in strength
and may experience failure.
Cracks and Crack-Like Discontinuities.
Cracks and other crack-like discontinuities are found in numerous parts and/or structures and are very
dangerous discontinuities. This is particularly true where structures are subjected to vibration or fatigue
loading, due to propagation of these crack-like discontinuities. Crack-like discontinuities will appear in a
radiograph as very straight and sharply outlined dark or black lines. Cracks may also appear as diffused
jagged lines. In some cases they have a tree-like pattern. Scatter radiation from the sides of a crack can act as
an amplifier of the crack image in a radiograph. This is the most difficult service type failure to detect by
radiography since these crack separations are usually not associated with other detectable conditions that
give clue to their presence. A crack-like discontinuity oriented at any