Rivet head cracking are acceptable under the following  conditions: and are not tipped in the same direction, tipping may have  occurred during some previous installation. Inspect rivets that are known to have been critically loaded,  but that show no visible distortion, by drilling off the head and  carefully punching out the shank. If upon examination, the  shank appears joggled and the holes in the sheet misaligned,  the rivet has failed in shear. In that case, determine what  is causing the stress and take necessary corrective action.  Countersunk rivets that show head slippage within the  countersink or dimple, indicating either sheet bearing failure  or rivet shear failure, must be replaced. Joggles in removed rivet shanks indicate partial shear failure.  Replace these rivets with the next larger size. Also, if the  rivet holes show elongation, replace the rivets with the next  larger size. Sheet failures, such as tearouts, cracks between  rivets, and the like, usually indicate damaged rivets, and the  complete repair of the joint may require replacement of the  rivets with the next larger size. The presence of a black residue around the rivets is not an  indication of looseness, but it is an indication of movement  (fretting). The residue, which is aluminum oxide, is formed  by a small amount of relative motion between the rivet and  the adjacent surface. This is called fretting corrosion, or  smoking, because the aluminum dust quickly forms a dark,  dirty looking trail, like a smoke trail. Sometimes, the thinning  of the moving pieces can propagate a crack. If a rivet is  suspected of being defective, this residue may be removed  with a general purpose abrasive hand pad, such as those  manufactured by Scotch Brite™, and the surface inspected  for signs of pitting or cracking. Although the condition  indicates the component is under significant stress, it does  not necessarily precipitate cracking.  [Figure 4-172] Airframe cracking is not necessarily caused by defective  rivets. It is common practice in the industry to size rivet  patterns assuming one or more of the rivets is not effective.  This means that a loose rivet would not necessarily overload  adjacent rivets to the point of cracking. The repair material must duplicate the strength of the original  structure. If an alloy weaker than the original material has  to be used, a heavier gauge must be used to give equivalent  cross-sectional strength. A lighter gauge material should not  be used even when using a stronger alloy. To prepare a damaged area for repair:  Corrosion is the gradual deterioration of metal due to a  chemical or electrochemical reaction with its environment.  The reaction can be triggered by the atmosphere, moisture,  or other agents. When inspecting the structure of an aircraft,  it is important to watch for evidence of corrosion on both the  outside and inside. Corrosion on the inside is most likely to  occur in pockets and corners where moisture and salt spray  may accumulate; therefore, drain holes must always be kept  clean. Also inspect the surrounding members for evidence  of corrosion.  All new sections fabricated for repairing or replacing  damaged parts in a given aircraft should be carefully laid  out to the dimensions listed in the applicable aircraft manual  before fitting the parts into the structure. 4-93 Repair Parts Layout  Repair Material Selection Damage Removal Inspection for Corrosion •  The depth of the crack is less than  1 ⁄ 8 of the shank  diameter. •  The width of the crack is less than  1 ⁄ 16 of the shank  diameter. •  The length of the crack is confined to an area  on the head within a circle having a maximum  diameter of 1 1 ⁄ 4 times the shank diameter. •  Cracks should not intersect, which creates the  potential for the loss of a portion of a head.  1. Remove all distorted skin and structure in  damaged area. 2.  Remove damaged material so that the edges of the  completed repair match existing structure and aircraft  lines. 3.  Round all square corners. 4.  Smooth out any abrasions and/or dents. 5.  Remove and incorporate into the new repair any  previous repairs joining the area of the new repair. Figure 4-172.  Smoking rivet.