Replace buckled or bent members or reinforce them by  attaching a splice over the affected area. A buckled part of the  structure shall not be depended upon to carry its load again,  no matter how well the part may be strengthened.  structure be investigated, since the damaging force may have  been transmitted over a large area, sometimes quite remote  from the point of original damage. Wrinkled skin, elongated  or damaged bolt or rivet holes, or distortion of members  usually appears in the immediate area of such damage, and  any one of these conditions calls for a close inspection of  the adjacent area. Check all skin, dents, and wrinkles for any  cracks or abrasions.  Nondestructive inspection methods (NDI) are used as  required when inspecting damage. NDI methods serve as  tools of prevention that allow defects to be detected before  they develop into serious or hazardous failures. A trained  and experienced technician can detect flaws or defects with  a high degree of accuracy and reliability. Some of the defects  found by NDI include corrosion, pitting, heat/stress cracks,  and discontinuity of metals. The material used in all replacements or reinforcements must  be similar to that used in the original structure. If an alloy  weaker than the original must be substituted for it, a heavier  thickness must be used to give equivalent cross-sectional  strength. A material that is stronger, but thinner, cannot be  substituted for the original because one material can have  greater tensile strength but less compressive strength than  another, or vice versa. Also, the buckling and torsional strength  of many sheet metal and tubular parts depends primarily on the  thickness of the material rather than its allowable compressive  and shear strengths. The manufacturer’s SRM often indicates  what material can be used as a substitution and how much  thicker the material needs to be.  Figure 4-168  is an example  of a substitution table found in an SRM.  Care must be taken when forming. Heat-treated and cold- worked aluminum alloys stand very little bending without  cracking. On the other hand, soft alloys are easily formed,  but they are not strong enough for primary structure. Strong  alloys can be formed in their annealed (heated and allowed  to cool slowly) condition, and heat treated before assembling  to develop their strength. The size of rivets for any repair can be determined by  referring to the rivets used by the manufacturer in the next  parallel rivet row inboard on the wing or forward on the  fuselage. Another method of determining the size of rivets to  be used is to multiply the thickness of the skin by three and  use the next larger size rivet corresponding to that figure. For  example, if the skin thickness is 0.040-inch, multiply 0.040- inch by 3, which equals 0.120-inch; use the next larger size  rivet,  1 ⁄ 8-inch (0.125-inch). The number of rivets to be used  for a repair can be found in tables in manufacturer’s SRMs or  in Advisory Circular (AC) 43.13-1 (as revised), Acceptable  Methods, Techniques, and Practices—Aircraft Inspection  and Repair.  Figure 4-169  is a table from AC 43.13-1 that is  used to calculate the number of rivets required for a repair.  Ensure that the cross-sectional area of a splice or patch is  at least equal to or greater than that of the damaged part.  Avoid abrupt changes in cross-sectional area. Eliminate  dangerous stress concentration by tapering splices. To  reduce the possibility of cracks starting from the corners of  cutouts, try to make cutouts either circular or oval in shape.  Where it is necessary to use a rectangular cutout, make the  radius of curvature at each corner no smaller than  1 ⁄ 2-inch. If  the member is subjected to compression or bending loads,  the patch should be placed on the outside of the member to  obtain a higher resistance to such loads. If the patch cannot  be placed there, material one gauge thicker than the original  shall be used for the repair.  Extensive repairs that are made too strong can be as  undesirable as repairs weaker than the original structure. All  aircraft structure must flex slightly to withstand the forces  imposed during takeoff, flight, and landing. If a repaired  area is too strong, excessive flexing occurs at the edge of  the completed repair, causing acceleration of metal fatigue. Certain fundamental rules must be observed if the original  strength of the structure is to be maintained. When investigating damage, proceed as follows: In any aircraft sheet metal repair, it is critical to: Maintaining Original Strength •  Remove all dirt, grease, and paint from the damaged  and surrounding areas to determine the exact condition  of each rivet, bolt, and weld. •  Inspect skin for wrinkles throughout a large area. •  Check the operation of all movable parts in the area. •  Determine if repair would be the best procedure. •  Maintain original strength, •  Maintain original contour, and •  Minimize weight. 4-87