Dent: A Dent is depressed or hollow deformations without removal of material or change in cross sectional area (See: scratch & crack definition below). Stress wrinkles caused by failure of a fastener are not considered a dent. Generally dents are caused by impact from a smoothly contoured object. One characteristic that all dents should have is a "pushed in surface" and a relatively smooth bottom where metal is not displaced, folded or creased. Many Aircraft Structural Repair Manuals specify that a "crease" be treated as a crack. Generally when evaluating dents, the width of the dent is the second longest distance across the dent, measured at 90 degrees to the direction of the length. Nicks: Broken edges without cracks, but with portions of material removed. Negligible damage limits will vary with structure, material, and loading. Scratches: Marks penetrating the surface that reduce the structural cross section of the material but do not penetrate the complete thickness. The depth of a scratch may be determined by use of an optical micrometer. Generally, scratchesin Alclad aluminum alloy sheet that do not penetrate the protective Alclad layer are classified as negligible. Cracks: Fractures that would not separate the material into two parts if the surrounding supports were removed; usually originating at edges, holes, or points where concentrated loads are applied or where abrupt changes in cross-sectional area occur. Cracks cause a significant cross-sectional area change. This damage usually has an irregular line and is often the result of fatigue in the material. The length of cracks that may be tolerated varies widely with material, structure, and application. No crack should be regarded as negligible until the damage limits for the affected structure have been determined. Holes: Punctures penetrations or cutouts that breach the complete thickness of the material and is fully surrounded by undamaged material. The size, shape, and distance from edges and supporting structures must be considered when evaluating hole damage. Abrasion: A damaged area that is the result of scuffing, rubbing, scraping, or other surface erosion. This type of damage is usually rough and has an irregular shape. Gouge: A damaged area where the result is a cross-sectional change caused by a sharp object and gives a continuous, sharp or smooth groove in the material. Corrosion: Deterioration of a metal because of an electrochemical reaction with its environment. Depending on the type of?corrosion, this deterioration may take the form cracking, exfoliation, or erosion of the corroding material.?Corrosion?damage is typically classified as light, moderate, or severe, depending on the extent of the corrosion and the loading requirements of the corroded part. Aircraft-specific structural manuals should be consulted for the correct classification of corrosion damage on a given part. Note: An initial accurate determination of the type of damage encountered can usually be made by the use of a 10X magnifying glass or an optical micrometer. True crack length determination will generally require some form of Non Destructive Testing such as?Eddy Current?or Fluorescent penetrants