Service Inspection

T.O. 33B-1-1 6.8.18.1 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. 6.8.18.2 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. 6.8.19 Service Inspection. 6.8.19.1 General. 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. 6.8.19.2 Wear. 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. 6.8.19.3 Corrosion. 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. 6.8.19.4 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 6-106 Change 3